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Title: Wireless Transmission of Photographs
       Second Edition, Revised and Enlarged 1919


Author: Marcus J. Martin



Release Date: October 9, 2010  [eBook #34052]

Language: English

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WIRELESS TRANSMISSION OF PHOTOGRAPHS

[Illustration: FIG. 10.]

       *       *       *       *       *


WIRELESS TRANSMISSION OF PHOTOGRAPHS

by

MARCUS J. MARTIN

SECOND EDITION
REVISED AND ENLARGED 1919







The Wireless Press, Ltd.
12-13 Henrietta Street, Strand
London, W.C. 2



{v}

PREFACE TO SECOND EDITION

Although during the last few years very little, in common with other
wireless work, has been possible in connection with the practical side of
the wireless transmission of photographs, yet, now that the prospect of
experimental work is once again occupying the minds of all wireless
workers, advantage has been taken of a reprint of this little volume to
amplify a few points that were insufficiently dealt with in the first
edition, and also to add some fresh matter.

To Chapter V. has been added a short description of the Nernst lamp, and
also some useful information regarding photographic films, and a few notes
relating to enlarging included in the Appendix B.

A fresh appendix dealing with the principles of optical lenses has also
been added. This is a subject that plays an important part in any system of
wireless photography, and to those experimenters whose knowledge of optics
is limited this section should prove useful.

To serious workers engaged on the problem of the wireless transmission of
photographs, attention {vi} is called to a series of articles which are
being published from time to time in the _Wireless World_, on the design
and construction of wireless photographic apparatus.

  M. J. M.

  MAIDSTONE, 1919.

{vii}

PREFACE

In these progressive times it is only reasonable to expect that some
attempt would be made to utilise the ether-waves for other purposes than
that of telegraphic communication, and already many clever minds are at
work trying to solve the problems of the wireless control of torpedoes and
airships, wireless telephony, and, last but not least, the wireless
transmission of photographs.

It may seem rather premature to talk about the wireless transmission of
photographs at a time when the ordinary systems are not fully developed;
but the prospects of wireless photography are of a very encouraging nature,
especially for long over-water distances, as there are great difficulties
to be overcome in long-distance transmission over ordinary land lines and
cables which will be entirely eliminated by wireless methods.

From a perusal of Chapter I. the reader will be able to understand
something of the difficulties that are to be encountered in working over
long distances, and he will also be able to appreciate something of the
advantages that would be derived {viii} from a reliable wireless system.
Apart from the value of such a system for transmitting news pictures, it
would also be of great advantage to transmit to ships at sea photographs of
criminals for identification purposes. In such a small volume as this it
would be impossible to deal with the working of wireless apparatus and the
many systems that have been devised for the transmission of photographs
over metallic circuits. The Author has taken it for granted that other
works have been studied in connection with these subjects, and will
therefore only describe such apparatus as is likely to be of use in
wireless transmission. At present the transmission of photographs by
wireless methods is in a purely experimental stage, and this book will have
served its purpose if it helps to put future experimenters on the right
track and prevent them from making expensive and fruitless experiments, by
showing them the right direction in which investigations are being carried
out. As there is no claim to originality in respect of a good many pieces
of apparatus, etc., described, I have not thought it necessary to state the
various sources from which the information has been obtained.

  M. J. M.

  ASHFORD, 1916.

       *       *       *       *       *


{ix}

CONTENTS

                                                                       PAGE
  PREFACE TO SECOND EDITION                                               v

  PREFACE                                                               vii

CHAPTER I

  INTRODUCTORY                                                            1

    Foreword--Early experiments--Advantages of
    Radio-Photography--Difficulties in Cable working--Bernochi's
    System--Knudsen's System.

CHAPTER II

  TRANSMITTING APPARATUS                                                 13

    Wireless Apparatus--Preparing the Photographs--Transmitting
    Machines--Transmitting Apparatus--Effects of
    Arcing--Spark-Gaps--Contact Breakers--Complete Station--Professor
    Korn's Apparatus--Poulsen Company's Photographic Recorder--Comparison
    of various systems--Practical applications.

CHAPTER III

  RECEIVING APPARATUS                                                    37

    Methods of Receiving--Author's Photographic Receiver--Decohering
    Apparatus--Description of Einthoven Galvanometer--Use of Galvanometer
    in Receiving--Belin's Application of Blondel's
    Oscillograph--Description of Charbonelle's Receiver--Use of Telephone
    Relay--Description of Telephone Relay--Telephotographic
    Receiver--Polarisation Receiver--Kathode-Ray Receiver--Electrolytic
    Receiver--Atmospherics in Long-Distance working.

{x}

CHAPTER IV

  SYNCHRONISING AND DRIVING                                              63

    Driving Motors--Isochronising the Electrolytic System--Professor Korn's
    method--Description of Hughes Governor--Author's Speed
    Regulator--Problem of Synchronising--Methods of Synchronising--Advances
    made in Radio-Photography.

CHAPTER V

  THE "TELEPHOGRAPH"                                                     74

    Author's System of
    Radio-Photography--Requirements--Advantages--Transmitting
    machine--Description of Differential Relay--Wireless Receiving
    Apparatus--Photo-Telegraphic Receiving Apparatus--Circuit
    Breaker--Friction Brake--Magnetic Clutch--Description of
    Isochroniser--Method of working--Types of Nernst Lamp--Action of Nernst
    Lamp--Comparison of Actinic Value--Inertia of Photographic
    Films--Choosing Films--Speed of Films--Standard of Speed--Comparative
    Film Speeds--Effects of Minimum Exposure--Effects of Maximum
    Exposure--Considerations in working and choosing Films.

APPENDIX A

  SELENIUM CELLS                                                        109

    Nature of Selenium--Preparation of Selenium--Forms of Selenium
    Cells--Action of Selenium Cells--Characteristics of Selenium
    Cells--Effects of Inertia in Photo-Telegraphy--Methods of counteracting
    Inertia--Sensitiveness of Selenium to Light--Effect of Heat on
    Selenium.

APPENDIX B

  PREPARING THE METAL PRINTS                                            115

    Outline of Process--Line Screens--Choice of Camera--Fixing Line Screen
    in Camera--Lenses and Stops--Taking the Photograph--Copying
    Stands--Choice of Photographic Plates--Sources of Illumination--Metal
    Prints--Coating the {xi} Metal Sheets--Sensitising Solution--Printing
    Operations--Developing--Intensifying--Precautions to be observed in
    working--Preparing Sketches on Metal--Apparatus for Reducing or
    Enlarging--Improvements to Copying Board--Lenses for Copying--Formula
    for Copying.

APPENDIX C

  LENSES                                                                126

    Action of Light--Law of Refraction--Lenses--Prisms--Action of
    Lenses--Focal Length of Lenses--Formation of Images--Apparent Magnitude
    of Objects--Real and Virtual Images--Formation of Virtual Images--Power
    of Magnification--Defects of Lenses--Aberration.

       *       *       *       *       *


{xiii}

ILLUSTRATIONS

  FIG.                                                                 PAGE

  1. Diagram showing effects of capacity on an intermittent current       5

  2. Bernochi's wireless apparatus                                        7

  3. Knudsen's wireless apparatus                                        10

  4. Wireless transmitting station                                       13

  5. Diagram of experiment illustrating principle of line photograph     16

  6. Drawing of transmitting machine                                     17

  7. Drawing of transmitting machine                                     18

  8. Drawing of stylus                                                   18

  9. Electrical connections of machine                                   19

  10. Photograph of Author's experimental machine            _Frontispiece_

  10a. End view of Author's experimental machine  }
                                                  }        _facing page_ 21
  10b. View of image broken up by a "cross" screen}

  11. Connections of complete transmitting apparatus                     23

  12. Drawing of ordinary type of spark-gap                              27

  13. Synchronous rotating spark-gap                                     28

  14. Non-synchronous rotating spark-gap                                 28

  15. Connections for complete wireless photographic station             30

  16. Connections of Professor Korn's apparatus                          31

  17. Connections of Poulsen's photographic recorder                     33

  18. Author's photographic receiver                                     38

  19. Enlarged drawing of cone                                           39

  20. End view of Author's photographic receiver                         39

  21. Connections of decohering apparatus                                41

  22. Connections for complete photographic receiver                     42

  {xiv}
  23. Arrangement of Einthoven galvanometer                              45

  24. Einthoven galvanometer arranged for receiving                      46

  25. Connection of telephone relay                                      49

  26. Drawing of Author's improved photographic receiver                 51

  27. Diagram giving ratio of vibrating arm                              51

  28. Arrangement of polarisation receiver                               53

  29. Arrangement of kathode-ray receiver                                54

  30. Connections of electrolytic receiver                               56

  31. Drawing of improved stylus for receiving                           58

  32. Drawing of Hughes telegraph governor                               66

  33. Arrangement of simple speed regulator                              68

  34. Diagram of connections of simple speed regulator                   68

  35. Author's arrangement for complete radio-photographic station       77

  36. Drawing of transmitting machine and circuit breaker                78

  37. Drawing of special transmitting stylus showing adjusting
      arrangements                                                       79

  37a. End view of transmitting stylus                                   79

  38. Connections of new type of relay designed by the Author            80

  39. Arrangement of mercury containers and dipping rods for relay       82

  40. Drawing of Author's receiver                                       84

  41. Enlarged drawing of diaphragm and steel point                      84

  41a. Drawing showing arrangement of bush and counter-weight            84

  42. Optical arrangements of receiver                                   85

  43. Optical arrangements of receiver                                   86

  44. Drawing of circuit breaker                                         88

  45. Drawing of friction brake                                          89

  46. Sectional drawing of magnetic clutch                               90

  47. Plan of magnetic clutch                                            90

  48. Details of Isochroniser                                            92

  49. Connections of Isochroniser                                        94

  50. Dial of Isochroniser                                               94

  51. Diagram of driving mechanism                                       96

  {xv}
  52. Diagram showing starting positions of machines                     97

  52a. Arrangement of small type Nernst lamp                             99

  52b. Ballasting resistances for Nernst lamps                          100

  52c. Arrangement of large type Nernst lamp                            101

  53. Connections of selenium cell elements                             110

  53a. Form of selenium cell used by Bell and Tainter                   110

  54. Diagram showing construction of modern cell                       111

  55. Resistance curve of selenium cell                                 111

  55a. Actual curve of selenium cell                                    112

  56. Diagram of Professor Korn's method for counteracting inertia      113

  57. Arrangement of plate sheath and line screen                       117

  58. Details of clips to hold line screen                              118

  59. Arrangement of apparatus for copying                              119

  60. Drawing showing method of arranging camera and copying stand for
     adjustment                                                         119

  61. Photograph of line screen and metal print  }
                                                 } _facing page_        124
  62. Photograph of sketch drawn upon metal foil }

  63. Method of marking out copying board                               124

  64. Diagram illustrating law of refraction                            127

  65. Forms of lenses                                                   128

  66. Action of light passed through a prism                            129

  67. Diagram illustrating action of a lens                             130

  68. Formation of principal focus of a lens                            130

  69. Formation of conjugate foci of a lens                             131

  70. Apparatus illustrating principle of camera                        132

  71. Formation of an image by a lens                                   133

  72. Diagram illustrating apparent magnitude                           134

  73. Formation of virtual image by a convex lens                       137

  74. Formation of virtual image by a concave lens                      138

  75. Diagram showing spherical aberration                              139

  76. Combination of plano-convex lenses                                139

  77. Combination of meniscus and convex lenses                         139

       *       *       *       *       *


{1}

RADIO-PHOTOGRAPHY

CHAPTER I

INTRODUCTORY

Those who desire to experiment on radio-photography, _i.e._ transmitting
photographs, drawings, etc., from one place to another without the aid of
artificial conductors, must cultivate at least an elementary knowledge of
optics, chemistry, mechanics, and electricity; photo-telegraphy calling for
a knowledge of all these sciences. There are, no doubt, many wireless
workers who are interested in this subject, but who are deterred from
experimenting owing to a lack of knowledge regarding the direction
developments are taking, besides which, information on this subject is very
difficult to obtain, the science of photo-telegraphy being, at the present
time, in a purely experimental stage.

The wireless transmission of photographs has, no doubt, a great commercial
value, but for any system to be commercially practicable, it must be
simple, rapid, and reliable, besides being able to work {2} in conjunction
with the apparatus already installed for the purpose of ordinary wireless
telegraphy.

As far back as 1847 experiments were carried out with a view to solving the
problem of transmitting pictures and writing by electrical methods over
artificial conductors, but no great incentive was held forth for
development owing to lack of possible application; but owing to the great
public demand for illustrated newspapers that has recently sprung into
being, a large field has been opened up. During the last ten years,
however, development has been very rapid, and some excellent results are
now being obtained over a considerable length of line.

The wireless transmission of photographs is, on the other hand, of quite
recent growth, the first practicable attempt being made by Mr. Hans Knudsen
in 1908. It may seem rather premature to talk about the wireless
transmission at a time when the systems for transmitting over ordinary
conductors are not perfectly developed, but everything points to the fact
that for long-distance transmission a reliable wireless system will prove
to be both cheaper and quicker than transmission over ordinary land lines
and cables.

The effects of capacity and inductance--properties inherent to all
telegraph systems using metallic conductors--have a distinct bearing upon
the two questions, how far and how quickly can {3} photographs be
transmitted? Owing to the small currents received and to prevent
interference from earth currents it is necessary to use a complete metallic
circuit. If an overhead line could be employed no difficulty would be
experienced in working a distance of over 1000 miles, but a line of this
length is impossible--at least in this country--and if transmission is
attempted with any other country, a certain amount of submarine cable is
essential. It has been found that the electrostatic capacity of one mile of
submarine cable is equal to the capacity of 20 miles of overhead line, and
as the effect of capacity is to retard the current and reduce the speed of
working, it is evident that where there is any great length of cable in the
circuit the distance of possible transmission is enormously reduced.

If we take for an example the London-Paris telephone line with a length of
311 miles and a capacity of 10.62 microfarads, we find that about half this
capacity, or 5.9 microfarads,[1] is contributed by the 23 miles of cable
connecting England with France.

In practice the reduction of speed due to capacity has, to a great extent,
been overcome by means of apparatus known as a line-balancer, which hastens
the slow discharge of the line and {4} allows each current sent out from
the transmitter--the current in several systems being intermittent--to be
recorded separately on the receiver. Photographs suitable for press work
can now be sent over a line which includes only a short length of cable for
a distance of quite 400 miles in about ten minutes, the time, of course,
depending upon the size of the photograph. In extending the working to
other countries where there is need for a great length of cable, as between
England and Ireland, or America, the retardation due to capacity is very
great. On a cable joining this country with America the current is retarded
four-tenths of a second. In submarine telegraphy use is made of only one
cable with an earth return, but special means have had to be adopted to
overcome interference from earth currents, as the enormous cost prohibits
the laying of a second cable to provide a complete metallic circuit. The
current available at the cable ends for receiving is very small, being only
1/200000th part of an ampere, and this necessitates the use of apparatus of
a very sensitive character. One system of photo-telegraphy in use at the
present time, employs what is known as an electrolytic receiver (see
Chapter III.) which can record signals over a length of line in which the
capacity effects are very slight, with the marvellous speed of 12,000 a
minute, but this speed rapidly decreases with an increase of distance
between the {5} [Illustration] two stations. The effect of capacity upon an
intermittent current is clearly shown in Fig. 1. If we were to send twenty
brief currents in rapid succession over a line of moderate capacity in a
given time, we should find that instead of being recorded separately and
distinctly as at _a_, each mark would be pointed at both ends and joined
together as shown at _b_, while only perhaps fifteen could be recorded. If
the capacity be still farther increased as at _c_, only perhaps half the
original number of currents could be recorded in the same time, owing to
the fact that with an increase of resistance, capacity, and inductance of
the line a longer time is required for it to charge up and discharge,
thereby materially lessening the rate at which it will allow separate
signals to pass; the number of signals that can therefore be recorded in a
given time is greatly diminished. If we were to attempt to send the same
number of signals over a line of great capacity, as could be sent, and
recorded separately and distinctly over a line of small capacity--the time
limit being of course the same in both instances--we should find that the
{6} signals would be recorded practically as a continuous line. The two
latter cases _b_, and _c_, Fig. 1, clearly shows the retardation that takes
place at the commencement of a current and the prolongation that takes
place at the finish. If the photo-telegraphic system previously mentioned
could be rendered sensitive enough to work on the Atlantic cables, we
should find that only about 1200 signals a minute could be recorded, and
this would mean that a photograph which could be transmitted over ordinary
land lines in about ten minutes would take at least fifty minutes over the
cable. This would be both costly and impracticable, and time alone will
show whether, for long-distance work, transmission by wireless will be both
cheaper and more rapid than any other method. At present wireless
telegraphy has not superseded the ordinary methods of communicating over
land, but there can be no doubt that wireless telegraphy, if free from
Government restrictions, would in certain circumstances very quickly
supersede land-line telegraphy, while it has proved a formidable commercial
competitor to the cable as a means of connecting this country with America.
Likewise we cannot say that no system of radio-photography will ever come
into general use, but where there is any great distance to be bridged,
especially over water, wireless transmission is really the only practical
solution. From the {7} foregoing remarks, it is evident that a reliable
system of radio-photography would secure a great victory in the matter of
time and cost alone, besides which, the photo-telegraphic apparatus would
be merely an accessory to the already existing wireless installation.

[Illustration: FIG. 2.]

There have been numerous suggestions put forward for the wireless
transmission of photographs, but they are all more or less impracticable.
One of the earliest systems was devised by de' Bernochi of Turin, but his
system can only be regarded interesting from an historical point of view,
and as in all probability it could only have been made to work over a
distance of a few hundred yards it is of no practical value. Fig. 2 will
help to explain the apparatus. A glass cylinder A' is fastened at one end
to a threaded steel shaft, which runs in two bearings, one bearing having
an internal thread corresponding with that on the {8} shaft. Round the
cylinder is wrapped a transparent film upon which a photograph has been
taken and developed. Light from a powerful electric lamp L, is focussed by
means of the lens, N, to a point upon the photographic film. As the
cylinder is revolved by means of a suitable motor, it travels upwards
simultaneously by reason of the threaded shaft and bearing, so that the
spot of light traces a complete spiral over the surface of the film. The
light, on passing through the film (the transmission of which varies in
intensity according to the density of that portion of the photograph
through which it is passing), is refracted by the prism P on to the
selenium cell S which is in series with a battery B and the primary X of a
form of induction coil. As light of different intensities falls upon the
selenium cell,[2] the resistance of which alters in proportion, current is
induced in the secondary Y of the coil and influences the light of an arc
lamp of whose circuit it is shunted. This arc lamp T is placed at the focus
of a parabolic reflector R, from which the light is reflected in a parallel
beam to the receiving station.

The receiver consists of a similar reflector R' with a selenium cell E
placed at its focus, whose resistance is altered by the varying light
falling upon it from the reflector R. The selenium cell {9} E is in series
with a battery F and the mirror galvanometer H. Light falls from a lamp D
and is reflected by the mirror of the galvanometer on to a graduated
aperture J and focussed by means of the aplanatic lens U upon the receiving
drum A^2, which carries a sensitised photographic film. The two cylinders
must be revolved synchronously. The above apparatus is very clever, but
cannot be made to work over a distance of more than 200 yards.

A system based on more practical lines was that invented and demonstrated
by Mr. Hans Knudsen, but the apparatus which he employed for receiving has
been discarded in wireless work, as it is not suitable for working with the
highly-tuned systems in use at the present time.

Knudsen's transmitter, a diagrammatic representation of which is given in
Fig. 3, consists of a flat table to which a horizontal to-and-fro motion is
given by means of a clockwork motor. Upon this table is fastened a
photographic plate which has been prepared in the following manner. The
plate upon which the photograph is to be taken has the gelatine film from
three to four times thicker than that commonly used in photography. In the
camera, between the lens and this plate, a single line screen is
interposed, which has the effect of breaking the picture up into parallel
lines. Upon the plate being developed and before it is {10} [Illustration]
completely dry, it is sprinkled over with fine iron dust. With this type of
plate the transparent parts dry much quicker than the shaded or dark parts,
and on the iron dust being sprinkled over the plate it adheres to the
darker portions of the film to a greater extent than it does to the lighter
portions; a picture partly composed of iron dust is thus obtained. A steel
point attached to a flat spring rests upon this plate and is made to travel
at right angles to the motion of the table. As the picture is partly
composed of iron dust, and as the steel needle is fastened to a delicate
spring it is evident that as the plate passes to and fro under the needle,
both the spring and needle are set in a state of vibration. This vibrating
spring makes {11} and breaks the battery circuit of a spark coil, which in
turn sets up sparking in the spark-gap of the wireless apparatus.

The receiver consists of a similar table to that used for transmitting, and
carries a glass plate that has been smoked upon one side. A similar spring
and needle is placed over this plate, but is actuated by means of a small
electro-magnet in circuit with a battery and a sensitive coherer. As the
coherer makes and breaks the battery circuit by means of the intermittent
waves sent out from the transmitting aerial, the needle is made to vibrate
upon the smoked glass plate in unison with the needle at the transmitting
end. Scratches are made upon the smoked plate, and these reproduce the
picture on the original plate. A print can be taken from this scratched
plate in a similar manner to an ordinary photographic negative.

The two tables are synchronised in the following manner. Every time the
transmitting table is about to start its forward stroke a powerful spark is
produced at the spark-gap. The waves set up by this spark operate an
ordinary metal filings coherer at the receiving end which completes the
circuit of an electro-magnet. The armature of this magnet on being
attracted immediately releases the motor used for driving, allowing it to
operate the table. The time taken to transmit a photograph, quarter-plate
size, is about fifteen minutes. {12} Although very ingenious this system
would not be practicable, as besides speed the quality of the received
pictures is a great factor, especially where they are required for
reproduction purposes. The results from the above apparatus are said to be
very crude, as with the method used to prepare the photographs no very
small detail could be transmitted.

       *       *       *       *       *


{13}

CHAPTER II

TRANSMITTING APPARATUS

Let us now consider the requirements necessary for transmitting photographs
by means of the wireless apparatus in use at the present time.

[Illustration: FIG. 4.]

The connections for an experimental syntonic wireless transmitting station
are shown in the diagram Fig. 4. A is the aerial; T, the inductance; E,
earth; L, hot-wire ammeter. The closed oscillatory circuit consists of an
inductance F, spark-gap G, and a block condenser C. H is a spark-coil for
supplying the energy, the secondary J being connected to the spark-gap. A
{14} mercury break N and a battery B are placed in the primary circuit of
the coil. The Morse key K is for completing the battery circuit for
signalling purposes. When the key K is depressed, the battery circuit is
completed, and a spark passes between the balls of the spark-gap G
producing oscillations in the closed circuit, which are transposed to the
aerial circuit by induction. For signalling purposes it is only necessary
for the operator by means of the key K to send out a long or short train of
waves in some pre-arranged order, to enable the operator at the receiving
station to understand the message that is being transmitted.

If a photograph could be prepared in such a manner that it would serve the
purpose of the key K, and could so arrange matters that a minute portion of
the photograph could be transmitted separately but in succession, and that
each portion of the photograph having the same density could be given the
same signal, then it would only be necessary to have apparatus at the
receiving station capable of arranging the signals in proper sequence (each
signal recorded being the same size and having the same density as the
transmitted portion of the photograph) in order to receive a facsimile of
the picture transmitted.

The following method of preparing the photograph[3] is one that has been
adopted in several {15} systems of photo-telegraphy, and is the only one at
all suitable for wireless transmission. The photograph or picture which is
to be transmitted is fastened out perfectly flat upon a copying-board. A
strong light is placed on either side of this copying board, and is
concentrated upon the picture by means of reflectors. The camera which is
used for copying has a single line screen interposed between the lens and
sensitised plate, and the effect of this screen is to break the picture up
into parallel lines. Thus a white portion of the photograph would consist
of very narrow lines wide apart, while the dark portion would be made up of
wide lines close together; a black part would appear solid and show no
lines at all. From this line negative it will be necessary to take off a
print upon a specially prepared sheet of metal. This consists of a sheet of
thick lead- or tinfoil, coated upon one side with a thin film of glue to
which bichromate of potash has been added; the bichromate possessing the
property of rendering the glue waterproof when acted upon by light. The
print can be taken off by artificial light (arc lamps being generally
used), but the exact time to allow for printing can only be found by
experiment, as it varies considerably according to the thickness of the
film. The printing finished, the metal print is washed under running water,
when all those parts not acted upon by light, _i.e._ the parts between the
lines, are {16} washed away, leaving the bare metal. We have now an image
composed of numerous bands of insulating material (each band varying in
width according to the density of the photograph at any point from which it
is prepared) attached to a metal base, so that each band of insulating
material is separated by a band of conducting material. It is, of course,
obvious that the lines on the print cannot be wider apart, centre to
centre, than the lines of the screen used in preparing it. A good screen to
use is one having 50 lines to the inch, but one is perhaps more suitable
for experimental work a little coarser, say 35 lines to the inch. To use a
screen having 50 or more lines to the inch, the transmitting apparatus, as
will be evident later on, will require to be very nearly perfect.

[Illustration: FIG. 5.]

Before proceeding further it will perhaps be as well to make an experiment.
If we take one of the metal prints or, more simple, draw a sketch in
insulating ink upon a sheet of metal A, Fig. 5, and connect a battery B and
the galvanometer D as shown, we shall find on drawing the free end of the
wire across the metal plate that all the time the wire is in contact with
the lines of insulating material the needle of the galvanometer will remain
{17} at zero, but where it is in contact with the metal plate the needle is
deflected.

From this experiment it will be seen that we have in our metal line print,
which consists of alternate lines of insulating and conducting material, a
method by which an electric circuit can be very easily made and broken. It
is, of course, necessary to have some arrangement whereby the whole of the
surface of the metal print is utilised for this purpose to the best
advantage. One type of transmitting machine used for this purpose is
represented by the diagram, Fig. 6. The cylinder A is fastened to the steel
shaft B, which runs in the two bearings D and D', the bearing D' having an
internal thread corresponding to that on the shaft. The stylus in this
class of machine is a fixture, the cylinder being given a lateral as well
as a revolving movement. As it is impossible to use a rigid drive, a
flexible coupling F is employed between the shaft B and the motor.

[Illustration: FIG. 6.]

Another type of machine is shown in Fig. 7. The drum in this case is
stationary, the table T moving laterally by reason of the screwed shaft
{18} [Illustration] and half nut F. The table, shown separate in Fig. 8,
carries a stiff brass spring A, to which is attached a holder B made to
take a hardened steel point. The holder is provided with a set screw P for
securing the steel point Z. The spring and needle are insulated from the
rest of the machine, as shown in the drawing. In working, the metal print
is wrapped tightly round the cylinder of the machine, the glue image being,
of course, uppermost. To fasten the print a little seccotine should be
applied to one edge, and the joint carefully smoothed down with the
fingers. [Illustration] If there is any tendency on the part of the print
to slip round on the drum, a couple of small spring clips placed over the
ends of the drum will act as a preventive. It is necessary to place the
print upon the drum in such a manner that the stylus draws away from the
edge of the lap and not towards it, and the metal prints should be of such
a size that when placed round the drum of the {19} machine a lap of about
3/16ths of an inch is allowed.

[Illustration: FIG. 9.]

The steel point Z (ordinary gramophone needles may be used and will be
found to answer the purpose admirably) is made to press lightly upon the
metal print, and while the pressure should be sufficient to make good
electrical contact, it should not be sufficient to cause the needle to
scratch the surface of the foil. The pressure is regulated by means of the
milled nut H. The electrical connections are given in Fig. 9. One wire from
the battery M is taken to the terminal T, and the other wires from M and F
lead to the relay R. The current flows from the battery M through the
spring Y, through the drum and metal print, the stylus Z, spring A, down to
the relay R, and from R back to the battery M. As the drum carrying the
single line half-tone print is revolved, the stylus, by reason of the
lateral movement given to the table or cylinder as the case may be, will
trace a spiral path over the entire surface of the print. As the stylus
traces over a conducting strip the circuit is completed, and the tongue of
the relay R is attracted, making contact with the stop S. {20} On passing
over a strip of insulation the circuit is broken and the tongue of the
relay R returns to its normal position.

As already stated, the conducting and insulating bands on the print vary in
width according to the density of the photograph from which it is prepared,
so that the length of time that the tongue of the relay R is held against
the stop S, is in proportion to the width of the conducting strip which is
passing under the stylus at any instant. The function of the transmitter is
therefore to send to the relay R an intermittent current of varying
duration.

The two photographs Figs. 10 and 10_a_ are of a machine designed and used
by the writer in his experiments. In this machine the drum is 3.5 inches
long and 1.5 inches in diameter. The lead screw has 30 threads to the inch,
and the reduction between it and the drum is 3:1, so that the table has a
movement of 1/90th inch per revolution of the drum.

From the brief description of the various types of machines that have been
given it will be apparent that in the design of the machine proper there is
nothing very complicated, although the addition of the driving and
synchronising apparatus complicates matters rather considerably. The
questions of driving and synchronising the machines at the two stations is
fully dealt with in Chapter IV.

[Illustration: FIG. 10a.]

[Illustration: FIG. 10b. Enlarged view of an image broken up by a cross
screen.]

{21} Although the design of the machines is rather simple great attention
must be paid both to accuracy of construction and accuracy of working, and
this applies, not only to the machines (whether for transmitting or
receiving) but for all the various pieces of apparatus that are used. Too
much care cannot be bestowed upon this point, as in the wireless
transmission of photographs there is a large number of instruments all
requiring careful adjustment, and which have to work together in perfect
unison at a high speed.

The machine shown in Figs. 10 and 10_a_ was designed and used by the writer
solely for experimental work. It will be noticed in the description given
in the appendix of the method of preparing the metal prints that a 5" x 4"
camera is recommended, while the machine, Fig. 10, is designed to take a
print procured from a quarter-plate negative. This size of drum was adopted
for several reasons, and although it will be found quite large enough for
general experimental work the writer has come to the conclusion that for
practical commercial work a drum to take a print 5" x 4" will give better
results.

In making a negative of a picture that is required for reproduction
purposes, the line screen in the camera is replaced by a "cross screen,"
_i.e._ two single line screens placed with their lines at an angle of 90deg
to one another, and this breaks the {22} image up into small squares
instead of lines. By looking at any ordinary newspaper or book illustration
through a powerful magnifying glass the effects of a cross screen will
readily be seen. With a cross screen a certain amount of detail is
necessarily lost, but with a single line screen the amount lost is much
greater. If there is any very small detail in the picture most of this
would be lost in a coarse screen, hence the necessity of employing as fine
a line screen as practicable in order to get as much detail in as possible.
It is mainly on this account that a 5" x 4" print is recommended, as, if
fairly bold subjects are used for copying, the small detail (this is, of
course, a very vague and indefinable term) will not be too fine, and the
time required for transmitting reasonable. For obvious reasons it is a
great advantage to put the print under pressure to cause the glue image to
sink into the soft metal base and leave a perfectly flat and smooth
surface. It is essential that the bands on the print lie along the axis of
the cylinder, so that the stylus traces its path across them, and not with
them.

We have now an arrangement that is capable of taking the place of the key
K, Fig. 4, and the diagram, Fig. 11, gives the connections for the complete
transmitter. A is the aerial, E earth, T inductance, L ammeter. The closed
oscillatory circuit consists of a spark-gap G, inductance F, {23}
[Illustration] and a condenser C. The secondary J of the coil H is
connected to the spark-gap, and the primary P is in circuit with the
mercury break N, the battery B, and the local contacts of the relay R. The
action is as follows. When contact is made between the stylus Z and the
drum V by means of the conducting bands on the line print, the circuit of
the relay R and the battery M is completed. The closing of the local
circuit of the relay R actuates the second relay R', allowing the primary
circuit of the coil H to be closed. As soon as the primary circuit of the
coil is completed sparks pass between the electrodes of the spark-gap G,
causing waves to radiate from the aerial. The duration of the wave-trains
radiated depends upon the duration of contact made by the relays {24} R and
R', and this in turn depends upon the width of the conducting strip that is
passing under the stylus. The battery M should be about 4 volts, and the
battery D about 2 volts. The two-way switch X is connected up so that the
relay R' can be thrown out and the key K switched in for ordinary
signalling purposes. If any sparking takes place at the point of the
stylus, a small condenser C' (about 1 microfarad capacity) should be
connected as shown. In the present instance the condenser should be used
more as a preventive than as a cure, as in all probability the voltage from
M will not be sufficient to cause destructive (if any) sparking; but, as
most wireless workers know, anything in the nature of a spark occurring in
the neighbourhood of a detector (this, of course, only applies when the
receiving apparatus is placed in close proximity to the transmitter) is
liable to destroy the adjustment.

In transmitting over ordinary conductors where the initial voltage is
fairly high and the self-induction of the circuit very great, the use of
the condenser will be found to be absolutely essential. It has also been
noted that the angle which the stylus presents to the drum has a marked
effect upon the sparking, an angle of about 60deg being found to give very
good results.

If the size of the single line print used is 5 inches by 4 inches, and a
screen having 50 lines {25} to the inch is used for preparing it, then the
stylus will have to make 250 contacts during one revolution of the drum.
Assuming the drum to make one revolution in three seconds, then the time
taken to transmit the complete photograph can be found from the equation T
= w x t x s, where w is the width of the print, t the travel of the stylus
during one revolution of the drum, and s the time required for one
revolution of the drum. In the present instance this will be T = 4 x 90 x 3
= 1080 seconds = 18 minutes. The number of contacts made by the stylus per
minute is 5000, and in working at this speed the first difficulty is
encountered in the use of the two relays. The relay R is lightly built, and
capable of working at a fairly high speed, but R' is a heavier pattern, and
consequently works at a slightly lower rate. This relay must necessarily be
heavier, as more substantial contacts are needed in order to pass the heavy
current taken by the spark-coil.

Relays sensitive and accurate enough to work at this speed will in all
probability be beyond the reach of the majority of workers, but there are
several types of relays on the market very reasonable in price that will
answer very well for experimental work, although the speed of working will
no doubt be slower.

For the best results the duration of the wave-trains sent out should be of
the same duration as {26} the contact made by R, and therefore equal to the
time taken by the stylus to trace over a conducting strip; but if the
duration of the contact made by R is t, then that made by R' and
consequently the duration of the groups of wave-trains would be t - v where
v equals the extra time required by R' to complete its local circuit. The
difference in time made by the two relays, although very slight, will be
found to affect very considerably the quality of the received pictures.
Renewing the platinum contacts is also a great expense, as they are soon
burnt out where a heavy current is passed. If the distance experimented
over is short so that the power required to operate the spark-coil is not
very heavy, one relay will be sufficient providing the contacts are massive
enough to carry the current safely. It is useless to expect any of the
ordinary relays in general use to work satisfactorily at such a high speed,
and in order to compensate for this we must either increase the time of
transmitting, or, as already suggested, make use of a coarser line screen
in preparing the photographs.

For reasons already explained, all points of make and break should be
shunted by a condenser. The effective working speed of an ordinary type of
relay may be anything from 1000 to 2500 dots a minute, depending upon
accuracy of design and construction.

In the wireless transmission of photographs it {27} is absolutely essential
to use some form of rotary spark-gap, as where sparks are passed in rapid
succession the ordinary type of gap is worse than useless. When a spark
passes between the electrodes of an ordinary spark-gap, Fig. 12, we find
that for a fraction of a second after the first spark has passed, the
normally high resistance of the gap has been lowered to less than one ohm.
If the column of hot gas which constitutes the spark is not instantly
dispersed, but remains between the electrodes, it will provide an easy path
for any further discharges, and if sparks are passed at all rapidly, what
was at first a disruptive and oscillatory discharge will degenerate into a
hot, non-oscillatory arc.[4]

[Illustration: FIG. 12.]

Two forms of rotating spark-gaps are shown in Figs. 13 and 14, and are
known as "synchronous" and "non-synchronous" gaps respectively. In the
synchronous gap the cog-wheel is mounted on the shaft of the alternator,
and a cog comes opposite the fixed electrode when the maximum of potential
is reached in the condenser, thus ensuring a discharge at every alternation
of current. With this type of gap a spark of pure tone is obtained which
{28} [Illustration] [Illustration] is of great value where the signals are
received by means of a telephone, but where the signals are to be
mechanically recorded the tone of the spark is of little consequence. In a
non-synchronous gap a separate motor is used for driving the toothed wheel,
and can either be mounted on the motor shaft or driven by means of a band,
there being no regard given to synchronism with the alternator. The fixed
electrode is best made long enough to cover about two of the teeth, as this
ensures regular sparking and a uniform sparking distance; the {29} spark
length is double the length of the spark-gap. The toothed wheel should
revolve at a high speed, anything from 5000 to 8000 revolutions per minute,
or even more being required. The shaft of the toothed wheel is preferably
mounted in ball-bearings.

Owing to the large number of sparks that are required per minute in order
to transmit a photograph at even an ordinary speed, it is necessary that
the contact breaker be capable of working at a very high speed indeed. The
best break to use is what is known as a "mercury jet" interrupter, the
frequency of the interruptions being in some cases as high as 70,000 per
second. No description of these breaks will be given, as the working of
them is generally well understood.

In some cases an alternator is used in place of the battery B, Fig. 4, and
when this is done the break M can be dispensed with. In larger stations the
coil H is replaced with a special transformer.

The writer has designed an improved relay which will respond to currents
lasting only 1/100th part of a second, and capable of dealing with rather
large currents in the local circuit.[5] This relay has not yet been tried,
but if it is successful the two relays R and R' can be dispensed with, and
the result will be more accurate and effective transmission.

{30}

[Illustration: FIG. 15.]

The connections for a complete experimental station, transmitting and
receiving apparatus combined, are given in Fig. 15. The terminals W, W are
for connecting to the photo-telegraphic receiving apparatus Q, being a
double pole two-way switch for throwing either the transmitting or
receiving apparatus in circuit. There is another system of transmitting
devised by Professor Korn, which employs an entirely different method from
the foregoing. By using the apparatus just described, the waves generated
are what are known as "damped waves," and by using these damped waves,
tuning, which is so essential to good commercial working, can be made to
reach a fairly high degree of efficiency. {31}

The question of damped _versus_ undamped waves is a somewhat burning one,
and no attempt will be made here to deal with the merits or demerits of the
claims made for the respective systems. A series of articles describing the
production of undamped waves and their efficiency in working compared with
damped waves will be found in the _Wireless World_, Nos. 3 and 4, 1913, and
are well worth reading by any one interested in the subject.

[Illustration: FIG. 16.]

A diagrammatic representation of the apparatus as arranged by Professor
Korn is given in Fig. 16. The undamped or "continuous" waves are generated
by means of a high-frequency alternator or Poulsen arc. In Fig. 16, X is
the generator, F inductance, C condenser; the aerial inductance T is
connected by the aerial A and earth E. By this means the waves are tuned to
a certain period. {32} A metal print, similar to what has already been
described, is wrapped round the drum D of the machine, and when the stylus
Z traces over an insulating strip the waves generated are in tune with the
receiving station, but when it traces over a conducting strip, a portion of
the inductance T is short-circuited, the period of the oscillations is
altered, and the two stations are thrown out of tune.

The receiving station is provided with an aperiodic circuit, which consists
of an inductance F', condenser C', and a thermodetector N. A string
galvanometer H (described in Chapter III.), and the self-induction coils B,
B' are connected as shown, the coils B, B' preventing the high-frequency
currents, which change their direction, from flowing through the
galvanometer. The manner in which the string galvanometer is arranged to
reproduce a transmitted picture is shown in Fig. 24.

The connections adopted by the Poulsen Company for photographically
recording wireless messages are given in Fig. 17, a string galvanometer of
the Einthoven type being used. The two self-induction coils S and S' are in
circuit with the detector D and the galvanometer G. The condenser C'
prevents the continuous current produced by the detector from flowing
through the high frequency circuit; P is the primary of the aerial {33}
inductance and F the secondary. The method of transmitting adopted by
Professor Korn appears to be a simple and reliable arrangement, provided
that an equally reliable method of producing the undamped waves can be
found. Owing to the absence of mechanical inertia it should be capable of
working at a good speed, while the absence of a number of pieces of
delicate apparatus all requiring careful adjustment add greatly to its
reliability.

[Illustration: FIG. 17.]

In any spark system with a properly designed aerial a coil taking ten
amperes is capable of transmitting signals over a distance of thirty to
fifty miles, but where the number of interruptions of the break required
per second is very high, as in radio-photography, it must be remembered
that a much higher voltage is needed to drive the requisite amount of
current through the primary winding of the coil than would be the case if
the interruptions were slower. It is possible to use platinum {34} contacts
for the relays, for currents up to ten amperes, but for heavier currents
than this some arrangement where contact is made with mercury will be found
to be more economical and reliable.

In the transmitter already described and given in Fig. 11, the best results
would be obtained by finding the speed at which the relay R' works best,
and regulating the number of contacts made by the stylus accordingly.

The method employed by De' Bernochi (see Chapter I.) of varying the
intensity of a beam of light by passing it through a photographic film,
which in turn alters the resistance of a selenium cell, has been very
successfully employed in at least one system of photo-telegraphy. Its
application has also been suggested for wireless transmission, and although
with any system using continuous waves this would not be very difficult, it
could hardly be adapted to work with the ordinary spark system. The
apparatus for receiving from this type of transmitter would, on the other
hand, necessarily be more elaborate than the methods that are described in
the next chapter, and as far as the writer's experience goes, experiments
along these lines would not prove very profitable, as simplicity is the
keynote of success in any radio-photographic system.

It has been suggested that in order to decrease the time of transmission a
cylinder capable of {35} taking a print 7 inches by 5 inches be employed,
the print being prepared from rather a coarse line screen--say 35 to the
inch--and a traverse of about 1/50 inch given to the stylus, thus reducing
the time of transmission to about twelve minutes. It is questionable,
however, whether the increase in speed would compensate for the loss of
detail, as only very bold subjects could be transmitted. As already pointed
out, wireless transmission would only be employed for fairly long
distances, and the extra time and expense required to receive a fairly good
detailed picture is negligible when compared with the enormous time it
would take to receive the original photograph by any ordinary means of
transit.

The public much prefer to have passable pictorial illustrations of current
events than wait several days for a more perfect picture--the original, and
the advantage of any newspaper being able to publish photographs several
days before its rivals is obvious. There can also be no doubt but that a
system of radio-photography, if fairly reliable and capable of working over
a distance of say thirty miles, would be of great military use for
transmitting maps and written matter with a great saving of time and even
life. Written matter could be transmitted with even greater safety than
messages which are sent in the ordinary way in Morse Code, as the signals
received in the receiver {36} of an hostile installation would be but a
meaningless jumble of sounds, and even were they possessed of
radio-photographic apparatus the received message would be unintelligible,
unless they knew the exact speed at which the machines were running and
could synchronise accurately.

       *       *       *       *       *


{37}

CHAPTER III

RECEIVING APPARATUS

There are only two methods available at present for receiving the
photographs, and both have been used in ordinary photo-telegraphic work
with great success. They have disadvantages when applied to wireless work,
however, but these will no doubt be overcome with future improvements. The
two methods are (1) by means of an ordinary photographic process, and (2)
by means of an electrolytic receiver.

In several photo-telegraphic systems the machine used for transmitting has
the cylinder twice the size of the receiving cylinder, thus making the area
of the received picture one-quarter the area of the picture transmitted.
The extra quality of the received picture does not compensate for the
disadvantage of having to provide two machines at each station, and in the
writer's opinion results, quite good enough for all practical purposes, can
be obtained by using a moderate size cylinder so that one machine answers
for both transmitting {38} and receiving, and using as fine a line screen
as possible for preparing the photographs.

[Illustration: FIG. 18.]

The writer, when first experimenting in photo-telegraphy, endeavoured to
make the receiving apparatus "self-contained," and one idea which was
worked out is given in Fig. 18. The electric lamp L is about 8 c.p., and is
placed just within the focus of a lens which has a focal length of 3/4
inch. When a source of light is placed at some point between a lens and its
principal focus, the light rays are not converged, but are transmitted in a
parallel beam the same size as the lens. It has been found that this
arrangement gives a sharper line on the drum than would be the case were
the light focussed direct upon the hole in the cone A. An enlarged drawing
of the cone is given in Fig. 19. The hole in the tip of the cone A is a
bare 1/90 inch in diameter--the size of this hole depends upon the travel
per revolution of the drum or table of the machine used--and in working,
the cone is run as close as possible to the {39} drum without being in
actual contact. The magnet M is wound full with No. 40 S.C.C. wire, and the
armature is made as light as possible. The spring to which the armature is
attached should be of such a length that its natural period of vibration is
equal to the number of contacts made by the transmitting stylus. The spring
must be stiff enough to bring the armature back with a fairly crisp
movement. The spring and armature is shown separate in Fig. 20.

[Illustration: FIG. 19.]

[Illustration: FIG. 20.]

The shutter C is about 1/4 inch square and made from thin aluminium. The
hole in the centre is 1/16 x 1/8 inch, and the movement of the armature is
limited to about 3/32 inch. In all arrangements of this kind there is a
tendency for the armature spring to vibrate, as it were, sinusoidally, if
the coil is magnetised and demagnetised at a higher rate than the natural
period of vibration of the spring. {40} This causes an irregularity in the
rate of the vibrations which affects the received image very considerably.
A photographic film is wrapped round the drum of the machine, being
fastened by means of a little celluloid cement smeared along one edge.

This device, although it will work well over artificial conductors, is not
suitable for wireless work, as it is too coarse in its action; it can be
made sensitive enough to work at a speed of 1000 to 1500 contacts per
minute, with a current of .5 milliampere. It is impossible to obtain a
current of this magnitude from the majority of the detectors in use, so
that if any attempt is made to use this device for radio-photography it
will be necessary to employ a Marconi coherer (filings), as this is
practically the only coherer from which so large a current can be obtained.

There have been many attempts made to receive with an ordinary filings
coherer, but as was pointed out in Chapter I. these have now been discarded
in serious wireless work, being only used in small amateur stations or
experimental sets. As the reasons for this are well known to the majority
of wireless workers there is no need to enumerate them here.

A method whereby a filings coherer can be decohered, the act of decohering
closing a local circuit which contains the photographic {41} receiving
apparatus, is given in the diagram Fig. 21.

[Illustration: FIG. 21.]

In the figure, the coherer C is fixed in rigid supports, one support being
provided with a platinum pin F. To the coherer is connected the sensitive
electro-magnet M, which becomes magnetised as soon as the incoming waves
act upon the coherer. To the armature B is attached a light aluminium arm
S, pivoted at K, and carrying at the other end the striker G, which is
fitted with a platinum contact. When the armature B is attracted the
coherer is decohered by the force of the impact between the contacts F and
G. To prevent damage to the coherer the force of the blow is taken off by
the ability of the striker to work back through a hole in the arm S, the
spring {42} N keeping it normally in a fixed position. T and P are
adjusting screws, and the terminals J are for connecting to the receiving
apparatus. With this arrangement a very short wave-train causes only one
tap of the contacts, so that only one mark is registered on the receiving
drum for every contact made on the transmitter.

[Illustration: FIG. 22.]

The drawing, Fig. 22, gives a diagrammatic representation of apparatus
arranged for another photographic method of receiving. The machine shown in
Fig. 6 is used in this case. A is the aerial, E earth, P primary of
oscillation-transformer, S secondary of transformer, C variable condenser,
C' block condenser, D detector, X two-way switch, T telephone.

A De' Arsonval galvanometer H is also connected to the switch X, so that
either the telephone or the galvanometer can be switched in. The {43}
galvanometer can be made sensitive enough to work with a current as small
as 10^{-7} of an ampere, with a period of about 1/150th of a second. The
screen J has a small hole about 1/8 inch diameter drilled in the centre.
Under the influence of the brief currents which pass through the detector
every time a group of waves is received, the mirror of the galvanometer
swings to-and-fro in front of the screen J, and allows the light reflected
from the source of light M to pass through the aperture in the screen, on
to the lens N.

Round the drum V of the machine is wrapped a sensitive photographic film,
and this records the movements of the mirror which correspond to the
contacts on the half-tone print used in transmitting. Every time current
passes through the galvanometer, the light that is received from M,[6]
passes through the aperture in the screen J, and is focussed by the lens N
to a point upon the revolving film. As soon as the current ceases, the
mirror swings back to its original position, and the film is again in
darkness. Upon being developed a photograph, similar to the negative used
for preparing the metal print is obtained. If desired the apparatus can be
so arranged that the received picture is a positive instead of a negative.

{44}

The detector used should be a Lodge wheel-coherer or a Marconi
valve-receiver, as these are the only detectors that can be used with a
recording instrument. If the swing of the galvanometer mirror is too great,
a small battery with a regulating resistance can be inserted in order to
limit the movement of the mirror to a very short range; the current of
course flowing in an opposite direction to the current flowing through the
coherer.

In this, as in all other methods of receiving, the results obtained depend
upon the fineness of the line screen used in preparing the metal prints;
and as already shown the fineness of the screen that can be used is
dependent upon the mechanical efficiency of the entire apparatus.

Another system, and one that has been tried as a possible means of
recording wireless messages, is as follows. The wireless arrangements
consist of apparatus similar to that shown in Fig. 22, but instead of a
Lodge coherer a Marconi valve is used, and an Einthoven galvanometer is
substituted for the reflecting galvanometer. The Einthoven galvanometer
consists of a very powerful electro-magnet, the pole pieces of which
converge almost to points. A very fine silvered quartz thread is stretched
between the pole pieces, as shown in Fig. 23, the tension being adjustable.
The period of swing is about 1/250th of a second. A hole is bored through
the poles, and one of them is fitted {45} [Illustration] with a sliding
tube which carries a short focus lens N. The light from M passes through
the magnets, and a magnified image of the quartz thread is thrown upon the
ebonite screen J. This screen is provided with a fine slit, and when the
galvanometer is at rest the shadow of the thread just covers the slit in
the screen and prevents any light from M reaching the photographic film.
Upon signals being received the shadow of the thread moves to one side for
a long or short period, uncovering the slit, and allowing light to pass
through. The lens R concentrates the collected light to a point upon the
revolving film. The connections for the complete receiver are given in Fig.
24.

The modified form of the Einthoven galvanometer, as arranged by Professor
Korn for use with his selenium machines for photo-telegraphy over ordinary
land lines, consists of two fine silver wires which are displaced in a
lateral direction between the pole pieces when traversed by a current; the
current passing through both wires in the same {46} direction. A small
shutter of aluminium foil is attached to the wires at the optical centre.
The silver wires used are 1/1000 inch in diameter, with a natural period of
about 1/120th of a second; the length of wires free to swing being usually
about 5 cm.

[Illustration: FIG. 24.]

The period of the wires depends to a great extent upon their length and
diameter, and also upon their tension. By using short fine wires the period
can be made much smaller, but a greater current is required to produce a
similar displacement. Where the current available, as in wireless
telegraphy, is very small, and a definite displacement of the wires is
required, it is at once apparent that with wires of a given diameter there
is a limit to their length and therefore to the period. Finer wires can be
used, but here again there is a practical limit to their fineness, although
galvanometers have been constructed with a single silvered quartz thread
1/12000th of an inch diameter, which, when placed in a powerful field, will
give a good displacement with a current as small as 10^{-8} ampere. {47}

With the apparatus arranged by the Poulsen Company, given in the diagram,
Fig. 17, for photographically recording wireless signals, the current
required to operate the galvanometer for signals transmitted at the rate of
1500 a minute is 1 x 10^{-6} ampere, while for signals up to 2500 a minute
a current about 5 x 10^{-6} ampere is necessary.

Another very sensitive instrument, employed by M. Belin, and known as
Blondel's oscillograph, consists of two fine wires stretched between the
poles of a powerful electro-magnet, a small and very light mirror being
attached to the centre of the wires. The current passes down one wire and
up the other, and the wires, together with the mirror, are twisted to a
degree depending upon the strength of the received current. In order to
render the instrument dead-beat the moving parts are arranged to work in
oil. The light reflected from the mirror is made use of in a manner similar
to that shown in Fig. 22.

In all photographic methods of receiving, the apparatus must be enclosed in
some way to prevent any extraneous light from reaching the film, or better
still placed in a room lighted only by means of a ruby light.

The following method is given more as a suggestion than anything else, as I
do not think it has been tried for wireless receiving, although it is
stated to have given some good results over {48} ordinary land lines. It is
the invention of Charbonelle, a French engineer, and is quite an original
idea. His method consists of placing a sheet of carbon paper between two
sheets of thin white paper, and wrapping the whole tightly round the drum
of the machine. A hardened steel point is fastened to the diaphragm of a
telephone receiver, and this receiver is placed so that the steel point
presses against the sheets of paper. As the diaphragm and steel point
vibrates under the influence of the received currents marks are made by the
carbon sheet on the bottom paper.

Over a line where a fair amount of current is available at the receiver,
the diaphragm would have sufficient movement to mark the paper, but the
movement would be very small with the current received from a detector.
This difficulty could no doubt be overcome to a certain extent by making a
special telephone receiver, with a large and very flexible diaphragm, and
wound for a very high resistance. The movement of an ordinary telephone
diaphragm for a barely audible sound is, measured at the centre, about
10^{-6} of a c.m. With a unit current the movement at the centre is about
1/700th of an inch. Greater movement of the diaphragm could be obtained by
connecting a _Telephone relay_ to the detector, and using the magnified
current from the relay to operate the telephone. {49}

[Illustration: FIG. 25.]

The telephone relay consists of a microphone C, Fig. 25, formed of the two
pieces of osmium iridium alloy. The contact is separated to a minute degree
partly by the action of the local current from F, which flows through it
and also through the winding W of the two magnet coils. The local current
from F assists in forming the microphone by rendering the space between the
contacts conductive. The vibrating reed P is fastened to the metal frame
(not shown) which carries a micrometer screw by which the distance between
the contacts can be accurately regulated. It will be seen from Fig. 25 that
the local circuit consists of a battery F (about 1.5 volts), the microphone
contacts C, the windings W, milliampere meter B, and the terminals T, for
connecting to the galvanometer or telephone, all in {50} series. On the top
of the magnet cores N, S is a smaller magnet D, wound with fine wire for a
resistance of about 4935 ohms, the free ends of the coils being connected
to the detector terminals. The working is as follows. Supposing the current
from the detector flows through D in such a way that its magnetism is
increased, the reed P will be attracted, the contacts opened, and their
resistance increased. It will be seen that the current from F is passed
through the coils W, in such a way as to increase the magnetism of the
permanent magnet, so that any opening of the microphone contact increases
their resistance, causes the current to fall, and weakens the magnets to
such an extent that the reed P can spring back to its normal position. On
the other hand, if the detector current flows through D in such a direction
as to decrease the magnetism in the permanent magnets, the reed P will rise
and make better contact owing to the removal of the force opposing the
stiffness of the reed. Owing to the decrease in the resistance of the
microphone, the strength of the local current will be increased, the
magnets strengthened, and the reed P will be pulled back to its original
position. This relay gives a greatly magnified current when properly
adjusted, the current being easily increased from 10^{-4} to 10^{-2}
amperes. It is also very sensitive, but needs careful adjustment in order
that the best results may {51} be obtained. A greater range of
magnification can be obtained by placing two or more relays in series.

[Illustration: FIG. 26.]

A very sensitive receiver designed by the writer is given in the figures 26
and 27. To the centre of a telephone diaphragm is fastened a light steel
point P, and the movement of this point is communicated to the aluminium
arm D, which is pivoted at C. As will be seen the telephone receiver is of
special construction, it containing only one coil and therefore only one
core; by this means the movement of the diaphragm is centralised. The coil
is wound for a resistance of about 200 ohms, and the diaphragm should be
fairly thin but very resillient.

[Illustration: FIG. 27.]

To the free end of D is fastened the mirror T, made from thin diaphragm
glass about 1-1/2 centimetres diameter, and having a focal length of 40
inches. Light from the lamp L is transmitted by the lens N in a parallel
beam to the mirror which {52} concentrates it to a point upon a hole
1/100th of an inch in diameter in the screen J. As the telephone diaphragm
vibrates under the influence of the received signals the arm, and
consequently the mirror, vibrates also, and the hole in the screen J is
constantly being covered and uncovered by the spot of light. It will be
seen from Fig. 27 that the ratio between the centre of the mirror and the
pivot C, and C and the steel point P is 10:1, so that if a movement of
1/20000th of an inch is obtained at the centre of the diaphragm the mirror
will move 1/2000th of an inch; and as the focal length of the mirror is 40
inches a movement of 1/50th inch is given to the spot of light.

This receiver is capable of working at a fairly high speed, as the inertia
of the moving parts is practically negligible; the weight of the arm and
mirror being less than 20 grains. The hole in the screen is made slightly
less in diameter than the traverse of the revolving cylinder, the slight
distance between the cylinder and the screen allowing the light to disperse
sufficiently to produce a line on the film of about the right thickness.

There are two other possible means of photographically receiving the
picture that upon investigation may yield some results; but it is doubtful
whether the current available, even that obtained from a telephone relay,
will be sufficient to produce the desired magnetic effect, and the {53}
insertion of a second relay would detract greatly from the efficiency by
decreasing the speed of working. If rays of monochromatic light from a lamp
L, Fig. 28, pass through a Nicol prism P (polarising prism), then through a
tube containing CS_2 (carbon bisulphide), afterwards passing through the
second prism P' (analysing prism), and if the two Nicol prisms are set at
the polarising angle, no light from L would reach the photographic film
wrapped round the drum V of the machine. Upon the tube being subjected to a
field produced by a current passing through the coil C, the refractive
index of the liquid will be changed, and light from L will reach the
photographic film.[7]

[Illustration: FIG. 28.]

The second method is rather more complicated, and is based upon the fact
that the kathode rays in a Crookes' tube can be deflected from their course
by means of a magnet. In Fig. 29 the kathode K of the X-ray tube sends a
kathode ray discharge through an aperture in the anode A, through a small
aperture in the ebonite screen J {54} on to the drum V of the machine,
round which is wrapped a photographic film; A and K being connected to
suitable electrical apparatus. Upon the coil M being energised, the
kathode-ray is deflected from its straight-line course, and the drum V is
left in darkness.

[Illustration: FIG. 29.]

The method which is now going to be described is very ingenious, as it
makes use of what is known as an electrolytic receiver. This method of
receiving has proved to be the most practical and simple of all the
photo-telegraphic systems that have been devised.

The application of this system to wireless reception is as follows. The
aerial A, and the earth E, are joined to the primary P of a transformer,
the secondary S being connected to a Marconi valve receiver C. The valve
receiver is connected to the battery B and silvered quartz thread K of an
Einthoven galvanometer (already described). The thread is 1/12000th of an
inch in diameter, and will respond to currents as small as 10^{-8} of {55}
an ampere. The light from M throws an enlarged shadow of the thread over a
slit in the screen J, and as the thread moves to one side under the
influence of a current, the slit in J is uncovered, and the light from M is
thrown upon a small selenium cell R. In the dark the selenium cell has a
very high resistance, and therefore no current can flow from the battery D
to the relay F. When the string of the galvanometer moves to one side and
uncovers the slit in the screen J, a certain amount of light is thrown upon
the selenium cell lowering its resistance, allowing sufficient current to
pass through to operate the relay.

Round the drum of the machine (shown in Fig. 7) is wrapped a sheet of paper
that has been soaked in certain chemicals that are decomposed on the
passage of an electric current through them. As soon as the local circuit
of the relay is closed, the current from the battery Z (about 12 volts)
flows through the paper and produces a coloured mark. The picture,
therefore, is composed of long or short marks which correspond to the
varying strips of conducting material on the single line print. In order to
render the marks short and crisp, a small battery Y, and regulating
resistance L, is placed across the drum and stylus. The diagram, Fig. 30,
gives the connections for the complete receiver. {56}

The paper used is soaked in a solution consisting of

  Ferrocyanide of potassium             1/4 oz.
  Ammoniac Nitrate                      1/2 oz.
  Distilled water[8]                      4 oz.

[Illustration: FIG. 30.]

The paper has to be very carefully chosen, as besides being absorbent
enough to remain moist during the whole of the receiving, the surface must
also remain fairly smooth, as with a rough paper the grain shows very
distinctly, and if there is an excess of solution the electrolytic marks
are inclined to spread and so cause a blurred image. The writer tried
numerous specimens of paper before one could be found that gave really
satisfactory results. It was also found that when working in a warm room
the paper became nearly {57} dry before the receiving was finished, and the
resistance of the paper being greatly increased (this may be anything up to
1000 ohms), the marking became very faint. A sponge moistened with the
solution and applied to the undecomposed portion of the paper, while still
revolving, was found to help matters considerably.

Another experience which happened during the writer's early experiments,
the cause of which I am still unable to explain, occurred in connection
with the stylus. The stylus used consisted of a sharply pointed steel
needle, and after working for about three minutes it was noticed that the
lines were becoming gradually wider, finally running into each other. Upon
examination it was found that the point of the needle had worn away
considerably, becoming in fact, almost a chisel point. Almost every needle
tried acted in a similar manner, and to overcome this difficulty the stylus
shown in Fig. 31 was devised.

It will be seen that it consists of a holder A, somewhat resembling a drill
chuck, fastened to the flat spring B in such a manner that the angle the
stylus makes to the drum can be altered. The needle consists of a length of
36-gauge steel wire, and as this wears away slowly the jaws of the holder
can be loosened and a fresh length pushed through. The wire should not
project beyond the face of the holder more than 1/8th inch. The gauge {58}
of wire chosen would not suit every machine, the best gauge to use being
found by trial, but in the writer's machine the pitch of the decomposition
marks is much finer than of those made by the commercial machines, and this
gauge, with the slight but unavoidable spreading of the marks, will produce
a mark of just the right thickness. As already mentioned, no explanation of
this peculiarity on the part of the stylus can be given, as there is
nothing very corrosive in the solution used, and the pressure of the stylus
upon the paper is so slight as to be almost negligible.

[Illustration: FIG. 31.]

No special means are required for fastening the paper to the drum, the
moist paper adhering quite firmly. Care should be taken, however, to fasten
the paper--which should be long enough to allow for a lap of about 1/4
inch--in such a manner that when working the stylus draws away from the
edge of the lap and not towards it.

The current required to produce electrolysis is very small, about one
milliampere being sufficient. {59} Providing that the voltage is
sufficiently high, decomposition will take place with practically "no
current," it being possible to decompose the solution with the discharge
from a small induction coil. The quantity of an element liberated is by
weight the product of time, current, and the electro-chemical equivalent of
that element, and is given by the equation W = zct, where

  W = quantity of element liberated in grammes.
  z = electro-chemical equivalent,
  c = current in amperes,
  t = time in seconds.

The chemical action that takes place is therefore very small, as the
intermittent current sent out from the transmitter in some cases only lasts
from 1/50th to 1/100th a second.

The decomposed marks on the paper are blue, and, as photographers know,
blue is reproduced in a photograph as a white, so that a photograph taken
of our electrolytic picture, which will of course be a blue image upon a
white ground, will be reproduced almost like a blank sheet of paper. If,
however, a yellow contrast filter is placed in front of the camera lens,
and an orthochromatic plate used, the blue will be reproduced in the
photograph as a dead black.

There is one other point that requires attention. It will be noticed that
the metal print used for {60} transmitting is a positive, since it is
prepared from a negative. The received picture will therefore be a
negative, making the final reproduction, if it is to be used for newspaper
work, a negative also. Obviously this is no good. The final reproduction
must be a positive, therefore the received picture must be also a positive.
To overcome this difficulty matters must be so arranged at the receiving
station that in the cases of Figs. 17, 18, 22, and 24, the film is kept
permanently illuminated while the stylus on the transmitter is tracing over
an insulating strip, and in darkness when tracing over a conducting strip.
In Fig. 30 the relay F should allow a continuous current from Z to flow
through the electrolytic paper, and only broken when the resistance of the
selenium cell is sufficiently reduced to allow the current from D to
operate the relay.

The author has endeavoured to make direct positives on glass of the picture
to be transmitted, so that a negative metal print could be prepared. The
results obtained were not very satisfactory, but the method tried is given,
as it may perhaps be of interest. The plate used in the camera has to be
exposed three or four times longer than is required for an ordinary
negative. The exposed plate is then placed in a solution of protoxalate of
iron (ferrous oxalate) and left until the image shows plainly through the
back of the plate. It {61} is then washed in water and placed in a solution
consisting of

  Distilled water                      1000 cc.
  Nitric acid                             2 cc.
  Sulphuric acid                          3 cc.
  Bichromate of potash                  105 grammes.
  Alum                                   80    "

After being in this bath for about fifteen minutes the plate is again well
washed in water, and developed in the ordinary way. The first two
operations should be performed in the dark room, but the remaining
operations can be performed in daylight, once the plate has been placed in
the bichromate bath. As already stated, the results obtained were not very
satisfactory, and such a method is not now worth following up, as it is
comparatively easy so to arrange matters at the receiving station that a
positive or negative image can be received at will.

It is necessary to connect the stylus of the receiving machine to the
positive pole of the battery Z, otherwise the marks will be made on the
underside of the paper. The electrolytic receiver, owing to the absence of
mechanical and electro-magnetic inertia, is capable of recording signals at
a very high speed indeed.

"Atmospherics," which are such a serious nuisance in long-distance wireless
telegraphy, will also prove a nuisance in wireless photography, {62} but
their effects will not be so serious in a photographic method of receiving
as they would be in the electrolytic system. In a photographic receiver
where the film is, under normal conditions, constantly illuminated, the
received signals (both the transmitted signals and the atmospheric
disturbances) will be recorded, after development, as transparent marks
upon the film, the remainder of the film being, of course, perfectly
opaque. By careful retouching the marks due to the disturbances can be
eradicated, a print upon sensitised paper having been first obtained to act
as a guide during the process.

       *       *       *       *       *


{63}

CHAPTER IV

SYNCHRONISING AND DRIVING

Clockwork and electro-motors are the source of driving power that are most
suitable for photo-telegraphic work, and each has its superior claims
depending on the type of machine that is being used. For general
experimental work, however, an electro-motor is perhaps the most
convenient, as the speed can be regulated within very wide limits. For a
constant and accurate drive a falling weight has no equal, but the
apparatus required is very cumbersome and the work of winding both tedious
and heavy. This method of driving was at one time universally employed with
the Hughes printing telegraph, but it has now been discarded in favour of
electro-motors, which are more compact, besides being cheaper to instal in
the first instance.

Synchronising and isochronising the two machines are the most difficult
problems that require solving in connection with wireless photography, and
as previously mentioned, the {64} synchronising of the two stations must be
very nearly perfect in order to obtain intelligible results. The limit of
error in synchronising must be about 1 in 500 in order to obtain results
suitable for publication.

The electrolytic system is perhaps the easiest to isochronise, as the
received picture is visible. On the metal print used for transmitting, and
at the commencing edge a datum line is drawn across in insulating ink. The
reproduction of this line is carefully observed by the operator in charge
of the receiving instrument, and the speed of the motor is regulated until
this line lies close against a line drawn across the electrolytic paper.
Although this may seem an ideal method there are one or two considerations
to be taken into account. Unless the decomposition marks are made the
correct length and are properly spaced, however good the isochronising may
be, the result will be a blurred image. Any one who has worked with a
selenium cell, will know that it cannot change from its state of high
resistance to that of low resistance with infinite rapidity, and the
effects of this inertia, or "fatigue" as it has been called, are more
pronounced when working at a high speed. In working, the effects of this
inertia would be to increase the time of contact of the relay F (Fig. 30)
as the current from D would flow for a slightly longer period through R to
F than the period of {65} illumination allowed by K. This, of course, would
mean a lengthening of the marks on the paper; results would also differ
greatly with different selenium cells. There is a method of compensation by
which the inertia of a cell can almost entirely be overcome, but it would
add greatly to the complicacy of the receiving apparatus.

In using an electro-motor with any optical method of receiving there are
two methods available. The first is an arrangement similar to that used by
Professor Korn in his early experiments with his selenium machines. The
motor used for driving has several coils in the armature connected with
slip rings, from which an alternating current may be tapped off; the motor
acting partially as a generator, besides doing good work as a motor in
driving the machine. This alternating current is conducted to a frequency
meter, which consists of a powerful electro-magnet, over which are placed
magnetised steel springs, having different natural periods of vibration. By
means of a regulating resistance the motor is run until the spring which
has the same period as the desired armature speed vibrates freely. The
speed of the motors at both stations can thus be adjusted with a fair
amount of accuracy. Another method is to make use of a governor similar to
those employed in the Hughes printing telegraph system. A drawing of the
governor is given in Fig. 32. It consists of a

[Illustration] {67} metal frame which supports an upright steel bar S,
whose ends turn on pivots. This bar is rectangular in section. The
gear-wheel G is fastened near the bottom of this rod and gears with a
similar wheel on the shaft of the driving motor (not shown). Suspended from
the broader sides of S are the two flexible arms D, each carrying a brass
ball T. These balls are not fastened to the arms, but can slide up and
down, being held in position by the wire springs M, one end of each spring
being fastened to the screws C. These screws work in a slot cut in the
upper part of S, and are connected to the adjusting screw E. When E is
turned the screws are raised or lowered accordingly, and also the balls on
the arms D.

Fastened to the arms are two brushes of tow B, and these revolve inside but
just clearing the inner surface of the steel ring Z. Upon the motor speed
increasing above the normal the arms D, and consequently the balls T, swing
out, making a larger circle, causing the brushes B to press against the
steel ring Z, setting up friction which, however, is reduced as soon as the
motor regains its ordinary working speed. By careful adjustment the speed
of the motors can be kept perfectly constant. The object of having the
balls T adjustable on D, is to provide a means of altering the motor speed,
as the lower the balls on D the slower the mechanism runs, and _vice
versa_. {68}

[Illustration]

A simple and effective speed regulator devised by the writer is given in
drawings 33 and 34. It comprises two parts, A and B, the part A being
connected to the driving motor, and the part B working independently. The
independent portion B consists of an ordinary clock movement M, a steel
spindle J being geared to one of the slower moving wheels, so that it makes
just one revolution in two seconds. This spindle, which runs in two coned
bearings, carries at its outer end a light [Illustration] pointer D, about
two inches long, to the underside of which is fastened the thin brass
contact spring S, which presses lightly upon the ebonite ring N. {69} The
portion A comprises a spindle, pointer, and contact spring similar to those
employed in B, the spindle J' being geared to the driving motor by means of
F, so that the pointer D' makes a little more than one revolution in two
seconds. By means of a special form of brake on the driving motor, the
speed is reduced, so that both pointers travel at the same rate, viz. one
revolution in two seconds. By careful adjustment the two pointers can be
made to revolve in synchronism,[9] and when this is obtained the contact
springs S, S', pass over the contacts C, C', completing the circuit of the
battery B and lamp L. When working properly the lamp L lights up regularly
once every second. This regulator is an excellent one to use for
experimental work, although it depends a great deal upon the skill of the
operator, but good adjustment should be obtained in about two minutes. It
is a good plan to insert a clutch of some description between the driving
motor and the machine, so that the regulator can be adjusted prior to the
act of receiving or transmitting, the machine being prevented from
revolving by means of a catch. The motor used should be powerful enough to
take up the work of driving the machine without any reduction in speed. The
clocks M can be regulated so that they only gain or lose a few seconds in
{70} twenty-four hours, which gives an accuracy in working sufficient for
all practical purposes.

Connection is made with the contact springs S, S', by means of the springs
T, T', which press against the spindles J, J'.

Another important point is the correct placing of the picture upon the
receiving drum. It is necessary that the two machines besides revolving in
perfect isochronism should synchronise as well, _i.e._ begin to transmit
and record at exactly the same position on the cylinders, viz. at the edge
of the lap, so that the component parts of the received image shall occupy
the same position on the paper or film as they do on the metal print. If
the receiving cylinder had, let us suppose, completed a quarter of a
revolution before it started to reproduce, the reproduction when removed
from the machine and opened out will be found to be incorrectly placed; the
bottom portion of the picture being joined to the top portion, or _vice
versa_, and this means that perhaps an important piece of the picture would
be rendered useless even if the whole is not spoilt. It is evident,
therefore, that some arrangement must be employed whereby synchronism, as
well as isochronism of the two instruments can be maintained.

There are several methods of synchronising that are in constant use in
high-speed telegraphy, in which the limit of error is reduced to a minimum,
{71} and some modification of these methods will perhaps solve the problem,
but it must be remembered that synchronism is far easier to obtain where
the two stations are connected by a length of line than where the two
stations are running independently.

In one system of ordinary photo-telegraphy synchronism is obtained in the
following manner. The receiving cylinder travels at a speed slightly in
excess of the transmitting cylinder, and as its revolution is finished
first is prevented from revolving by a check, and when in this position the
receiving apparatus is thrown out of circuit and an electro-magnet which
operates the check is switched in. When the transmitting cylinder has
completed its revolution (about 1/100th of a second later) the transmitting
apparatus, by means of a special arrangement, is thrown out of circuit for
a period, just long enough for a powerful current to be sent through the
line. This current actuates the electro-magnet. The check is withdrawn and
the receiving cylinder commences a fresh revolution in perfect synchronism
with the transmitting cylinder. As soon as the check is withdrawn the
receiving apparatus is again placed in circuit until another revolution is
completed. As the receiver cannot stop and start abruptly at the end of
each revolution a spring clutch is inserted between the driving motor and
the machine. {72}

Although a method of synchronising similar to this may later on be devised
for wireless photography, the writer, from the result of his own
experiments, is led to believe that results good enough for all practical
purposes can be obtained by fitting a synchronising device whereby the two
machines are started work at the same instant, and relying upon the perfect
regulation of the speed of the motors for correct working.

The method of isochronism must, however, be nearly perfect in its action,
as it is easy to see that with only a very slight difference in the speed
of either machine this error will, when multiplied by 40 or 50 revolutions,
completely destroy the received picture for practical purposes.

From what has been written in this and in the preceding chapters it will be
evident that the successful solution of transmitting photographs by
wireless methods will necessitate the use of a great many pieces of
apparatus all requiring delicate adjustment, and depending largely upon
each other for efficient working. As previously stated, there is at present
no real system of wireless photography, the whole science being in a purely
experimental stage, but already Professor Korn has succeeded in
transmitting photographs between Berlin and Paris, a distance of over 700
miles. If such a distance could be worked over successfully, there is no
reason to doubt that before long {73} we shall be able to receive pictures
from America with as great reliability and precision as we now receive
messages.

In nearly all wireless photographic systems devised up to the present the
chief portion of the receiver consists of a very sensitive galvanometer,
and although very good results have been obtained by their use they are
more or less a nuisance, as the extreme delicacy of their construction
renders them liable to a lot of unnecessary movement caused by external
disturbances. A galvanometer of the De' Arsonval pattern, used by the
writer, was constantly being disturbed by merely walking about the room,
although placed upon a fairly substantial table; and for the same reason it
was impossible to attempt to place the driving motor of the machine on the
same table as the galvanometer. For ship-board work it will be evident that
the use of such a sensitive instrument presents a great difficulty to
successful working, and a good opening exists for some piece of
apparatus--to take the place of the galvanometer--that will be as sensitive
in its action but more robust in its construction.

       *       *       *       *       *


{74}

CHAPTER V

THE "TELEPHOGRAPH"

In the present chapter it is proposed to give a brief description of a
system of radio-photography devised by the author, and which includes a
greatly improved method of transmitting and receiving, as well as an
ingenious arrangement for synchronising the two stations; the whole being
an attempt to produce a system that would be capable of working
commercially over fairly long distances.

The system about to be described, and which I have designated the
"telephograph," is the outcome of several years' original experimental
work, many difficulties that were manifest in the working of the earlier
systems having been overcome by apparatus that has been expressly designed
for the purpose.

In any practical system of radio-photography the following points are of
great importance: (1) the speed of transmission; (2) the quality of the
received picture; (3) the method of synchronising {75} the two machines so
that transmission and reception begin simultaneously; (4) the correct
regulation of the speed of the driving motors; (5) the simplicity and
reliability of the entire arrangement. Points 1 and 2 are dependent upon
several factors; the number of contacts made by the stylus per minute; the
size of the metal print used; the number of lines per inch on the screen
used in preparing the print; and the accurate and harmonious working of the
various pieces of apparatus employed.

In the system under discussion the size of the metal print used is 5 inches
by 7 inches, and a screen having 50 lines to the inch is used for preparing
it. With the drum of the machine making one revolution in four seconds, the
stylus makes 87 contacts per second, or 5220 a minute, the time for
complete transmission being twenty-five minutes. By the use of ordinary
relays not more than 2000 contacts a minute can be obtained, and in the
present system it is only by means of a specially designed relay that such
a high rate of working has been made possible. Similarly, too, with the
receiving of such a large number of signals transmitted at such a high
speed, a special instrument has been devised that can record this number of
signals without any trouble, and could even record up to 8000 signals a
minute, provided that a suitable transmitter could be designed. {76}

In the present system the writer does not claim to have completely solved
the problem of the wireless transmission of photographs, but it is a great
advance on any system previously described, and the following advantages
are put forward for recognition: (1) a greatly improved method of
transmitting and receiving; (2) a simple method of regulating the speed of
the driving motors and maintaining isochronism with a limit of error of
less than 1 in 800; (3) an arrangement for synchronising the two machines
whereby transmitting and receiving begin simultaneously; (4) the use of one
machine only at each station.

TRANSMITTING APPARATUS

A diagrammatic representation of the apparatus required for a complete
station, transmitting and receiving combined, is given in Fig. 35, the
usual wireless equipment having been omitted from the diagram to avoid
confusion.

_The Machine._--This, as will be seen from Fig. 36, consists of a
base-plate M, to which are attached the two bearings B and B'. The bearing
B' is fitted with an internal thread to correspond with the threaded
portion of the shaft D. The drum V is a brass casting, being fastened to
the shaft by set screws. The shaft is threaded 75 to the inch. The bearings
are preferably of the concentric type. The circuit breaker C is so arranged
that when {77} the drum has traversed the required distance, the end of the
shaft pushes back the spring M, breaking the circuit of the driving gear
and stopping the machine. The machine is connected to the driving gear by
the flexible coupling A.

[Illustration: FIG. 35.

M, motor; Y, isochroniser; F, clutch; A, machine; R, stylus; S, relay; X,
gearing; O, circuit breaker; T, receiver; C, condenser; U, telephone relay;
K, polarised relay; L, contact breaker; D, D^1, D^2, D^3, batteries; P,
friction brake; B, B^1, double-pole two-way switches; N, N^1, N^2, single
switches; W, key; E, electric clock; J, telephones.]

The drum measures 5 inches long by 2-1/8 inches diameter, and this takes a
metal print 5 inches by 7 inches, which allows for a lap of about 1/4 inch.
In working, the print is wrapped tightly round the drum, being secured by
means of a little seccotine smeared along one edge. Care must be taken that
the edge of the lap draws away from the point of {78} the stylus and not
towards it. A margin of bare foil, about 1/8 inch wide, should be left on
the print at the commencing edge, the purpose of which will be explained
later.

[Illustration: FIG. 36.]

_The Stylus._--As the drum of the machine travels laterally, by reason of
the threaded shaft and bearing, the stylus must necessarily be a fixture.
It consists of a holder B, drilled to take a hardened steel point S,
attached to the spring M. The spring is arranged to work in the guide F,
which is provided with an adjusting screw W for regulating the pressure of
the stylus upon the print; the pressure being sufficient to enable good
contact to be made, but must not be heavy enough to scratch the soft foil.
The needle should present an angle of about 60deg to the surface of the
print, as this angle has been found to give the best results in working.

To eliminate any sparking that may take place at the point of make and
break, due to the self-induction of the relay coils, a condenser C, about 1
microfarad capacity, should be connected across {79} the drum and stylus.
The complete stylus is given in the drawings, Figs. 37, 37_a_, and also in
the diagrams Figs. 8 and 9.

[Illustration: FIG. 37.

Showing the arrangement for sliding the stylus to or from the machine.]

[Illustration: FIG. 37a.]

_The Relay._--As will be seen from the diagram, Fig. 38, this consists of
two electro-magnets having very soft iron cores, the magnet M being wound
in the usual manner, while the magnet N is wound differentially. The
armature A is made as light as possible, and is pivoted at P, and when
there is no current flowing through any of the coils, is held midway
between the magnet cores by the two spiral springs S and T, which are under
slight but equal tension. The connections are as follows. The wires from
the winding on M are connected directly to the relay terminals F and H, as
are also the wires from one winding on N. The other winding on N is
connected in series with the battery C, ammeter B, and regulating
resistance R. {80}

[Illustration: FIG. 38.]

When the circuit of the battery C is completed, the coil of N, to which it
is connected, is energised, and the armature A is attracted against the
stop V. When in this position the tension of the spring S is released,
while the tension of the spring T is increased. As soon as the circuit of
the battery D is completed by means of the metal line print on the
transmitting machine, the current divides at the terminals F and H, a
portion flowing through the magnet coil M, and a portion through the
remaining winding on N. The current which flows through the winding on N
produces a magnetising effect equal to that caused by the other winding on
N, but since the two windings are of equal length and resistance, and since
the current flowing through the two windings is of equal strength but in
opposite directions, the result is to neutralise {81} the magnetising
effects produced by each winding, and consequently no magnetism is produced
in the cores.

The other portion of the current from D flows through the coil M, and it
becomes magnetised at the same time that the coil N becomes demagnetised.
The armature A is attracted by M against the stop X, and this attraction is
assisted by the spring T, which was under increased tension. The conditions
of the springs are now reversed, the spring S being under increased
tension, while the tension of the spring T is released.

As soon as the current from D is broken, the magnetism disappears from M,
the neutralising current in N ceases, and N once more becomes magnetised,
owing to the current which still flows through one winding from C; the
armature is therefore again attracted by N, assisted by the spring S. The
current flowing through the two windings of N must be perfectly equal, and
the regulating resistance R, and ammeters B and B', are inserted for
purposes of adjustment. The current from C must flow in a direction
opposite to that which flows from D.

[Illustration: FIG. 39.

H, H', containers; M, mercury; E, paraffin oil; T, T', terminals; C,
suspending rod; D, base; F, F', dipping rods.]

The local circuit of the relay is completed by means of a copper dipper in
mercury, somewhat resembling an ordinary mercury break, but modified to
suit the present requirements. The arrangement will be seen from Fig. 39.
The whole of the {82} moving parts are made as light as possible, and for
this reason the rod C and the dippers F, F' should be made as short as
convenient. The containers H, H' are separate, of cast iron, and
rectangular in shape. The dipper is of very thin copper tube--an advantage
where alternating current is to be used--and is made adjustable for height
on the suspending rod C. The leg F is of such a length that permanent
contact is made with the mercury in the container H, while the leg F'
clears the surface of the mercury by about 1/4 inch, when the armature of
the relay is in its normal position. To prevent undue churning of the
mercury, which would necessarily take place if the dipper entered and left
the mercury at each movement of the armature, a pointed ebonite plug is
inserted in the end of the tube. This will be found to give good results at
a high speed, the mercury being practically undisturbed, and the production
of "sludge" reduced to a minimum. To prevent oxidation of the mercury, and
to prevent arcing, the surface is covered with paraffin oil. If this is not
sufficient to prevent arcing a condenser should be shunted across the {83}
containers. The volume of mercury, and the area of the dippers, should be
sufficient to carry the current used for a considerable period without
heating up to any extent. An adjustable weight J is provided in order to
balance the armature and dipping rod.

The remaining transmitting apparatus consists of the battery D^2 and the
usual wireless apparatus. The double-pole two-way switch B' is to enable
the photo-telegraphic set to be switched out and the hand key W switched in
for ordinary signalling purposes. The battery D^2 should be about 12 volts.

RECEIVING APPARATUS

The wireless portion of the receiver is similar to that given in Fig. 22,
is of the usual syntonic type, and comprises an oscillation transformer, S
being the secondary, and P the primary; C' is a block condenser, and C a
variable condenser. The detector D is of the carborundum crystal or
electrolytic pattern. A two-way switch B is provided so that the relay U
can be switched out and the telephones J switched in for ordinary receiving
purposes. The relay U is a Brown's telephone relay.

[Illustration: FIG. 40.]

_The Receiver._--The magnified current from the relay U is taken to a
special telephone receiver, the construction of which is given in Fig. 40.
The diaphragm F is about 2-1/2 inches diameter, and should be fairly thin
but very resilient. Only one {84} [Illustration] [Illustration] coil is
provided, and this should be wound with No. 47 S.S.C. copper wire for a
resistance of about 2000 ohms. By using only one coil and therefore only
one core, the movement of the diaphragm is centralised. To the centre of
the diaphragm a light steel point is fastened, about 1/2 inch long, and
provided with a projecting hook H. An enlarged view of this pin is given in
Fig. 41. The movement of the diaphragm and consequently of the steel point
P is communicated to a pivoted rod R, which is of special construction. A
piece of aluminium tube 3-3/4 inches long, and of the section given at B,
is bushed at one end with a piece of brass of the shape shown in Fig. 41a.
A stiff steel wire T about 1 inch long (20 gauge) is screwed into the end
of Z, and carries a counterbalance weight C. A hardened {85} steel spindle,
pointed at both ends, is fastened at D, and runs between two coned
bearings, one of which is adjustable. The underside of Z is flattened, and
a small coned depression is made for the reception of the pointed end of
the pin. By means of the spring J the two pieces, Z and P, are held firmly
together, at the same time allowing perfect freedom of movement. The bridge
G is made from a piece of sheet aluminium placed in a slot cut in the tube
R, the end of the tube being pressed tight upon G, and secured by means of
a small rivet.

The optical arrangements are as follows. By means of the Nernst lamp L, and
the lenses B and B', Figs. 42 and 43, a magnified shadow of G is thrown
upon the screen J. When the shutter G is in its normal position (_i.e._ at
rest), its shadow is just above the small hole in J, and light from L
reaches the photographic film wrapped round the drum V of the machine.

[Illustration: FIG. 42.

J, screen; L, Nernst lamp; G, shutter; B, condensing lens; B_1, focussing
lens.]

When, however, signals are sent out from the transmitting apparatus, the
magnified current from the relay U energises the coil of the special
telephone S, attracting the diaphragm F, and consequently giving movement
to the pivoted rod R. As by means of the optical arrangements a {86}
magnified movement as well as a magnified image of G is thrown upon the
screen J, the shadow of G will, when the telephone S is actuated, cover the
hole in the screen, and prevent any light from reaching the film on V,
until current from the relay U ceases to flow. Therefore, when the stylus
of the transmitter traces over a conducting strip on the metal print, no
light reaches the film on V, but when tracing over an insulating strip the
shadow of G on the screen J rises, and the light from L reaches the film.
By this means a positive picture is received, which is a great advantage
where the photographs are required for reproduction. Atmospherics would be
represented by irregular transparent marks on the film after development,
and these can be easily eradicated by retouching.

[Illustration: FIG. 43. E, ebonite screen; F, focussing lens; G, shutter;
O, condensing lens; L, Nernst lamp.]

The drum of the machine moves laterally 1/75th of an inch per revolution,
and the hole in the screen is 1/90th of an inch in diameter. As the screen
J is not in direct contact with the film, the slight diffusion of the light
that takes place will produce {87} a mark of about the right thickness.
With a movement of the diaphragm of only 1/40000th of an inch, the actual
movement of G will be 1/4000th of an inch. If the optical arrangements have
a magnifying power of 100, then the movement of the shadow upon the screen
will be 1/40th of an inch, which will be ample to cover the aperture.

The aluminium rod R, minus the counter-weight, can be made to weigh not
more than 12 grains. It is necessary to enclose the optical parts in a
light tight box, indicated by the dotted lines in Fig. 43, in order to
prevent any extraneous light from reaching the film.

_The Contact Breaker._--The contact breaker (L, Fig. 35), as will be seen
from Fig. 44, consists of an electro-magnet N, the windings of which are
connected with the battery B and the polarised relay K. The armature which
is supported by the spring G carries a contact arm A, which in its normal
position makes permanent contact with the contact screw T, and completes
the circuit between the relay K and the telephone relay U (Fig. 35). As
soon as the transmitter sends out the first signal, the magnified current
from the telephone relay actuates the relay K, which in turn completes the
circuit of the contact breaker. Directly the armature M has been attracted,
the contact with T is broken, and A makes fresh contact with the screw H,
by means of the spring Z {88} fastened to the underside of A. The armature,
once it has been attracted, is held in permanent contact with H by the
catch S, independent of the magnets N. As soon as contact is made with H,
the clutch (F, Fig. 35) circuit is completed, and the circuit of the relay
K is broken. When the circuit of the clutch F is broken by means of the
circuit breaker C on the machine (Fig. 36), the stop S is pulled back by
hand, allowing the contact arm A to rise, and again make fresh contact with
the contact screw T.

[Illustration: FIG. 44.]

DRIVING APPARATUS

_The Friction Brake._--This consists of a steel disc A, Fig. 45, about
2-1/2 inches diameter and 3/8 inch or 1/2 inch wide on the face, secured to
the main shaft of the driving motor. The arm H, pivoted at C, carries at
one end the curved block B, which is faced with a pad of tow F. The other
extremity is pivoted to the steel rod P, which slides {89} [Illustration]
in holes bored in the standards J. One end of the rod P is screwed with a
fine thread, about 75 to the inch, and is fitted with a regulating wheel T,
by means of which the block B can be made to press upon the disc A with any
required degree of pressure. A fairly stiff steel spring R is placed upon
the rod P, between one standard J and the collar N. As the speed of the
driving motor is slightly in excess of that required by the machine, the
block B, by means of the wheel, is made to press upon the disc A, setting
up friction which reduces the motor speed until the isochroniser indicates
that the correct working speed has been attained.

_The Clutch_.--The details of this will be seen from Figs. 46 and 47. It
consists of a steel shaft coned at both ends running between two
countersunk bearings, one of which is adjustable. This shaft carries the
two portions of the clutch A and B, the portion A being a fixture on the
shaft, and the portion B running free upon it. The portion B is a gun-metal
casting bored to run accurately upon the steel shaft. A soft iron annular
ring is fastened to the face.

[Illustration: FIG. 46.

E, spindle; R, bobbins; P, iron cores; D, copper rings; T, brushes; N, back
plate; V, front plate; J, gearing; S, spring; H, collar; Z, iron ring; F,
fixed bearing; C, insulating bush.]

The portion A consists of a gun-metal casting {90} [Illustration] bored a
tight fit for the shaft E, secured by means of a set screw. The two magnet
cores P are screwed into the front plate V, which is also of gun-metal, and
after the bobbins R have been slipped on, the shanks of the cores are
passed through holes drilled in the flange N of the main casting and held
in place with nuts. The faces of both A and B must be turned perfectly
square with the shaft, so that they run accurately together. The portion B
is {91} kept in contact with A by means of a spring S, the pressure being
regulated by the collar H. Current is taken to the magnets by means of the
two insulated copper rings D mounted upon the body of A. The gear-wheels on
both portions have teeth of very fine pitch, the number of teeth on each
being regulated by the speed of the driving motor and the required machine
speed. Connection with the circuit breaker L and the battery B^2 is made
with the collecting rings D by the brushes T. The complete connections are
given in the diagram Fig. 51.

_The Isochroniser._--This is a device for ensuring the correct speed
regulation of the driving motors, and is shown in detail in Fig. 48. It
comprises two portions, one portion being rotated at a definite speed by
electrical means, and the other portion rotated by the driving motor.

The main portion consists of a metal tube N, bushed at both ends, the
bottom end of the tube being arranged to work on ball-bearings. An ebonite
bush C carries three copper rings T, T^1, T^2, and the brushes R, R^1, R^2
are in electrical contact with them. The ebonite plate J, 3-1/2 inches
diameter, is secured to the top end of N, and carries a contact piece Q,
shown separate at E. As will be seen this is a block of ebonite with three
contacts arranged on the top surface. The middle contact P is 1/64th of an
inch wide, and the contacts P^1 {92} and P^2 are placed on either side at a
distance of 1/16 inch; the contact strips P^1, P^2 carry the brass pins D,
which are about 1/16 inch diameter, and spaced 3/8 inch apart. A connecting
wire is carried from the contact P to the copper ring T, another from P^1
to T^1, and one from P^2 to T^2.

[Illustration: FIG. 48.

N, brass tube; S, bushes; G, ball-bearing; H, gear-wheel; T, T^1, T^2,
copper rings; C, insulating block; R, R^1, R^2, brushes; J, ebonite disc;
Q, contact block; D, metal pins; O, pulley, P, P^1, P^2, contact plates; K,
needle; Z, spring; W, steel rod; E, countersunk bearing.]

The bushes S are bored a running fit for the steel rod W (shown separate at
A), which is coned at both ends, and runs between two countersunk bearings,
the bottom bearing E being fixed while {93} the top bearing (not shown) is
adjustable. A needle K is fastened near the end of the rod W, and attached
to this needle is the spring Z, which presses lightly but firmly upon the
contact block Q. To provide a level surface for Z to work over, the spaces
between the contact pieces are filled in with an insulating material, and
the whole surface finished off perfectly smooth. The spring Z is 1/8 inch
wide for portion of its length, but at the point where it presses upon Q it
is reduced in width to 1/64th of an inch (see Fig. 48). The driving
arrangements are as follows. A counter-shaft Q, Fig. 51, fitted with a
grooved pulley, is run in bearings parallel with the shaft W, and is
connected by suitable gearing to the shaft of the driving motor, so that
the needle K makes one revolution in about 2-1/2 seconds. A belt passing
over the pulleys connects the two shafts, and the tension of the belt is
regulated by means of an adjustable jockey pulley.

The tube N, carrying the disc J, must be rotated at a fixed speed, and this
is accomplished in the following manner. An ordinary electric clock impulse
dial, actuated from a master clock, is connected by suitable gearing H, so
that the tube N makes exactly one revolution in 2 seconds; it being
possible to adjust an electric clock of the "Synchronome" type, so that it
only gains or loses about 1 second in 24 hours, and this provides {94} an
accuracy sufficient for all practical purposes. The connections are given
in Fig. 49, and the face of the instrument in Fig. 50. It will be seen that
a connecting wire is run from the steel spindle W to one terminal each of
the lamps L, L^1, L^2, and from the other terminal of the lamps to one
terminal of the batteries J, the battery comprising a set of three 4-volt
accumulators. The other terminals of the batteries are joined one to each
of the brushes R, R^1, R^2.

[Illustration: FIG. 49.]

[Illustration: FIG. 50.

M, terminals for connecting to electric clock; L, white lamp; L^1, blue
lamp; L^2, red lamp.]

The lamps are coloured, the lamp L being white, and the lamps L^1 and L^2
blue and red respectively, and care must be taken in connecting up that
when the needle K makes contact with the stud P the white lamp L is in
circuit. When the machines are working, the operator, by means of the brake
(already described), reduces the speed of the driving motor until the
needle K travels in unison with the disc J, making permanent contact with P
on the contact {95} block Q, which is evidenced by the lamp L remaining
alight. If, however, the needle travels faster than the disc J, contact
with P is broken and fresh contact is made with P^2, the lamp L is
extinguished and the red lamp L^2 lights up, and remains alight until the
operator reduces the speed. Similarly, too, if the needle travels slower
than J, contact is made with P^1, and the circuit of the blue lamp L^1 is
completed. When the speed is either above or below the normal, the needle K
engages with one or the other of the pins D, and as the tension of the
driving belt is only such as is required to drive the needle, the belt
slips on the pulleys until the normal speed is regained.

METHOD OF WORKING

The clockwork motor M, Fig. 51, should be capable of running for several
hours with one winding, and powerful enough to take up the work of driving
the machine without any appreciable effort. The main spindle of the motor
is so arranged that it makes one revolution in two minutes, and the
reduction in speed between the motor shaft and the shaft to which the
coupling A is attached is 30:1. The metal line print having been wrapped
round the drum of the machine, the stylus is put into position, at the edge
of the lap, and with the needle resting about half-way on {96} the margin
of the bare foil left at the commencing edge of the print. Now, when the
two stations are in perfect readiness for work, the motors are started and
the speed adjusted; the speed of the machine being just under one
revolution in four seconds.

[Illustration: FIG. 51.

M, clockwork motor; S, isochroniser; E, friction break; T, brushes; F,
electric clutch; X, gearing; D, D^1, switches; A, flexible coupling; K,
polarised relay; L, circuit breaker; B_1, B_2, B_3, batteries; P, electric
clock; W, terminals for connection to telephone relay; H, terminals for
connection to terminals J, on transmitting machine.]

The switch D is then closed, and the arm of the switch D^1 placed on the
contact stud (1), at the transmitting station only. As soon as the switches
are closed the clutch F comes into action, and the transmitting machine
begins to revolve. When the whole of the line print wrapped round the drum
of the machine has passed under the stylus, the end of the shaft D, Fig.
36, engages {97} with the spring _m_, breaking the clutch circuit and
allowing the motor to run free. As soon as the machine stops, the switch D
is opened and the machine run back to its starting position by hand.

At the receiving station the switch D is also closed, and the arm of the
switch D^1 placed on the contact stud (2). The closing of these switches
does not bring the clutch F into operation until current from the telephone
relay U connected to the wireless receiving apparatus works the sensitive
polarised relay K, which in turn completes the circuit of the
circuit-breaker L. When the armature of L is attracted, the circuit of the
relay K is broken, the circuit of the clutch F is completed, and the
machine starts revolving.

[Illustration: FIG. 52.]

The current from the relay U, due to the transmitting stylus passing over
_one_ contact strip on the metal print, is too brief to actuate the heavier
mechanism of the relay K, hence the need of the margin of bare foil at the
commencing edge of the metal print, so that a practically continuous
current will flow to the relay K until the armature is attracted. As,
however, the relay is not actuated at the receipt of the first signal, and
as it is necessary for the machine to start recording at a certain point on
the film, viz. {98} at the edge of the lap--the reason for this was given
in Chapter IV.--the starting position of the receiving drum will be similar
to that given in the diagram Fig. 52, where X indicates the lap of the
photographic film, and the arrow the direction of rotation.

It is, of course, obvious that a somewhat similar adjustment must be made
with regard to the position of the stylus on the metal print at the
transmitting machine.

In the present system, as in almost every photographic method of receiving
that has been described, the Nernst lamp is invariably mentioned as the
source of illumination. Since the advent of the high-voltage metal-filament
lamps the Nernst lamp has fallen somewhat into disuse for commercial
purposes, but it possesses certain characteristics that render it eminently
suitable for the purpose under discussion.

The main principle of this type of lamp depends upon the discovery made by
Professor Nernst in 1898, after whom the lamp is named, that filaments of
certain earthy bodies when raised to a red heat became conductive
sufficiently well to pass a current which raised it to a white heat, and
furthermore that the glowing filament emitted a brighter light for a given
amount of current than carbon filaments.

[Illustration: FIG. 52a.]

Nernst lamps are made in two sizes, the larger {99} being intended for the
same work as usually done by arc lamps, and the smaller to replace
incandescent lamps; the smaller type being made to fit into the ordinary
bayonet lampholders. The principal parts of a Nernst lamp consist of the
filament, the heater, the automatic cut-out, and the resistance, and their
arrangement in the smaller type of lamp is given in the diagram, Fig. 52a.
The current enters at the positive terminal, passes through the heater M,
and out through the negative terminal. The filament B, which consists of a
short length of an infusible earth made of the oxides of several rare
minerals, of which zirconia is one, is a non-conductor at first, but
becomes a conductor upon being raised to a high temperature by means of the
heater M. As soon as the filament becomes conductive the current then
passes through the automatic cut-out H, and the armature D is attracted,
thus breaking the heater circuit. The current then flows from the positive
terminal {100} [Illustration] through the cut-out H, resistance J, and
filament B, and from thence out of the lamp. Since the resistance of the
filament decreases the hotter it gets, it is necessary to insert a
ballasting resistance in series with it which has the opposite property of
increasing its resistance as it gets hotter, to prevent the filament taking
too much current and destroying itself. Such a resistance, J, consists of a
filament of fine iron wire, which, to prevent oxidation from exposure to
the air, is enclosed in a glass bulb filled with hydrogen gas. Fig. 52_b_
shows the form of ballast resistance used in the small and large type of
lamp respectively.

Either direct or alternating current can be used with these lamps, and with
direct current the polarity must be strictly observed, and that the
positive wire is connected to the positive and the {101} negative wire to
the negative terminal. With the smaller type of lamp once it has been
correctly placed in its holder it is essential that it should not be
turned, as a change in the direction of the current will rapidly destroy
the filament.

[Illustration: FIG. 52c.]

The arrangement of the larger type of Nernst lamp can be readily seen from
the drawing, Fig. 52c.

Care must be taken to see that the voltage required by the burner and
resistance equals the voltage of the supply circuit, and that only parts of
the same amperage are used together on the same lamp. No advantage is
obtained by over-running a Nernst lamp, this only shortening its life
without increasing the light. Under normal conditions the average life of
the burner is about 700 hours.

The efficiency of the Nernst lamp is fairly high, being only 1.45 to 1.75
watts per c.p. The light given is remarkably steady, and the lamps are
adaptable for all voltages from 100 to 300. In one of the large type of
lamps for use on a 235-volt {102} circuit the burner takes 0.5 ampere at
215 volts, and the resistance 0.5 ampere at 20 volts, while one of the
smaller lamps for use on the same circuit takes 0.25 ampere at 215 volts
and 0.25 ampere at 20 volts for the burner and resistance respectively. The
burner and heater are very fragile, and should never be handled except by
the porcelain plate to which they are attached. The lamps burn in air and
emit a brilliant white light of high actinic power, the intrinsic
brilliancy (c.p./square inch) varying from 1000 to 2500, as compared with
1000 to 1200 for ordinary metal filament lamps, and 300 to 500 for carbon
filament lamps.

The chief advantage of the Nernst lamp from a photographic point of view
lies in the fact that it produces abundantly the blue and violet rays which
have the greatest chemical effect upon a photographic plate or film. These
rays are known as chemical or actinic rays, and are only slightly produced
in some types of incandescent electric lamps. Carbon-filament lamps are
very poor in this respect.

Because a light is visually brilliant it must by no means be assumed that
it is the best to use for purposes of photography, and this is a point over
which many photographers stumble when using artificial light. Many sources
of light, while excellent for illumination, have very low actinic powers,
while others may have low illuminating but high {103} actinic powers. A
lamp giving a light yellowish in colour has usually low actinic power,
while all those lamps giving a soft white light are generally found to be
highly actinic.

In addition to the actinic value of the source of illumination, the
photographic film used must be very carefully chosen, as the chemical
inertia of the sensitised film plays an important part in the successful
reproduction of the picture, and also, to a certain extent, affects the
speed of transmission. The length of exposure, the amount of light admitted
to the film, and the characteristics of the film itself, are all factors
which have a decided bearing upon the quality of the results obtained, and
the film found to be most suitable in one case will perhaps give very
unsatisfactory results in another.

In photo-telegraphy the length of exposure is determined by the time taken
by the transmitting stylus to trace over a conducting strip on the metal
print, and this time, of course, varies with the density of the image and
also with the speed of transmission.

The film in ordinary photography is chosen with regard to the subject and
the existing light conditions, and the amount of light admitted to the film
and the length of exposure are regulated accordingly. No such latitude is,
however, possible in photo-telegraphy. With each set of apparatus {104} the
various factors, such as the light value, the amount of light admitted to
the film, and the length of exposure, will be practically fixed quantities,
and the film that will give the most satisfactory results under these fixed
conditions can only be found by the rough-and-ready method of "trial and
error."

The films in common use are manufactured in four qualities, namely,
ordinary, studio, rapid, and extra rapid. These terms should really relate
to the light sensitiveness of the film (or, as it is technically termed,
the speed), but at the best they are a rough and very unsatisfactory guide,
for the reason that some unscrupulous makers, purely for business purposes,
do not hesitate to label their films and plates as slow, rapid, etc.,
without troubling to make any tests for correct classification.

The speed of photographic films and plates is generally indicated by a
number, and the system of standardisation adopted by the majority of makers
in this country is that originated by Messrs. Hurter & Driffield,
abbreviated H. & D. In their system the speed of the film and the exposure
varies in geometrical proportion, a film marked H. & D. 50 requiring double
the exposure of one marked H. & D. 100. The highest number always denotes
the highest speed, and the exposure varies inversely with the speed.

Besides the Hurter & Driffield method of {105} obtaining the speed numbers
of plates and films adopted by a large number of makers in this country,
there are also two standard English systems known as the W.P. No. (Watkin's
power number) and Wynne F. No., both of which are used to a fair extent.

The "Actinograph" number or speed number of a plate in the H. & D. system
is found by dividing 34 by a number known as the Inertia, the Inertia,
which is a measure of the insensitiveness of the plate, being determined
according to the directions laid down by Hurter & Driffield--that is, by
using pyro-soda developer and the straight portion only of the density
curve. If, for instance, the Inertia was found to be one-fifth, then the
speed number would be 34 / 1/5 = 170, and the plate is H. & D. 170. The
W.P. No. is found by dividing 50 by the Inertia. Thus 50 / 1/5 = 250, and
the plate is W.P. 250, but for all practical purposes the W.P. No. can be
taken as one and a half times H. & D. The Wynne F. numbers may be found by
multiplying the square root of the Watkins number by 6.4. Thus

  [sqrt]250 = 15.81, and 15.81 x 6.4 = W.F. 101.

For those photographers who are in the habit of using an actinometer giving
the plate speeds in H. & D. numbers, the following table, taken from the
_Photographer's Daily Companion_, is given, {106} which shows at a glance
the relative speed numbers for the various systems. The Watkins and Wynne
numbers only hold good, however, when the inertia has been found by the H.
& D. method.

TABLE OF COMPARATIVE SPEED NUMBERS FOR PLATES AND FILMS

  ------------------------------------------------------
  |H. & D.|W.P. No.|W.F. No.||H. & D.|W.P. No.|W.F. No.|
  --------+--------+-----------------+--------+---------
  |   10  |   15   |   24   ||  220  |  323   |  114   |
  |   20  |   30   |   28   ||  240  |  352   |  120   |
  |   40  |   60   |   49   ||  260  |  382   |  124   |
  |   80  |  120   |   69   ||  280  |  412   |  129   |
  |  100  |  147   |   77   ||  300  |  441   |  134   |
  |  120  |  176   |   84   ||  320  |  470   |  138   |
  |  140  |  206   |   91   ||  340  |  500   |  142   |
  |  160  |  235   |  103   ||  380  |  558   |  150   |
  |  200  |  294   |  109   ||  400  |  588   |  154   |
  ------------------------------------------------------

Although theoretically the higher the speed of the film the less the
duration of exposure required, there is a practical limit, as besides the
intensity and actinic value of the light admitted to the film a definite
time is necessary for it to overcome the chemical inertia of the sensitised
coating and produce a useful effect. With every make of film it is possible
to give so short an exposure that although light does fall upon the film it
does no work at all--in other words, we can say that for every film there
is a minimum amount of light action, and anything below this is of no use.
The exposure that enables the smallest amount of light action to take place
is termed the limit of the smallest useful exposure. {107}

There is also a maximum exposure in which the light affects practically all
the silver in the film, and any increased light action has no increased
effect. This is the limit of the greatest useful exposure.

In photo-telegraphy the duration of exposure, as already pointed out, is
determined by certain conditions connected with the transmitting apparatus,
and with conditions similar to those mentioned on page 75 the length of
exposure will vary roughly from 1-50th to 1-150th of a second.

The most suitable film to use for purposes of photo-telegraphy is one
having a fairly slow speed in which the range of exposure required comes
well within the limits of the film. There is no advantage in using a film
having a speed of, say, H. & D. 300 if good results can be obtained from
one with a speed of, say, H. & D. 200, as the use of the higher speed
increases the risk of overexposure. With the high-speeded films the
difficulties of development are also greatly increased, there being more
latitude in both exposure and development with the slower speeds, and
consequently a better chance of obtaining a good negative.

Another point, often puzzling to the beginner, and which increases the
difficulty of choosing a suitable make of film, is that, although one make
of film marked H. & D. 100 will give good results, another make, also
marked H. & D. 100, will give {108} very poor results. This is owing, not
to a poor quality film, as many suppose, but to the almost insurmountable
difficulty of makers being able to employ exactly the same standard of
light for testing purposes, so that although various makes may all be
standardised by the H. & D. method, films bearing the same speed numbers
may vary in their actual speed by as much as 30 to 50 per cent.

       *       *       *       *       *


{109}

APPENDIX A

SELENIUM CELLS

Selenium is a non-metallic element, and was first discovered by Berzelius
in 1817, in the deposit from sulphuric acid chambers, which still continues
the source from which it is obtained for commercial purposes, although it
is found to a small extent in native sulphur. Its at. wt. is 79.2, and its
sp. gr. 4.8. Symbol, Se.

In its natural state selenium is practically a non-conductor of
electricity, its resistance being forty thousand million times greater than
copper. Its practical value lies in the property which it possesses, that
when in a prepared condition it is capable of varying its electrical
resistance according to the amount of light to which it is exposed, the
resistance decreasing as the light increases.

Selenium is prepared by heating it to a temperature of 120deg C., keeping
it there for some hours, and allowing it to cool slowly, when it assumes a
crystalline form and changes from a bluish grey to a dull slate colour. A
selenium cell in its simplest form consists merely of some prepared
selenium placed between two or more metal electrodes, the selenium acting
as a high resistance conductor between them. The form given by Bell and
Tainter to the cells used in their experiments is given in Figs. 53 and
53a. It consists of a number of rectangular brass plates P, P', separated
by very thin sheets of mica M, the mica sheets being slightly narrower than
the brass plates, the whole being clamped together in the frame F by the
two bolts B. {110} By means of a sand-bath the cell is raised to the
desired temperature, and selenium is rubbed over the surface, which melts
and fills the small spaces between the brass plates. All the plates P are
connected together to form one terminal, and the plates P' to form the
other. By using very thin mica sheets, and a large number of elements, a
very narrow transverse section of selenium, together with a large active
surface, can be obtained.

The cell used for commercial purposes is usually constructed as follows. A
small rectangular piece of porcelain, slate, mica, or other insulator, is
wound with many turns of fine platinum wire. The wire is wound double, as
shown in Fig. 54, the spaces between the turns being filled with prepared
selenium. A thin glass cover is sometimes placed over the cell to protect
the surface from injury.

[Illustration: FIG. 53.

P, P', plates; M, mica; S, selenium.]

[Illustration: FIG. 53a.]

A strong light falling upon a cell lowers its resistance, and _vice versa_,
the resistance of a cell being at its highest when unexposed to light; the
light is apparently absorbed and made to do work by varying the electrical
resistance of the selenium. Selenium cells vary very considerably as
regards their quality as well as in their electrical resistance, it being
possible to obtain cells of the same size for any resistance between 10 and
1,000,000 ohms, and also, a cell may remain in good working condition for
several months, while another will become useless in as many weeks.

The ability of a cell to respond to very rapid changes in the illumination
to which it is exposed is determined largely upon its inertia, it being
taken as a general rule {111} that the higher the resistance of a cell the
less the inertia, and _vice versa_, and also, that the higher the
resistance the greater the ratio of sensitiveness. Inertia plays an
important part in the working of a cell, slightly opposing the drop in
resistance when illuminated, and opposing to a [Illustration] much greater
degree the return to normal for no-illumination. The effects of inertia or
"lag," as it is termed, can readily be seen by reference to Fig. 55. It
will be noticed that the current value rapidly increases when the cell is
first illuminated, but if after a short time _t_ the light is cut off, the
current value, instead of returning at once to normal for no-illumination,
only partially rises owing to the interference of the inertia, and some
time elapses before the cell returns to its normal condition; the time
varying from a few seconds to several minutes, depending upon the
characteristics of the cell and the amount of light to which it is exposed.
An actual curve is given in Fig. 55a. The inertia or "lag" of a cell
produces upon an intermittent current an effect similar to that produced by
the capacity [Illustration] of a line, as was noted in Chapter I.,
preventing the incoming signals from being recorded separately, and
distinctly. To obtain the best results in photo-telegraphy, the resistance
of a cell should only be decreased to an extent sufficient to pass the
current required to operate the recording apparatus, and the illumination
should be regulated so that this condition of the cell takes place.

The comparative slowness of selenium in responding to {112} any great
changes in the illumination offers a serious difficulty to its use in
photo-telegraphy, but various methods have been devised whereby the effects
of inertia can be counteracted. In the system of De' Bernochi (see Chapter
I.) the changes in the illumination are neither very rapid nor very great,
and the inertia effects would therefore be very slight; but in any
photo-telegraphic system in which a metal line print is used for
transmitting, where the source of illumination is constant and the
resistance of the cell is required to drop to a definite value and return
to normal instantly, many times in succession, the inertia effects are very
pronounced. The most successful method of counteracting the inertia is that
adopted by Professor Korn of always keeping the cell sufficiently
illuminated to overcome it, so that any additional light acts very rapidly.
Another method worked out and patented by Professor Korn, and known as the
"compensating cell" method, gives a practically dead beat action, the
resistance returning to its normal condition as soon as the illumination
ceases. The arrangement is given in the diagram Fig. 56.

[Illustration: FIG. 55a.]

Light from the transmitting or receiving apparatus, as the case may be,
falls upon the selenium cell S^1, which is {113} placed on one arm of a
Wheatstone bridge, a second cell S^2 being placed on the opposite arm. The
selenium cell S^1 should have great sensitiveness and small inertia, the
compensating cell S^2 having proportionally small sensitiveness and large
inertia. Two batteries B, B', of about 100 volts, are connected as shown, B
being provided with a compensating variable resistance W; W' is also a
regulating resistance. When no light is falling upon the cell S^1, light
from L is prevented from reaching the second cell S^2 by a small shutter
which is fastened to the strings of the Einthoven galvanometer (described
in Chapter III.), and the piece of apparatus C--relay or galvanometer as
the case may be--remains in a normal condition. When, however, light falls
upon the cell S^1, the balance of the bridge is upset, and light from L
falls a fraction of a second later upon the second cell S^2, and the
current flowing through C completes the circuit. Needless to say it is
necessary that the two cells be well matched, as it is very easy to have
over-compensation, in which case the current is brought below zero.

[Illustration: FIG. 56.]

It is also stated that by enclosing the cells in exhausted glass tubes,
their inertia can be greatly reduced and their life considerably prolonged.
The sensitiveness of a cell is the ratio between its resistance in the dark
and its resistance when illuminated. The majority of cells have a ratio
between 2:1 and 3:1, but Professor Korn has shown mathematically that by
conforming to certain conditions regarding the construction the ratio of
sensitiveness may be between 4:1 and 5:1. Thus a cell of R = 250,000 ohms
can be reduced to 60,000 ohms from the light of a 16 c.p. lamp placed only
a short distance away; the resistance may be still {114} further decreased
by continuing the illumination, but this produces a permanent defect in the
cells termed "fatigue," the cells becoming very sluggish in their action
and their sensitiveness gradually becoming less, the ratio between their
resistance in the dark and their resistance when illuminated being reduced
by as much as 30 per cent.

Excessive illumination will also produce similar results. The inertia of a
cell is practically unaffected by the wavelength of the light used, but the
maximum sensitiveness of a cell is towards the yellow-orange portion of the
spectrum.

In addition to light, heat has also been found to vary the electrical
resistance of selenium in a very remarkable manner. At 80deg C. selenium is
a non-conductor, but up to 210deg C. the conductivity gradually increases,
after which it again diminishes.

       *       *       *       *       *


{115}

APPENDIX B

PREPARING THE METAL PRINTS

Electricians who desire to experiment in photo-telegraphy, but who have no
knowledge of photography, may perhaps find the following detailed
description of preparing the metal prints of some value. The would-be
experimenter may feel somewhat alarmed at the amount of work entailed, but
once the various operations are thoroughly grasped, and with a little
patience and practice, no very great difficulty should be experienced. The
simpler photographic operations, such as developing, fixing, etc., cannot
be described here, and the beginner is advised to study a good text-book on
the subject.

The method to be given of preparing the photographs is practically the only
one available for wireless transmission, and although the manner given of
preparing is perhaps not strictly professional, having been modified in
order to meet the requirements of the ordinary amateur experimenter, the
results obtained will be found perfectly satisfactory.

As will have been gathered from Chapter II., the camera used for copying
has to have a single line screen placed a certain distance in front of the
photographic plate, and the object of this screen is to break the image up
into parallel bands, each band varying in width according to the density of
the photograph from which it has been prepared. Thus a white portion of the
photograph would consist of very narrow lines wide apart, while a dark
portion would be made up of wide lines close together; a black part would
appear solid and show no lines at all. It is, of course, obvious {116} that
the lines on the negative cannot be wider apart, centre to centre, than the
lines of the screen. A good screen distance has been found to be 1 to 64,
_i.e._ the diameter of the stop is 1/64th of the camera extension, and the
distance of the screen lines from the photographic plate is 64 times the
size of the screen opening. The following table shows what this distance is
for the screen most likely to be used. The line screens used consist of
glass plates upon which a number of lines are accurately ruled, the width
of the lines and the spaces between being equal; the lines are filled in
with an opaque substance. These ruled screens are very expensive, and are
only made to order,[10] a screen half-plate size costing from 21s. to 27s.
6d. An efficient substitute for a ruled screen can be made by taking a
rather large sheet of Bristol board and ruling lines across in pure black
drawing ink, the width of the lines and the spaces between being 1/12th of
an inch respectively. A photograph must be taken of this card, the
reduction in size determining the number of lines to the inch. A card 20 x
15 inches, with 12 lines to the inch, would, if reduced to 5 x 4 inches,
make a screen having 48 lines to the inch. Preparing the board is rather a
tedious operation, but the line negative produced will be found to give
results almost as good as those obtained from a purchased screen.

DIAMETER OF STOP USED 1/64TH OF CAMERA EXTENSION.

  --------------------------------------------------------------
  |Screen ruling  |Actual space| Distance of |In 1/32|In milli-|
  |lines per inch.| in inches. |screen ruling| inches|  metres.|
  |               |            |  in inches. |       |         |
  |---------------+------------+-------------+-------+---------|
  |       35      |    1/70    |      .91    |  28.8 |   21.8  |
  |       50      |    1/100   |      .64    |  20.5 |   16.2  |
  |       65      |    1/130   |      .49    |  15.7 |   12.4  |
  --------------------------------------------------------------

As it is impossible for many to have the use of professional apparatus
designed for this particular kind of work, {117} the fixing of the screen
into an ordinary camera must be left to the ingenuity of the worker. A
half-plate back focussing camera will be found suitable for general
experimental work, but if this is not available, a large box camera can be
pressed into service.

[Illustration: FIG. 57.]

The writer has never seen a half-plate box camera, but one taking a 5 x 4
inch plate can be obtained second-hand very cheaply. It is a comparatively
simple matter to fix the line screen into a camera of this description, the
drawings Figs. 57 and 58 showing the method adopted by the writer. The two
clips D, made from fairly stout brass about 1/2 inch wide, are bent to the
shape shown (an enlarged section is given at C) and soldered at the top and
bottom of one of the metal sheaths provided for holding the plates. The
distance between the front of the photographic plate (the film side) and
the back of the line screen (also the film side), indicated by the arrow at
A, is determined by the number of lines on the screen. As will be seen from
the table given, the distance for a screen having 50 lines to the inch will
be 41/64ths of an inch.

[Illustration: FIG. 58.

M, sheath; P, photographic plate; D, clips; S, line screen.]

In all probability there will be enough clearance between the top of the
sheath and the top of the camera to allow for the thickness of the clip,
but if not, a shallow groove a little wider than the clip should be
carefully cut in the top of the camera, so that it will slide in easily.
The screen should be placed between the clips, the film side on the {118}
inside, _i.e._ facing the photographic plate. As with a box camera the
extension is a fixture, the size of stop to be used is a fixture also. The
extension of a camera (this term really applies to a bellows camera) is
measured from the front of the photographic plate to the diaphragm, and if
this distance in our camera is 8 inches, then the diameter of the stop to
give the best results would be 1/64th of this, or 1/8th inch. Although for
all ordinary experimental work the lens fitted to the camera will be
suitable, the best type of lens for process work of all kinds is the
"Anastigmat."

The picture or photograph from which it is desired to make a print should
be fastened out perfectly flat upon a board with drawing pins, and if a
copying stand is not available it must be placed upright in some convenient
position. The diagram Fig. 59 gives the disposition of the apparatus
required for copying. A simple and inexpensive copying stand is shown in
Fig. 60. The blackboard A should be about 30 inches square, and must be
fastened perfectly upright upon the base-board B. The stand C should be
made so that it slides without any side play between the guides D, and
should be of such a height that the lens of the camera comes exactly
opposite the {119} [Illustration] [Illustration] centre of the board A. The
camera, if of the box type, can be secured to the stand by means of a screw
and wingnut, the screw being passed from the inside as shown. The beginner
is advised to photograph only very bold and simple subjects, such as black
and white drawings or enlargements. It is not safe to trust to the
view-finders as to whether the whole of the picture is included on the
plate, a piece of ground glass the same size as the plate sheaths, and used
as a focussing screen, being much more reliable. It is a good plan to focus
the camera for a number of different-sized pictures, marking the board A,
and the {120} guides D, so that adjustment is afterwards a very simple
matter.

The make of plate used is also a great factor in getting a good negative,
and Wratten Process Plates will be found excellent. As already mentioned,
such subjects as the exposure and the development of the plate cannot be
dealt with here, these subjects having been exhaustively treated in several
text-books on photography. With an arc lamp the exposure is about twice as
long as in daylight, but the exposure varies with the amount of light
admitted to the plate, character of the source of light, and the
sensitiveness of the plate used, etc. The writer has used acetylene gas
lamps for this purpose with great success. The beginner is advised to use
artificial light, as this can be kept perfectly even. With daylight,
however, the light is constantly fluctuating, and this renders the use of
an actinometer a necessity for correct exposure. After development, if the
plate is required for immediate use, it can be quickly dried by soaking for
a few minutes in methylated spirit.

Having obtained a good negative, the next operation is to prepare what is
known as a metal print. For this we shall require some stout tin-foil or
lead-foil, about 12 or 15 square feet to the pound, and this should be cut
into pieces of such a size that it allows a lap of 3/16 inch when wrapped
round the drum of the transmitting machine. Obtain some good fish-glue and
add a saturated solution of bichromate of potash in the proportion of 4
parts of potash to 40 or 50 parts of glue. Pour a little of this glue into
a shallow dish, lay a sheet of foil upon a flat board, and with a fairly
stiff brush (a flat hog's-hair as wide as possible) proceed to coat the
sheet of foil with a thin but perfectly even coating of glue. The thickness
of the coating can only be found by trial, for if the coating is too thick
a longer time will be required for printing; but it must not be thin enough
to show interference colours. After the coating has been laid on, a soft
brush, such as photographers use for dusting dry {121} plates, should be
passed up and down, and across and across, with light, even strokes to
remove any unevenness. A glue solution used by professional photo-engravers
is as follows:

  Fish-glue               12 oz.
  Bichromate of Ammonia  3/4 oz.
  Water                   18 to 24 oz.
  Ammonia .880            30 minims.

The bichromate should be dissolved in the water, and, when added to the
glue, stir very thoroughly in order that complete mixing may take place.
The coating may be done in a good light, not bright sunlight, but _it must
be dried in the dark_, because, although insensitive while in a moist
condition, it becomes sensitive immediately on desiccation. If allowed to
dry in the light the whole coating will become insoluble, and for this
reason the brushes used should be washed out as soon as they are finished
with. The sheets will take about 15 minutes to dry in a perfectly dry room,
but it is not advisable to prepare many sheets at once, as they will not
keep for more than two or three days.

The prepared negative must now be placed in an ordinary printing frame, and
a print taken off upon one of the metal sheets in the same way as a print
is taken off upon ordinary sensitised paper. In daylight the exposure
varies from 5 to 20 minutes, but in artificial light various trials will
have to be made in order to get the best results, the exposure varying with
the amount of bichromate in the coating; the proportion of the bichromate
to the glue should remain about 6 per cent. Light from a 25 ampere arc lamp
for 2 to 5 minutes, at a distance of 18 inches, will generally suffice to
"print" the impression on the metal sheets. The printing finished, the
metal print should be laid upon a sheet of glass and held under a running
stream of water. The washing is complete as soon as the unexposed parts of
the glue coating have been entirely washed away leaving the bare metal, and
this will take anything from 3 to 7 {122} minutes, depending upon the
thickness of the film. As soon as it is dry the print is ready for use.

As already mentioned, the negative from which the metal print is made
requires that the lines be perfectly sharp and opaque, and the spaces
between perfectly transparent. Ordinary dry plates are too rapid, a rather
slow plate being required. Wratten Process Plates give excellent results,
and the following is a good developer to use with them:

  Glycin                15 grammes       1     oz.
  Sulphite of Soda      40  ,,           2-1/2 ,,
  Carbonate of Potash   80  ,,           5     ,,
  Water               1000 c.c.         60     ,,

This developer should be used for 6 minutes at a temperature of 50deg F.,
3-1/2 minutes at 65deg, and 1-3/4 minutes at 80deg. It is best only used
once. If an intensifier is required, the following formula will be found to
give satisfactory results:

  Bichloride of Mercury   1 oz.    60 grammes.
  Hot Water              16 ,,   1000 c.c.

Allow to cool, completely pour off from any crystals, and add:

  Hydrochloric Acid      30 minims   4 c.c.

Allow negative to bleach thoroughly, wash well in water, and blacken in 10
per cent ammonia .880, or 5 per cent sodium sulphide.

In preparing the negatives and metal prints the following points should be
observed:

A good negative should have the lines perfectly sharp and opaque; there
should be no "fluff" between the lines even when they are close together.

A properly exposed and developed negative should not require any reducing
or intensifying.

If the lamps used for illuminating the copying board are placed 2 feet
away, and the exposure required is 5 minutes, the exposure, if the lamps
are placed 4 feet away, will be {123} 20 minutes, as the amount of light
which falls upon an object decreases as the inverse square of the distance.

Get the coating on the foil as thin as possible, and err on the side of
over-exposure, for if the coating is thick and has been under-exposed,
excessive washing will dissolve the whole coating; for, unless
insolubilisation has taken place right up to the metal base, the under
parts will remain in a more or less soluble condition.

On no account must the unexposed sheets be placed near a fire, otherwise
they will be spoilt, the whole coating becoming insoluble; heat acting in
the same manner as light.

In washing, keep the print moving so that the stream of water does not fall
continually in one place. It is best to hold the print so that the water
runs off in the direction of the lines.

To dry the prints after washing they can be laid out flat in a moderately
warm oven, or before a stove, the heat of course not being sufficient to
cause the coating to peel.

To render the glue image more distinct the print should be immersed for a
few seconds in an aniline dye solution, the glue taking up the colour
readily. These dyes are soluble in either water or alcohol. A dye known as
"magenta" is very good.

The process of coating the metal sheets must be performed as quickly as
possible (about 10 seconds), as owing to the peculiar nature of the
bichromated glue it soon sets, and once this has taken place it is
impossible to smooth down any unevenness.

See that the negative and metal sheet make good contact while printing.

If the glue solution does not adhere to the surface of the foil in a
perfectly even film, but assumes a streaky appearance, a little liquid
ammonia, or a weak solution of nitric acid, rubbed over the surface of the
foil, which is afterwards gently scoured with precipitated chalk on a tuft
of cotton {124} wool, will remove the grease which is the cause of the
difficulty.

A photograph of a picture prepared from a line negative is given in Fig.
61. For a great many experiments, and in order to save time, trouble, and
expense, sketches drawn upon stout lead-foil in an insulating ink will
answer the purpose admirably, but if any exact work is to be done a single
line print is of course absolutely necessary. The insulating ink can be
prepared by dissolving shellac in methylated spirit, or ordinary gum can be
used. A very fine brush should be used in place of a pen, as the gum will
not flow freely from an ordinary nib unless greater pressure than the foil
will safely stand be applied. A sketch prepared in this manner is shown in
Fig. 62. A little aniline dye should be added to the gum to render it more
visible, or a mixture of gum and liquid indian ink will be found suitable.

[Illustration: FIG. 63.]

With the copying arrangement already described it is only possible to
employ it for reducing, it being necessary to employ a bellows camera with
a back focussing attachment for purposes of enlarging, and this constitutes
the chief drawback to the use of a fixed focus camera. By replacing the box
camera with a focussing camera of the same size, we shall have a piece of
apparatus capable of reducing or enlarging, only in this case the camera
should be a fixture and the board, A, arranged to slide backwards and
forwards instead.

[Illustration: FIG. 61.

Portions of photographs (full size) of single line screen, and single line
print. Screen 40 lines to the inch.]

[Illustration: FIG. 62.]

{125} An extra improvement would be to rule the surface of the copying
board, A, in a manner similar to that shown in the diagram, Fig. 63. The
rulings should be marked off from the centre of the board, and should
enclose parallelograms of the various plate sizes ranging from 3-1/4 x
4-1/4 inches up to the full size of the board. By fastening the picture or
photograph to be copied in the space on the board corresponding in size, we
can ensure that it is in the correct position for the whole to be included
on the photographic plate, providing, of course, that the centre of lens
and board coincide.

With regard to the lens required, the practice adhered to by most
photographers is to use a lens having a focal length equal to the diagonal
of the plate used. Thus for a 1/4-plate camera a 5-inch lens should be
used, and for a 1/2-plate an 8-inch lens, and so on. For a 5 x 4 inch
camera a 6-inch lens will be required. The following is a simple rule for
finding the conjugate foci of a lens, and is useful in obtaining the
distance from the lens to the photographic plate and the picture to be
copied. Let us suppose that we wish to make a 1-1/2 times enlarged line
negative from a 4-1/4 x 3-1/4 inch print. Add 1 to the number of times it
is required to enlarge and multiply the result by the focal length of the
lens in inches. In the present case this will be 1-1/2 + 1 = 2-1/2; and if
a 6-inch lens is used, 2-1/2 x 6 = 15 inches will be the distance of the
lens from the plate. Divide this number by the number of times it is
desired to enlarge, and the distance of the lens from the picture to be
copied is obtained; in this instance 15 / 1-1/2 = 10 inches. The same rule
can be followed when it is required to reduce any given number of times,
only in this case the greater number will represent the distance between
the lens and the picture to be copied, and the lesser number the distance
between the lens and the plate.

In reducing, a 1/4-plate lens will be found to fully cover a 5 x 4 inch
plate, providing the reduction is not greater than three to one.

       *       *       *       *       *


{126}

APPENDIX C

LENSES

In this small volume it is not desirable, neither is it intended, to give
an exhaustive treatment on the subject of lenses and their action, but as
optics plays an important part in the transmission of photographs, both by
wireless and over ordinary conductors, the following notes relating to a
few necessary principles have been included as likely to prove of interest.

Light always travels in straight lines when in a medium of uniform density,
such as water, air, glass, etc., but on passing from one medium to another,
such as from air to water, or air to glass, the direction of the light rays
is changed, or, to use the correct term, _refracted_. This refraction of
the rays of light only takes place when the incident rays are passed
obliquely; if the incident rays are perpendicular to the surface separating
the two media they are not refracted, but continue their course in a
straight line.

All liquid and solid bodies that are sufficiently transparent to allow
light rays to pass through them possess the power of bending or refracting
the rays, the degree of refraction, as already explained, depending upon
the nature of the body.

The law relating to refraction will perhaps be better understood by means
of the following diagram. In Fig. 64 let the line AB represent the surface
of a vessel of water. The line CD, which is perpendicular to the surface of
the {127} water, is termed the _normal_, and a ray of light passed in this
direction will continue in a straight line to the point E. If, however, the
ray is passed in an oblique direction, such as ND, it will be seen that the
ray is bent or refracted in the direction DM; if the ray of light is passed
in any other oblique direction, such as JD, the refracted ray will be in
the direction DK. The angle NDC is called the _angle of incidence_ and MDE
the _angle of refraction_. If we measure accurately the line NC, we shall
find that it is 1-1/3, or more exactly 1.336, times greater than the line
EM. If we repeat this measurement with the lines JH and PK we shall find
that the line JH also bears the proportion of 1.336 to the line PK. The
line NC is called the _sine of the angle of incidence_ NDC, and EM the
_sine of the angle of refraction_ MDE.

[Illustration: FIG. 64.]

Therefore in water the sine of the angle of incidence is to the sine of the
angle of refraction as 1.336 is to 1, and this is true whatever the
position of the incident ray with respect to the surface of the water. From
this we say that _the sines of the angles of incidence and refraction have
a constant proportion or ratio to one another_.

The number 1.336 is termed the _refractive index_, or _coefficient_, or the
_refractive power_ of water. The refractive power varies, however, with
other fluids and solids, and a complete table will be found in any good
work on optics.

Glass is the substance most commonly used for refracting the rays of light
in optical work, the glass being worked up into different forms according
to the purpose for which it {128} is intended. Solids formed in this way
are termed _lenses_. A lens can be defined as a transparent medium which,
owing to the curvature of its surfaces, is capable of converging or
diverging the rays of light passed through it. According to its curvature
it is either spherical, cylindrical, elliptical, or parabolic. The lenses
used in optics are always exclusively spherical, the glass used in their
construction being either crown glass, which is free from lead, or flint
glass, which contains lead and is more refractive than crown glass. The
refractive power of crown glass is from 1.534 to 1.525, and of flint glass
from 1.625 to 1.590. Spherical surfaces in combination with each other or
with plane surfaces give rise to six different forms of lenses, sections of
which are given in Fig. 65.

[Illustration: FIG. 65.]

All lenses can be divided into two classes, convex or converging, or
concave or diverging. In the figure, _b_, _c_, _g_ are converging lenses,
being thicker at the middle than at the borders, and _d_, _e_, _f_, which
are thinner at the middle, being diverging lenses. The lenses _e_ and _g_
are also termed meniscus lenses, and _a_ represents a prism. The line XY is
the axis or _normal_ of these lenses to which their plane surfaces are
perpendicular.

Let us first of all notice the action of a ray of light when passed through
a prism. The prism, Fig. 66, is represented by the triangle BBB, and the
incident ray by the line TA. {129} Where it enters the prism at A its
direction is changed and it is bent or refracted towards the base of the
prism, or towards the normal, this being always the case when light passes
from a rare medium to a dense one, and where the light leaves the opposite
face of the prism at D it is again refracted, but away from the normal in
an opposite direction to the incident ray, since it is passing from a dense
to a rare medium. The line DP is called the _emergent_ or refracted ray. If
the eye is placed at T, and a bright object at P, the object is seen not at
P, but at the point H, since the eye cannot follow the course taken by the
refracted rays. In other words, objects viewed through a prism always
appear deflected towards its summit.

[Illustration: FIG. 66.]

In considering the action of a lens we can regard any lens as being built
up of a number of prisms with curved faces in contact. Such a lens is shown
in Fig. 67, the light rays being refracted towards the base of the prisms
or towards the normal, as already explained; while the top half of the lens
will refract all the light downwards, the bottom half will act as a series
of inverted prisms and refract all the light upwards.

[Illustration: FIG. 67.]

[Illustration: FIG. 68.]

If a beam of parallel light--such as light from the sun--be passed through
a double convex lens L, Fig. 68, we shall find that the rays have been
refracted from their parallel course and brought together at a point F.
This point F is {130} termed the principal focus of the lens, and its
distance from the lens is known as the focal length of that lens. In a
double and equally convex lens of glass the focal length is equal to the
radius of the spherical surfaces of the lens. If the lens is a plano-convex
the focal length is twice the radius of its spherical surfaces. If the lens
is unequally convex the focal length is found by the following rule:
multiply the two radii of its surfaces and divide twice that product by the
sum of the two radii, and the quotient will {131} be the focal length
required. Conversely, by placing a source of light at the point F the rays
will be projected in a parallel beam the same diameter as the lens. If,
however, instead of being parallel, the rays proceed from a point farther
from the lens than the principal focus, as at A, Fig. 69, they are termed
divergent rays, but they also will be brought to a focus at the other side
of the lens at the point a. If the source of light A is moved nearer to the
principal focus of the lens to a point A^1 the rays will come to a focus at
the point _a_^1, and similarly when the light is at A^2 the rays will come
to a focus at the point _a_^2. It can be found by direct experiment that
the distance _fa_ increases in the same proportion as AF diminishes, and
diminishes in the same proportion as AF increases. The relationship which
exists between pairs of points in this manner is termed the _conjugate
foci_ of a lens, and though every lens has only one principal focus, yet
its conjugate foci are innumerable.

[Illustration: FIG. 69.]

The formation of an image of some distant object in its principal focus is
one of the most useful properties of a convex lens, and it is this property
that forms the basis of several well-known optical instruments, including
the camera, telescope, microscope, etc.

If we take an oblong wooden box, AA, and substitute a sheet of ground
glass, C, for one end, and drill a small pinhole, H, in the centre of the
other end opposite the {132} glass plate, we shall find that a tolerably
good image of any object placed in front of the box will be formed upon the
glass plate. The light rays from all points of the object, BD, Fig. 70,
will pass straight through the hole H, and illuminate the ground glass
screen at points immediately opposite them, forming a faint inverted image
of the object BD. The purpose of the hole H is to prevent the rays from any
one point of the object from falling upon any other point on the glass
screen than the point immediately opposite to it, therefore the smaller we
make H, the more distinct will be the image obtained. Reducing the size of
H in order to produce a more distinct image has the effect of causing the
image to become very faint, as the smaller the hole in H, the smaller the
number of rays that can pass through from any point of the object. By
enlarging the hole H gradually, the image will become more and more
indistinct until such a size is reached that it disappears altogether.

[Illustration: FIG. 70.]

If in this enlarged hole we place a double convex lens, LL, Fig. 71, whose
focal length suits the length of the box, the image produced will be
brighter and more distinct than that formed by the aperture, H, since the
rays which proceed from any point of the object will be brought by the lens
to a focus on the glass screen, forming a bright {133} distinct image of
the point from which they come. The image owes its increased distinctness
to the fact that the rays from any one point of the object cannot interfere
with the rays from any other point, and its increased brightness to the
great number of rays that are collected by the lens from each point of the
object and focussed in the corresponding point of the image. It will be
evident from a study of Fig. 71 that the image formed by a convex lens must
necessarily be inverted, since it is impossible for the rays from the end,
M, of the object to be carried by refraction to the upper end of the image
at _n_. The relative positions of the object and image when placed at
different distances from the lens are exactly the same as the conjugate
foci of light rays as shown in Fig. 69.

[Illustration: FIG. 71.]

The length of the image formed by a convex lens is to the length of the
object as the distance of the image is to the distance of the object from
the lens. For example, if a lens having a focal length of 12 inches is
placed at a distance of 1000 feet from some object, then the size of the
image will be to that of the object as 12 inches to 1000 feet, or 1000
times smaller than the object; and if the length of the object is 500
inches, then the length of the image will be the 1/1000th part of 500
inches, or 1/2 inch. {134}

The image formed by the convex lens in Fig. 71 is known as a _real image_,
but in addition convex lenses possess the property of forming what are
termed _virtual images_. The distinction can be expressed by saying, _real
images are those formed by the refracted rays themselves, and virtual
images those formed by their prolongations_. While a real image formed by a
convex lens is always inverted and smaller than the object, the virtual
image is always erect and larger than the object. The power possessed by
convex lenses of forming virtual images is made use of in that useful but
common piece of apparatus known as a reading or magnifying glass, by which
objects placed within its focus are made larger or magnified when viewed
through it; but in order to properly understand how objects seem to be
brought nearer and apparently increased in size, we must first of all
understand what is meant by the expression, _the apparent magnitude of
objects_.

[Illustration: FIG. 72.]

The apparent magnitude of an object depends upon the angle which it
subtends to the eye of the observer. The image at A, Fig. 72, presents a
smaller angle to the eye than the angle presented by the object when moved
to B, and the image therefore appears smaller. When the object is moved to
either B or C, it is viewed under a much {135} greater angle, causing the
image to appear much larger. If we take a watch or other small circular
object and place it at A, which we will suppose is a distance of 50 yards,
we shall find that it will be only visible as a circular object, and its
apparent magnitude or the angle under which it is viewed is then stated to
be very small. If the object is now moved to the point B, which is only 5
feet from the eye, its apparent magnitude will be found to have increased
to such an extent that we can distinguish not only its shape, but also some
of the marking. When moved to within a few inches from the eye as at C, we
see it under an angle so great that all the detail can be distinctly seen.
By having brought the object nearer the eye, thus rendering all its parts
clearly visible, we have actually magnified it, or made it appear larger,
although its actual size remains exactly the same. When the distance
between the object and the observer is known, the apparent magnitude of the
object varies inversely as the distance from the observer.

Let us suppose that we wish to produce an image of a tree situated at a
distance of 5000 feet. At this distance the light rays from the tree will
be nearly parallel, so that if a lens having a focal length of 5 feet is
fastened in any convenient manner in the wall of a darkened room the image
will be formed 5 feet behind the lens at its principal focus. If a screen
of white cardboard be placed at this point we shall find that a small but
inverted image of the tree will be focussed upon it. As the distance of the
object is 5000 feet, and as the size of the received image is in proportion
to this distance divided by the focal length of the lens, the image will be
as 5000 / 5, or 1000 times smaller than the object.

If now the eye is placed six inches behind the screen and the screen
removed, so that we can view the small image distinctly in the air, we
shall see it with an apparent magnitude as much greater than if the same
small image were equally far off with the tree, as 6 inches is to 5000
{136} feet, that is 10,000 times. Thus we see that although the image
produced on the screen is 1000 times less than the tree from one cause, yet
on account of it being brought near to the eye it is 10,000 times greater
in apparent magnitude; therefore its apparent magnitude is increased as
10,000 / 1000, or 10 times. This means that by means of the lens it has
actually been magnified 10 times. This magnifying power of a lens is always
equal to the focal length divided by the distance at which we see small
objects most distinctly, viz. 6 inches, and in the present instance is 60 /
6, or 10 times.

When the image is received upon a screen the apparatus is called a _camera
obscura_, but when the eye is used and sees the inverted image in the air,
then the apparatus is termed a _telescope_.

The image formed by a convex lens can be regarded as a new object, and if a
second lens is placed behind it a second image will be formed in the same
manner as if the first image were a real object. A succession of images can
thus be formed by convex lenses, the last image being always treated as a
fresh object, and being always an inverted image of the one before. From
this it will be evident that additional magnifying power can be given to
our telescope with one lens by bringing the image nearer the eye, and this
is accomplished by placing a short focus lens between the image and the
eye. By using a lens having a focal length of 1 inch, and such a lens will
magnify 6 times, the total magnifying power of the two lenses will be 10 x
6 = 60 times, or 10 times by the first lens and 6 times by the second. Such
an instrument is known as a _compound or astronomical telescope_, and the
first lens is called the object glass and the second lens the magnifying
glass, or eye-piece.

We are now in a position to understand how virtual images are formed, and
the formation of a virtual image by means of a convex lens will be readily
followed from a {137} study of Fig. 73. Let L represent a double convex
lens, with an object, AB, placed between it and the point F, which is the
principal focus of the lens. The rays from the object AB are refracted on
passing through the lens, and again refracted on leaving the lens, so that
an image of the object is formed at the eye, N. As it is impossible for the
eye to follow the bent rays from the object, a virtual image is formed and
is seen at A^1B^1, and is really a continuation of the emergent rays. The
magnifying power of such a lens may be found by dividing 6 inches by the
focal length of the lens, 6 inches being the distance at which we see small
objects most distinctly. A lens having a focal length of 1/4 inch would
magnify 24 times, and one with a focal length of 1/100th of an inch 600
times, and so on. The magnifying power is greater as the lens is more
convex and the object near to the principal focus. When a single lens is
applied in this manner it is termed a _single microscope_, but when more
than one lens is employed in order to increase the magnifying power, as in
the telescope, then the apparatus is termed a _compound microscope_.

[Illustration: FIG. 73.]

Unlike a convex lens, which can form both real and virtual images, a
concave lens can only produce a virtual image; and while the convex lens
forms an image larger {138} than the object, the concave lens forms an
image smaller than the object. Let L, Fig. 74, represent a double concave
lens, and AB the object. The rays from AB on passing through the lens are
refracted, and they diverge in the direction RRRR, as if they proceeded
from the point F, which is the principal focus of the lens, and the
prolongations of these divergent rays produce a virtual image, erect and
smaller than the object, at A^1B^1. The principal focal distance of concave
lenses is found by exactly the same rule as that given for convex lenses.

[Illustration: FIG. 74.]

Up to the present we have assumed that all the rays of light passed through
a convex lens were brought to a focus at a point common to all the rays,
but this is really only the case with a lens whose aperture does not exceed
12deg. By aperture is meant the angle obtained by joining the edges of a
lens with the principal focus. With lenses having a larger aperture the
amount of refraction is greater at the edges than at the centre, and
consequently the rays that pass through the edges of the lens are brought
to a focus nearer the lens than the rays that pass through the centre.
Since this defect arises from the spherical form of the lens it is termed
_spherical aberration_, and in lenses that {139} are used for photographic
purposes the aberration has to be very carefully corrected.

The distortion of an image formed by a convex lens is shown by the diagram,
Fig. 75. If we receive the image upon a sheet of white cardboard placed at
A, we shall find that while the outside edges will be clear and distinct,
the inside will be blurred, the reverse being the case when the cardboard
is moved to the point B.

[Illustration: FIG. 75.]

[Illustration: FIG. 76.]

[Illustration: FIG. 77.]

Aberration is to a great extent minimised by giving to the lens a meniscus
instead of a biconvex form, but as it is desirable to reduce the aberration
to below once the {140} thickness of the lens, and as this cannot be done
by a single lens, we must have recourse to two lenses put together. The
thickness of a lens is the difference between its thickness at the middle
and at the circumference. In a double convex lens with equal convexities
the aberration is 1-67/100ths of its thickness. In a plano-convex lens with
the plane side turned towards parallel rays the aberration is 4-1/2 times
its thickness, but with the convex side turned towards parallel rays the
aberration is only 1-17/100ths of its thickness.

By making use of two plano-convex lenses placed together as at Fig. 76, the
aberration will be one-fourth of that of a single lens, but the focal
length of the lens, L^1, must be half as much again as that of L. If their
focal lengths are equal the aberration will only be a little more than half
reduced. Spherical aberration, however, may be entirely destroyed by
combining a meniscus and double convex lens, as shown in Fig. 77, the
convex side being turned to the eye when used as a lens, and to parallel
rays when used as a burning glass or condenser.

       *       *       *       *       *


{141}

INDEX

  Aberration, 139
    spherical, 138, 140
  Accuracy of working, 70, 72
  Acetylene gas lamps, 120
  Actinic power, 102
  Actinograph, 105
  Actinometer, 120
  Alternating current, 82, 100
  Ammonia, 123
  Angle of stylus, 24, 78
  Aniline dye, 123
  Arcing, 27, 82
  Arc lamps, 15, 120, 121
  Atmospherics, 61, 85

  Ballasting resistance, 100
  Belin, 47
  Bernochi, 7, 112
    system of, 7, 34
  Berzelius, 109
  Bichromate of potash, 120
  Blondel's oscillograph, 47

  Camera obscura, 136
    extension, 116, 118
    choice of, 117
  Capacity of condenser, 24, 78
    electrostatic, 3, 5
    of cable, 3
    of London-Paris telephone line, 3
  Carbon bisulphide, 53
  Charbonelle, 48
    receiver of, 48
  Chemical solution, 56
  Circuit breaker, 76
  Clutch, details of, 88, 89, 91
    spring, 71
  Coating the metal sheets, 120
  Coherer, 11, 40
  Collecting rings, 91
  Commercial value of photo-telegraphy, 1
  Compensating selenium cell, 112
  Contact breaker, 37
  Copying arrangements, 118, 125
  Cross screen, 21

  De' Arsonval galvanometer, 47, 73
  Decoherer, 41
  Design of machines, 21
  Detectors, 83
  Developing solutions, 105, 122
  Diaphragm, movement of, 48, 52, 84, 87
  Dipping rods, 81, 83
  Distance of transmission, 33
  Duration of wave-trains, 22, 25

  Early experiments, 2
  Einthoven galvanometer, 32, 44, 45, 54, 113
  Electric clock, 93
  Electrolytic receiver, 4, 37, 54, 61, 64
  Enlarging arrangements, 124, 125
  Experimental machine, 20
  Extraneous light, 47

  Fastening electrolytic paper, 58
  Fatigue of selenium cell, 64, 114
  Fish glue, 120
  Flexible couplings, 77
  Frequency meter, 65
  Friction brake, 88

  {142}
  High speed telegraphy, 70
  Hughes governor, 65
  Hughes printing telegraph, 63
  Hurter and Driffield, 104
  Hydrogen, 100

  Incidence, angle of, 127
  Inertia, 64, 65, 111
    effects in photo-telegraphy, 110
    method of counteracting, 103, 112, 113
    effect of wave-length of light on, 114
  Intensifying solution, 122
  Isochroniser, 89, 91
    details of, 91, 92, 95
  Isochronism, 64, 69, 70, 71

  Kathode rays, 53
  Knudsen, 2
    apparatus of, 9
  Korn, 30, 33, 45, 65, 72
    apparatus of, 31

  Lamps, coloured, 94
  Lenses, 85, 125, 128
    principal focus of, 130
    conjugate foci of, 131
    action of, 129
    convex, 128, 131, 136
    concave, 128, 138
    focal length of, 130, 138
    aperture, 138
    meniscus, 139
  Light, diffusion of, 86
    extraneous, 87
  Limit of error in synchronising, 64
  Line balancer, 3
  Line screens, 9, 15, 16, 116
    making, 116

  Magnifying power, 136, 137
  Marconi valve, 44, 54
    coherer, 40
  Mechanical inertia, 33
  Mercury break, 81
    churning of, 82
    containers, 82
  Mercury jet interrupter, 29
  Metal prints, 15, 18, 32, 59, 64, 95, 120, 124
    drying the, 121, 123
    exposure of, 121
    size of, 22, 24, 75, 77
    pressing the, 22
  Microscope, 131, 137
  Military uses, 35
  Mirror galvanometer, 9, 42, 73
  Mirror, 47, 51
  Morse code, 35
  Motor speed, 89, 95
    driving, 91, 93, 95
    clockwork, 63
    electric, 63

  Nernst lamps, 43, 85, 98
    heater of, 99
    filament of, 99
    principle of, 98
    resistance of, 100
    efficiency of, 101, 102
    overrunning, 101
  Nicol prism, 53

  Paper for electrolytic receiver, 56
  Parabolic reflector, 8
  Period of galvanometer, 43, 44, 46
  _Photographic Daily Companion_, 105
  Photographic films, 40, 43, 45, 53, 54, 62, 85, 86, 98
    process, 37
    chemical inertia, 103
    exposure of, 103, 107
    speed of, 104, 105
    plates, orthochromatic, 59
    plates, 120
  Points to be observed in preparing metal prints, 123
  Poulsen Company, 32, 47
    arc, 31
  Preparing selenium, 109
    photographs for transmitting, 15, 115
    sketches on metal foil, 124
  Prism, 128
    action of, 129
  Process plates, 122
  Professor Nernst, 98

  {143}
  Radio-photography, requirements of, 74
  Refraction, angle of, 127
  Refractive power, 127
  Relay, 25, 39, 49, 53, 55, 60, 75
    differential, 79
    polarised, 97
    working speed of, 26, 75
  Reproducing for newspapers, 60
  Resistance of selenium, 109
    of selenium cells, 110
    regulating, 113
  Retardation of current, 6
  Retouching, 62
  Rotary spark-gap, 28

  Selenium, 99
    cells, 8, 34, 55, 60, 64, 109, 110
    machines, 45
  Self-induction, 24, 78
  Sensitiveness of selenium cells, 113
    ratio of, 113
  Silvered quartz threads, 44, 46
  Spark-gap, 27
  Speed regulator, 68
    adjustments of, 69
  Spring clutch, 71
  Starting position of machines, 98
  String galvanometer, 32
  Stylus, 17, 18, 57, 61, 78, 95, 103
    sparking at, 24
  Stylus, angle of, 24, 78
    defects of, 57
  Submarine cable, 4
  Synchronism, 11, 20, 36, 64, 69, 71

  Telephograph, 74
    advantages of, 76
    method of working, 96
  Telephone receiver, 83, 85
    diaphragm, 48
    improved, 51
  Telephone relay, 48, 50, 52, 83, 85, 97
  Telescope, 131, 136
  Thermodetector, 32
  Tow, 88
  Transmission, distance of, 35, 72
    speed of, 25, 35, 75

  Vibration, natural period of, 39

  Watkins, 105
    power number, 105
  Waves, damped, 30
    undamped, 30, 31
  Wheatstone bridge, 113
  Wireless apparatus, 13
  _Wireless World_, 31
  Wynne, 105

  Zirconia, 99



THE END



_Printed by_ R. & R. CLARK, LIMITED, _Edinburgh_.

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Franqueo, 1 peseta extra. America Latina, $2.25, oro, neto; Franqueo, 25
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THE ELEMENTARY PRINCIPLES OF WIRELESS TELEGRAPHY.

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Volume, price 7S. net. (POSTAGE 6D.) Used by H.M. Government for
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PRINCIPES ELEMENTAIRES DE TELEGRAPHIE SANS FIL.

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  Book V. THE OSCILLATION VALVE.

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Notes

[1] These measurements only apply to a single line. Where a double line is
employed the capacity is halved.

[2] See Appendix A.

[3] See Appendix B.

[4] In wireless telegraphy "arcing" is principally caused by the
continuation of the supply current in the spark-gap after the capacity has
been charged to a potential sufficient to break down the insulation of the
gap.

[5] See Chapter V.

[6] Nernst lamps are the best to use, as they produce abundantly the blue
and violet rays which have the greatest chemical effect upon a photographic
film. Carbon filament lamps are very poor in this respect.

[7] A description of the apparatus required will be found in Ganot's
_Physics_.

[8] Great care must be exercised in using this solution, as it is
exceedingly poisonous.

[9] Two clocks would isochronise if their hands travelled at precisely the
same rate round the dials, but would not synchronise unless they both
registered the same time as well.

[10] Line screens can be obtained from Messrs. Penrose, 109 Farringdon
Street, London; or Messrs. Fallowfield, 146 Charing Cross Road, London.



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