PpSMISSION
WIRELESS TRANSMISSION OF PHOTOGRAPHS
WIEELESS TEANSMISSION
OF
PHOTOGEAPHS
BY
MAECUS J. MARTIN
SECOND EDITION
REVISED AND ENLARGED 1919
THE WIEELESS PEESS, LTD.
12-13 HENRIETTA STREET, STRAND
LONDON, W.C. 2
PEEFACE 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 in-
sufficiently 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 informa-
tion 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 experi-
menters 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 WIRELESS PHOTOGRAPHY
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 photo-
graphic apparatus.
M. J. M.
MAIDSTONE, 1919,
PBEFACE
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 pro-
blems of the wireless control of torpedoes and air-
ships, 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 over-
come in long-distance transmission over ordinary
land lines and cables which will be entirely elimi-
nated 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 WIKELESS PHOTOGRAPHY
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 experi-
ments, 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.
ASHFOBD, 1916.
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 — Trans-
mitting Machines — Transmitting Apparatus — Effects of
Arcing— Spark - Gaps— Contact Breakers — Complete Station
—Professor Korn's Apparatus — Poulsen Company's Photo-
graphic Recorder — Comparison of various systems — Practical
applications.
CHAPTER III
RECEIVING APPARATUS . . . . .37
Methods of Receiving — Author's Photographic Receiver —
Decohering Apparatus— Description of Einthoven Galvano-
meter—Use of Galvanometer in Receiving— Belin's Applica-
tion 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.
ix
x WIRELESS PHOTOGRAPHY
CHAPTER IV
PAGE
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
CONTENTS xi
PAGE
Metal Sheets— Sensitising Solution — Printing Operations —
Developing — Intensifying — Precautions to be observed in
working — Preparing Sketches on Metal — Apparatus for Re-
ducing 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 Magnifi-
cation— Defects of Lenses — Aberration.
ILLUSTRATIONS
no. 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 photo-
graph ........ 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 1 .
\ facing page 21
10&. 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
xiii
xiv WIRELESS PHOTOGRAPHY
no. I-A..K
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
4 la. 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
61. Diagram of driving mechanism . . . .96
ILLUSTRATIONS xv
PAGE
52. Diagram showing starting positions of machines . . 97
52a. Arrangement of small type Nernst lamp . .99
526. 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
56. 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
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 facing page 124
RADIO-PHOTOGRAPHY
CHAPTER I
INTRODUCTORY
THOSE who desire to experiment on radio-photo-
graphy, 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 experi-
menting 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 WIRELESS PHOTOGRAPHY
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 trans-
mitting 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, develop-
ment 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 every-
thing 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 — pro-
perties inherent to all telegraph systems using
metallic conductors — have a distinct bearing upon
the two questions, how far and how quickly can
INTRODUCTORY 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 sub-
marine 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 contri-
buted 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
1 These measurements only apply to a single line. Where a double
line is employed the capacity it* halved.
4 WIRELESS PHOTOGRAPHY
allows each current sent out from the transmitter
— the current in several systems being intermittent
—to be recorded separately on the receiver. Photo-
graphs 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 inter-
ference 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 ^yaVtftftn 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
INTRODUCTORY 5
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 &, while only perhaps fifteen
could be recorded. If the capacity be still farther
increased as at c, only perhaps half the original
(L
FIQ. 1.
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 WIRELESS PHOTOGRAPHY
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 men-
tioned 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 im-
practicable, 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 super-
sede 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 fore-
INTRODUCTORY 7
going 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.
There have been numerous suggestions put for-
ward for the wireless transmission of photographs,
Fia. 2.
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 WIRELESS PHOTOGRAPHY
shaft. Round the cylinder is wrapped a trans-
parent 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 re-
fracted by the prism P on to the selenium celfc
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,1
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
1 See Appendix A.
INTKODUCTORY 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
A2, 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 repre-
sentation 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
WIRELESS PHOTOGRAPHY
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
A V \/A
N
FIO. 3.
A, aerial ; B, batteries ; C, coherer ; E, earth ; D, spark-gap ; M, spark-coil ;
N, magnet ; P, plates ; 8, springs ; T, tables.
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
INTKODUCTOEY 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 photo-
graph, quarter-plate size, is about fifteen minutes.
12 WIRELESS PHOTOGRAPHY
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 trans-
mitted.
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.
The connections
~~* T
for an experimental
syntonic wireless
transmitting station
are shown in the dia-
gram Fig. 4. A is
the aerial ; T, the in-
ductance ; E, earth ;
L, hot - wire am-
meter. The closed
oscillatory circuit
consists of an induct-
ance 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
13
FIG. 4.
14 WIRELESS PHOTOGEAPHY
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 trans-
posed 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 photo-
graph l is one that has been adopted in several
1 See Appendix B.
TEANSMITTING APPARATUS 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 print-
ing 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 WIRELESS PHOTOGRAPHY
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.
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
A /
/ 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
TRANSMITTING APPARATUS
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 where-
by 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
FIG. 6.
MOTOR
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 correspond-
ing 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.
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
c
18
WIRELESS PHOTOGRAPHY
no. 7.
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 insu-
lated 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 secco-
tine should be applied to one edge, and the
joint carefully smoothed down with the fingers.
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 pre-
ventive. 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
Fm g
TRANSMITTING APPARATUS
19
machine a lap of about T^ths of an inch is
allowed.
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 con-
nections are given
in Kg. 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.
FlQ. 9.
20 WIEELESS PHOTOGRAPHY
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 10a 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 ^th inch per revolution of the
drum.
From the brief description of the various types
of machines that have been given it will be ap-
parent 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.
FIG. 10a
• • • •'.'
• • • •• •
• • • • *
I • • • •
• •• • • *
> • • • *;n
• » • • »
i • • • •
FIG. 106.
hilarged view of an image broken up by a " cross " screen.
TRANSMITTING APPARATUS 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 100 was
designed and used by the writer solely for experi-
mental 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 recom-
mended, 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 90° to one another, and this breaks the
22 WIRELESS PHOTOGRAPHY
image up into small squares instead of lines. By
looking at any ordinary newspaper or book illustra-
tion 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 neces-
sarily 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 indefin-
able term) will not be too fine, and the time re-
quired 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,
TRANSMITTING APPARATUS
23
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
FIO. 11.
circuit of the relay R and the battery M is com-
pleted. 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 WIRELESS PHOTOGRAPHY
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 60° 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
TRANSMITTING APPARATUS 25
to the inch is used for preparing it, then the stylus
will have to make 250 contacts during one revolu-
tion 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=wxtxs, 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 en-
countered 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 sub-
stantial 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 reason-
able in price that will answer very well for experi-
mental 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 WIRELESS PHOTOGRAPHY
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 trans-
mitting, 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
TRANSMITTING APPARATUS 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
EJ=
CD
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.1
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 dis-
charge at every alternation of current. With this
type of gap a spark of pure tone is obtained which
1 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.
28
WIRELESS PHOTOGRAPHY
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
•nlternaTor
Fia. 13.
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
FIO. 14.
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
TRANSMITTING APPARATUS 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 photo-
graph 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 x^^th
part of a second, and capable of dealing with rather
large currents in the local circuit.1 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 trans-
mission.
1 See Chapter V.
30
WIRELESS PHOTOGRAPHY
The connections for a complete experimental
station, transmitting and receiving apparatus com-
bined, are given in Fig. 15. The terminals W, W
are for connecting to the photo-telegraphic re-
ceiving apparatus Q, being a double pole two-way
switch for throwing either the transmitting or receiv-
ing apparatus in circuit. There is another system
of transmitting devised by Professor Korn, which
employs an entirely different method from the fore-
going. 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.
TRANSMITTING APPARATUS
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 pro-
duction of undamped waves and their efficiency in
working compared with damped waves will be
found in the Wireless World, Nos. 3 and 4, 1913,
V A V A'
F1H
FIG. 16.
and are well worth reading by any one interested
in the subject.
A diagrammatic representation of the apparatus
as arranged by Professor Korn is given in Fig. 16.
The undamped or " continuous " waves are gener-
ated 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 WIRELESS PHOTOGRAPHY
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 ar-
ranged to reproduce a transmitted picture is shown
in Fig. 24.
The connections adopted by the Poulsen Com-
pany for photographically recording wireless mes-
sages 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 con-
denser C' prevents the continuous current produced
by the detector from flowing through the high
frequency circuit ; P is the primary of the aerial
TRANSMITTING APPARATUS
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
V A
Hhc'
FIG 17.
requiring careful adjustment add greatly to its
reliability.
In any spark system with a properly designed
aerial a coil taking ten amperes is capable of trans-
mitting 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 interrup-
tions were slower. It is possible to use platinum
D
34 WIRELESS PHOTOGRAPHY
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 applica-
tion has also been suggested for wireless trans-
mission, 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 receiv-
ing 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, experi-
ments 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
TRANSMITTING APPARATUS 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 -^ 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 WIRELESS PHOTOGRAPHY
of an hostile installation would be but a meaning-
less jumble of sounds, and even were they pos-
sessed of radio - photographic apparatus the re-
ceived message would be unintelligible, unless they
knew the exact speed at which the machines were
running and could synchronise accurately.
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
37
38
WIRELESS PHOTOGRAPHY
and receiving, and using as fine a line screen as
possible for preparing the photographs.
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
| inch. When a source of light is placed at some
FIG. is.
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 3^ 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 work-
ing, the cone is run as close as possible to the
RECEIVING APPARATUS
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.
The shutter C is about J inch square and made
from thin aluminium. The
hole in the centre is ^ x |
inch, and the movement of the
armature is limited to about
•$% inch. In all arrangements
of this kind there is a ten-
dency for the armature spring
to vibrate, as it were, sinu-
FIQ. 20. . _ .
soidally, if the coil is mag-
netised and demagnetised at a higher rate than
the natural period of vibration of the spring.
40 WIRELESS PHOTOGRAPHY
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 de-
tectors 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
RECEIVING APPARATUS
41
receiving apparatus, is given in the diagram
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 mag-
netised as soon as the incoming waves act upon
VA
M
M
K
FIG. 21.
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
WIRELESS PHOTOGRAPHY
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.
The drawing, Fig. 22, gives a diagrammatic
PIG. 22.
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 - trans-
former, S secondary of transformer, C variable
condenser, C' block condenser, D detector, X two-
way switch, T telephone.
A De' Arson val galvanometer H is also con-
nected to the switch X, so that either the telephone
or the galvanometer can be switched in. The
RECEIVING APPARATUS 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 T^th of a second. The
screen J has a small hole about ^ 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 trans-
mitting. Every time current passes through the
galvanometer, the light that is received from M,1
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 de-
veloped 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.
1 Nernst lamps arc the best to use, as they produce abundantly the
blue and violet rays which have the greatest chemical effect upon a
y>hotographic film. Carbon filament lamps are very poor in this respect.
44 WIRELESS PHOTOGRAPHY
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 record-
ing instrument. If the swing of the galvanometer
mirror is too great, a small battery with a regulat-
ing 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 direc-
tion 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 ^J^jth of a second. A hole is
bored through the poles, and one of them is fitted
RECEIVING APPARATUS
45
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,
M
FIG. 23.
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
Kg. 24.
The modified form of the Einthoven galvano-
meter, 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
WIRELESS PHOTOGRAPHY
direction. A small shutter of aluminium foil is
attached to the wires at the optical centre. The
silver wires used are 10100 inch in diameter, with
a natural period of about Ti^n °f a second ; the
length of wires free to swing being usually about 5 cm.
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 displace-
=S x
FlQ. 24.
ment. Where the current available, as in wireless
telegraphy, is very small, and a definite displace-
ment 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 i2ioo^n °f an ^h diameter, which, when
placed in a powerful field, will give a good dis-
placement with a current as small as 10 ~8
ampere.
RECEIVING APPARATUS 47
With the apparatus arranged by the Poulsen
Company, given in the diagram, Fig. 17, for photo-
graphically 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 ~8 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 sug-
gestion 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 WIRELESS PHOTOGRAPHY
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 tele-
phone 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
Tthyth of an inch. Greater movement of the dia-
phragm could be obtained by connecting a Tele-
phone relay to the detector, and using the magnified
current from the relay to operate the telephone.
RECEIVING APPARATUS
49
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
w
5=
w
B
FIG. 25.
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, milli-
ampere meter B, and the terminals T, for con-
necting to the galvanometer or telephone, all in
E
50 WIRELESS PHOTOGRAPHY
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 resist-
ance 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 cur-
rent 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 de-
tector 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
toats 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 care-
ful adjustment in order that the best results may
RECEIVING APPARATUS
51
be obtained. A greater range of magnification
can be obtained by placing two or more relays in
series.
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 communi-
cated to the aluminium
N
FIG. 26*
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 cen-
tralised. The coil is wound for a resistance of
about 200 ohms, and the diaphragm should be
fairly thin but very
H j{ jjei'-* resillient.
To the free end of
— D is fastened the
mirror T, made from
thin diaphragm glass about Ij centimetres dia-
meter, 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
FIG. 27.
52 WIRELESS PHOTOGRAPHY
concentrates it to a point upon a hole
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 ^j^TOtii of an
inch is obtained at the centre of the diaphragm
the mirror will move ^^th of an inch ; and as
the focal length of the mirror is 40 inches a move-
ment of ^jth 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 photo-
graphically receiving the picture that upon in-
vestigation may yield some results ; but it is doubt-
ful whether the current available, even that
obtained from a telephone relay, will be sufficient
to produce the desired magnetic efiect, and the
RECEIVING APPARATUS 63
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, Kg. 28, pass through a Nicol prism P
(polarising prism), then through a tube containing
CS2 (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
FIQ. 28.
by a current passing through the coil C, the re-
fractive index of the liquid will be changed,
and light from L will reach the photographic
film.1
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 Kg. 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
1 A description of the apparatus required will be found in Canot's
Physics.
54 WIRELESS PHOTOGRAPHY
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.
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 re-
TO Coil.
To
FIG. 29.
ceiving 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 re-
ception 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 rzlWi^h °* an ^^ ^n diameter,
and will respond to currents as small as 10 "8 of
RECEIVING APPARATUS 55
an ampere. The light from M throws an en-
larged 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 WIRELESS PHOTOGRAPHY
The paper used is soaked in a solution consisting
ol
Ferrocyanide of potassium
Ammoniac Nitrate
Distilled water 1
oz.
4 oz.
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
FIG. 30.
inclined to spread and so cause a blurred image.
The writer tried numerous specimens of paper
before one could be found that gave really satis-
factory results. It was also found that when
working in a warm room the paper became nearly
1 Groat care must be exercised in using this solution, as it is exceed-
ingly poisonous.
RECEIVING APPARATUS 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 con-
siderably, 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 Jth inch. The gauge
58
WIRELESS PHOTOGRAPHY
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 peculi-
arity on the part of the stylus can be given, as
there is nothing very corrosive in the solution used,
FIG. si.
and the pressure of the stylus upon the paper is
so slight as to be almost negligible.
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 J 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.
RECEIVING APPARATUS 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
^jih to Tthjth of 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 WIRELESS PHOTOGRAPHY
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 diffi-
culty 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 suffici-
ently 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
RECEIVING APPARATUS 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 WIEELESS PHOTOGRAPHY
but their effects will not be so serious in a photo-
graphic method of receiving as they would be in
the electrolytic system. In a photographic re-
ceiver where the film is, under normal conditions,
constantly illuminated, the received signals (both
the transmitted signals and the atmospheric dis-
turbances) 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 disturb-
ances can be eradicated, a print upon sensitised
paper having been first obtained to act as a guide
during the process.
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 photo-
graphy, and as previously mentioned, the syn-
63
64 WIRELESS PHOTOGRAPHY
chronising 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 publica-
tion.
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. Al-
though 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
SYNCHRONISING AND DRIVING 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 avail-
able. 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
66 WIRELESS PHOTOGRAPHY
no. 32.
SYNCHRONISING AND DRIVING 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 con-
nected 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 mechan-
ism runs, and vice versa.
68
WIRELESS PHOTOGRAPHY
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 inde-
pendently. The
independent
portion B con-
sists of an ordi-
nary 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
FIG. 34.
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,
SYNCHRONISING AND DRIVING 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,1 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 ex-
perimental 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 pre-
vented 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
1 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.
70 WIRELESS PHOTOGRAPHY
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 re-
ceived 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, com-
pleted 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,
SYNCHRONISING AND DRIVING 71
and some modification of these methods will
perhaps solve the problem, but it must be re-
membered 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 re-
volving 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 ytWth °f 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 with-
drawn 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 in-
serted between the driving motor and the machine.
72 WIRELESS PHOTOGRAPHY
Although a method of synchronising similar to
this may later on be devised for wireless photo-
graphy, the writer, from the result of his own ex-
periments, 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 photo-
graphy, the whole science being in a purely ex-
perimental 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 success-
fully, there is no reason to doubt that before long
SYNCHRONISING AND DRIVING 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 galvano-
meter 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 ap-
paratus— to take the place of the galvanometer—
that will be as sensitive in its action but more
robust in its construction.
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 re-
ceiving, 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 out-
come 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
74
THE " TELEPHOGRAPH ' 75
the two machines so that transmission and recep-
tion begin simultaneously ; (4) the correct regula-
tion 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 har-
monious 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 pre-
paring 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 trans-
mitted 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 WIRELESS PHOTOGRAPHY
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 syn-
chronising the two machines whereby transmitting
and receiving begin simultaneously ; (4) the use
of one machine only at each station.
TEANSMITTING 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
THE " TELEPHOGRAPH
77
the drum lias 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.
The drum measures 5 inches long by 2^ inches
1
,D
n S
J
X
n
n r
N
a
^ l \
f B r-S
, I
/
^~ N
To WniLtsi
A*
]
I
- 1
r TJ
K
d
1
c
1
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, D1, D2, Da, batteries : P, friction brake ;
B, Bi, double-pole two-way switches ; N, N1, N2, single switches ; W, key ; E,
electric clock ; J, telephones.
diameter, and this takes a metal print 5 inches by
7 inches, which allows for a lap of about J 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
WIRELESS PHOTOGRAPHY
the stylus and not towards it. A margin of bare
foil, about £ inch wide, should be left on the print
at the commencing edge, the purpose of which will
be explained later.
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
Fia. 36.
is provided with an adjusting screw W for regulat-
ing 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 60° 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
THE " TELEPHOGRAPH
79
FIO. 37.
Showing the arrangement for sliding the stylus to
or from the machine.
the drum and stylus. The complete stylus is given
in the drawings, Figs. 37, 37a, and also in the
diagrams Figs. 8 and 9.
The Relay. — As will be seen from the diagram,
Fig. 38, this con-
sists 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 flow-
ing 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 ter-
minals 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 resist-
ance R.
FIG. 37er.
80
WIRELESS PHOTOGRAPHY
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
AAAA/VW H
9 • • — • A
J p
1 — 1
M
V
. To T*A*S
I
FIO. 38.
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
THE " TELEPHOGRAPH " 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 con-
ditions 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.
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 arrange-
ment will be seen from Fig. 39. The whole of the
G
82
WIEELESS PHOTOGKAPHY
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 con-
tainers 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,
A while the leg F' clears
the surface of the
mercury by about J
inch, when the arma-
ture of the relay is
its normal posi-
FIG. 39.
H, H', containers ; M, mercury ; E, paraffin
oil ; T, T', terminals ; C, suspending rod ; D, in
base ; F, F, dipping rods. . ^
tion. To prevent
undue churning of the mercury, which would neces-
sarily 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 undis-
turbed, and the production of " sludge " reduced
to a minimum. To prevent oxidation of the mer-
cury, 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
THE " TELEPHOGRAPH " 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 D2 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 D2 should be about 12 volts.
RECEIVING APPAEATUS
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.
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 2j inches diameter, and
should be fairly thin but very resilient. Only one
84 WIEELESS PHOTOGEAPHY
coil is provided, and this should be wound with
No. 47 S.S.C. copper wire for a resistance of about
R D
FIO. 41.
FIQ. 40.
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 | 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 E,
which is of special
construction. A "R
piece of alu- »PU " < ~C
minium tube 3|
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
THE " TELEPHOGRAPH " 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. A y
When, however, L \J G ^
signals are sent out B
from the transmitting Jfmm.Jit^^l G,shutter;
apparatus, the mag- B> condensins fens^ focussing lens.
nified 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
WIRELESS PHOTOGRAPHY
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 photo-
graphs are required
for reproduction.
A t m ospherics
would be represented by irregular transparent
marks on the film after development, and these
can be easily eradicated by retouching.
The drum of the machine moves laterally -^th
of an inch per revolution, and the hole in the
screen is ^th 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
Fio. 43.
E, ebonite screen ; F, focussing lens ; Q, shutter
O, condensing lens ; L, Nernst lamp.
THE " TELEPHOGRAPH " 87
a mark of about the right thickness. With a
movement of the diaphragm of only ^o^th of
an inch, the actual movement of G will be ^W^h
of an inch. If the optical arrangements have a
magnifying power of 100, then the movement of
the shadow upon the screen will be -^ih 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 con-
tact with the screw H, by means of the spring Z
88
WIRELESS PHOTOGRAPHY
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
FIO. 44.
arm A to rise, and again make fresh contact with
the contact screw T.
DRIVING APPARATUS
The Friction Brake. — This consists of a steel
disc A, Fig. 45, about 2| inches diameter and f
inch or J 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
THE " TELEPHOGRAPH ': 89
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 work-
ing speed has been
I . * FIG. 45.
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 counter-
sunk 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.
The portion A consists of a gun-metal casting
90 WIRELESS PHOTOGRAPHY
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,
i\ V. .2
FlQ. 40.
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 bear-
ing ; C, insulating bush.
and after the bobbins R have been slipped on, the
shanks of the cores are passed through holes drilled
F B
B
—
s ... r
w
=
\\\\\\1 f
J
M l
Fio. 47.
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
THE " TELEPHOGRAPH " 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 in-
sulated 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. Connec-
tion with the circuit breaker L and the battery B2
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, T1, T2,
and the brushes R, R1, R2 are in electrical contact
with them. The ebonite plate J, 3J inches dia-
meter, 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 ^th of an inch wide, and the contacts P1
92
WIRELESS PHOTOGEAPHY
and P2 are placed on either side at a distance of
^ inch ; the contact strips P1, P2 carry the
brass pins D, which are about T\ inch diameter,
and spaced f inch apart. A connecting wire is
! & ii
Sfa E
A
VKW/flA
W
FIQ. 48.
N, brass tube ; 8, bushes ; G, ball-bearing ; H, gear-wheel : T, T1, T2, copper
rings ; C, insulating block ; E, E1, E2, brushes ; J, ebonite disc ; Q, contact block ;
D, metal pins ; O, pulley ; P. pi, P2, contact plates ; K, needle ; Z, spring ; W,
steel rod ; E, countersunk bearing.
carried from the contact P to the copper ring T,
another from P1 to T1, and one from P2 to T2.
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
THE " TELEPHOGRAPH " 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
| inch wide for portion of its length, but at the
point where it presses upon Q it is reduced in
width to ^jth 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 con-
nected by suitable gearing to the shaft of the
driving motor, so that the needle K makes one
revolution in about 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
WIRELESS PHOTOGRAPHY
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 connect-
ing wire is run from the steel
spindle W to one terminal each
of the lamps L, L1, L2, and
from the other terminal of the
lamps to one terminal of the
batteries J, the battery com-
prising a set of three 4 -volt
accumulators. The other
terminals of the batteries
joined one to each of
FlO. 49.
© ©
are
the brushes R, R1, R2.
The lamps are coloured, the lamp L being white,
and the lamps L1 and L2 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 de- no. 50.
•r j\ -i ji j M, terminals for connecting to
scribed), reduces tne speed electric clock; L, white iamP; LI.
e i-i J • • j_'i blue lamp ; L2, red lain]).
of the driving motor until
the needle K travels in unison with the disc J,
making permanent contact with P on the contact
THE " TELEPHOGRAPH " 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 P2, the
lamp L is extinguished and the red lamp L2 lights
up, and remains alight until the operator reduces
the speed. Similarly, too, if the needle travels
slower than J, contact is made with P1, and the
circuit of the blue lamp L1 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 ar-
ranged 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
WIRELESS PHOTOGRAPHY
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.
The switch D is then closed, and the arm of
FIO. 51.
M, clockwork motor ; S, isochroniser ; E, friction brake ; T, brushes ; F, electric
clutch ; X, gearing ; D, D1, switches ; A, flexible coupling ; K, polarised relay ; L,
circuit breaker ; BI, Ba, B3, batteries ; P, electric clock ; W, terminals for connec-
tion to telephone relay ; H, terminals for connection to terminals J, on trans-
mitting machine.
the switch D1 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
THE " TELEPHOGEAPH " 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 D1 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.
The current from the relay U, due to the trans-
mitting 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 practic-
ally 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.
93 WIEELESS PHOTOGRAPHY
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 cur-
rent which raised it to a white heat, and further-
more that the glowing filament emitted a brighter
light for a given amount of current than carbon
filaments.
Nernst lamps are made in two sizes, the larger
THE " TELEPHOGEAPH
99
being intended for the same work as usually done
by arc lamps, and the smaller to replace incan-
descent lamps ; the smaller type being made to
fit into the ordinary bayonet landholders. The
principal parts of a Nernst
lamp consist of the fila-
ment, the heater, the auto-
matic cut - out, and the
resistance, and their ar-
rangement 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
FIG. 52a.
100
WIRELESS PHOTOGRAPHY
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
cu
XX
Fio. 525.
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. 52b 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
THE " TELEPHOGRAPH
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 de-
stroy the filament.
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 cir-
cuit, 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
FIG. 52c.
102 WIRELESS PHOTOGRAPHY
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 excel-
lent for illumination, have very low actinic powers,
while others may have low illuminating but high
THE "TELEPHOGRAPH" 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 trans-
mission. 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 condi-
tions, and the amount of light admitted to the film
and the length of exposure are regulated accord-
ingly. No such latitude is, however, possible in
photo-telegraphy. With each set of apparatus
104 WIKELESS PHOTOGKAPHY
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 quanti-
ties, and the film that will give the most satis-
factory 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 troub-
ling 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
THE "TELEPHOGRAPH" 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 -f- £ = 170, and the plate is H. & D. 170.
The W.P. No. is found by dividing 50 by the
Inertia. Thus 50 -^ % = 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
x/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
WIRELESS PHOTOGRAPHY
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.
THE "TELEPHOGRAPH" 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 ap-
paratus, and with conditions similar to those
mentioned on page 75 the length of exposure will
vary roughly from l-50th to l-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 over-
exposure. With the high-speeded films the diffi-
culties of development are also greatly increased,
there being more latitude in both exposure and
development with the slower speeds, and conse-
quently 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 WIRELESS PHOTOGRAPHY
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.
APPENDIX A
SELENIUM CELLS
SELENIUM is a non- metallic element, and was first dis-
covered 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-con-
ductor 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
120° 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.
109
110
WIRELESS PHOTOGRAPHY
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 con
structed 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
? M
P
Al 2
' M
P'
M
' M
P
s [-[
B U
FlQ. 53a.
FIO. 53.
P, P', plates ; M, mica ;
6, selenium.
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.
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
APPENDIX A
111
/////////
X
x
X X
FIG. 54.
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 im-
portant part in the working of a cell, slightly opposing the
drop in resistance when illuminated, and opposing to a
much greater degree the
return to normal for no- t 3 6- 7
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 interfer-
ence 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
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
T/7716.
FIG. 55.
112
WIRELESS PHOTOGRAPHY
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
Zf,600
19.000
2.1,0 tO
,^-— •
.^•^
— -
-— -
•— *^"*
iy,eoo
^
^
\
x
17,000
1
^
/
\
/
lf,»oo
v
^
1
o
2
^
,
6
6
/
9
/
*,
/
t
/
5
S*« 6ipo3fcd---)k llTi-tx^osed ^
T/fH6 /H ^dCOTlds.
Fio. 55a.
instantly, many times in succession, the inertia effects are
very pronounced. The most successful method of counter-
acting the inertia is that adopted by Professor Korn of
always keeping the cell sufficiently illuminated to over-
come 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 return-
ing to its normal condition as soon as the illumination
ceases. The arrangement is given in the diagram Fig. 56.
Light from the transmitting or receiving apparatus, as
the case may be, falls upon the selenium cell S1, which is
APPENDIX A 113
placed on one arm of a Wheatstone bridge, a second cell
S2 being placed on the opposite arm. The selenium cell
S1 should have great sensitiveness and small inertia, the
compensating cell S2 having proportionally small sensitive-
ness 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 regulat-
ing resistance. When no light is falling upon the cell S1,
light from L is prevented from reaching the second cell S2
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 w
condition. When, however,
light falls upon the cell S1,
the balance of the bridge is
upset, and light from L falls
a fraction of a second later
upon the second cell S2, 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.
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 be-
tween 2 : 1 and 3:1, but Professor Korn has shown mathe-
matically 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 K = 250,000 ohms can be re-
duced 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 WIRELESS PHOTOGRAPHY
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 wave-
length 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 80° C. selenium is a non-conductor, but up to
210° C. the conductivity gradually increases, after which
it again diminishes*
APPENDIX B
PREPARING THE METAL PRINTS
ELECTRICIANS who desire to experiment in photo- tele-
graphy, 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 experi-
menter 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 photo-
graphic 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
115
116
WIRELESS PHOTOGRAPHY
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 ^\ th 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
DlAMETEB OF STOP USED ^TH OF CAMERA EXTENSION.
Screen ruling
lines per inch.
Actual space
in inches.
Distance of
screen ruling
in inches.
In A inches.
In
millimetres.
35
50
A
•91
•64
28-8
20-5
21-8
16-2
65
rii
•49
15-7
12-4
expensive, and are only made to order,1 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 be-
tween being T\th 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.
As it is impossible for many to have the use of profes-
sional apparatus designed for this particular kind of work,
1 Line screens can be obtained from Messrs. Fenrose, 109 Fani
Street, London ; or Messrs. Fallowfield, 140 Charing Cross Road, London.
APPENDIX B
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 experi-
mental work, but if this is not available, a large box camera
can be pressed into service.
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 draw-
ings Figs. 57 and 58 showing the method adopted by the
writer. The two clips
D, made from fairly
stout brass about J inch
wide, are bent to the
shape shown (an en-
larged 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
|J ths of an inch.
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
FIG. 57.
118
WIRELESS PHOTOGRAPHY
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
D camera is 8 inches, then
| the diameter of the stop
to give the best results
would be ^j-th of this, or
0 Jth 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 photo-
graph 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.
Fig. 59
gives the disposition of
the apparatus required for copying. A simple and in-
expensive copying stand is shown in Fig. 60. The black-
board 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
D
FIG. 58.
M. sheath ; P, Photographic plate ; D, cliP3 ; The diagram
APPENDIX B
119
centre of the board A. The camera, if of the box type,
can be secured to the stand by means of a screw and wing-
nut, the screw being passed from the inside as shown. The
p s
FIG. 59.
L, L, lamps ; A, board with picture ; S, line screen ; P, photographic plate.
beginner is advised to photograph only very bold and
simple subjects, such as black and white drawings or en-
largements. 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,
FIG. 60.
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 WIRELESS PHOTOGRAPHY
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 -fa 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
APPENDIX B 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 . . £ 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 WIRELESS PHOTOGRAPHY
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 trans-
parent. 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$ „
Carbonate of Potash . . 80 „ 6 „
Water .... 1000 c.c. 60 „
This developer should be used for 6 minutes at a tempera-
ture of 50° F., 3J minutes at 65°, and If minutes at 80°.
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
APPENDIX B 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 appear-
ance, 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
WIRELESS PHOTOGRAPHY
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 pre-
pared 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.
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
FIG. 63. case the camera should
be a fixture and the
board, A, arranged to slide backwards and forwards instead.
FIG. 01.
Portions of photographs (full size) of single line screen,
and single line print. Screen 40 lines to the inch.
'10. 62.
APPENDIX B 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 J x 4J inches up to
the full size of the board. By fastening the picture or photo-
graph 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, pro-
viding, 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
J-plate camera a 5-inch lens should be used, and for a
J-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 1J times enlarged line negative from
a 4J x 3 J 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=2J; and if a 6-inch lens is used, 2Jx6 = 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 J = 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 J-plate lens will be found to fully cover a
5x4 inch plate, providing the reduction is not greater
than three to one.
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 trans-
parent 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
126
APPENDIX C
127
H
P
£
FIG. 64.
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 NO, we
shall fiad that it is 1 J, or more
exactly 1-336, times greater
than the line EM. If we re-
peat 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 NO is called the sine of the angle of
incidence NDC, and EM the sine of the angle of refraction
MDE.
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
WIRELESS PHOTOGRAPHY
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 con-
verging 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 con-
struction 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.
Fio. 65.
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.
All lenses can be divided into two classes, convex or
converging, or concave or diverging. In the figure, 6, c, g
are converging lenses, being thicker at the middle than at
the borders, and d> e, /, 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.
APPENDIX C 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 direc-
tion 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
FIG. 66.
rays. In other words, objects viewed through a prism
always appear deflected towards its summit.
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.
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
WIRELESS PHOTOGRAPHY
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
Fio. 67.
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
APPENDIX C 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 A1 the rays will come
to a focus at the point a1, and similarly when the light is
at A2 the rays will come to a focus at the point a2. 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.
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 WIRELESS PHOTOGRAPHY
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 immedi-
ately opposite to it, therefore the smaller we make H, the
more distinct will be the image obtained. Reducing the
FIG. 70.
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 en-
larging the hole H gradually, the image will become more
and more indistinct until such a size is reached that it
disappears altogether.
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
APPENDIX C
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 corre-
sponding 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 refrac-
tion to the upper end of the image at n. The relative
Fio. 71.
positions of the object and image when placed at different
distances from the lens are exactly the same as the con-
jugate foci of light rays as shown in Fig. 69.
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 rsV^th part of 500 inches, or J inch.
134
WIRELESS PHOTOGRAPHY
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
eye V*^"
A
Fio 72.
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.
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
APPENDIX C 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 -r 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 WIRELESS PHOTOGRAPHY
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 -i- 1000, or 10 times. This means that by means
of the lens it has actually been magnified 10 times. This
ma'gnifying 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 -j- 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 i
by means of a convex lens will be readily followed from u
APPENDIX C
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 A1B1, and is really a con-
tinuation of the emergent rays. The magnifying power
of such a lens may be found by dividing 6 inches by the
A'
B'
Fio. 73.
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 J inch would magnify 24 times, and one
with a focal length of Twth 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 micro-
scope, 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.
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 WIRELESS PHOTOGRAPHY
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 prolonga-
tions of these divergent rays produce a virtual image,
erect and smaller than the object, at A1B1. The principal
focal distance of concave lenses is found by exactly the
same rule as that given for convex lenses.
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
12°. 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 splierical aberration, and in lenses that
APPENDIX C
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
L
FIG. 75.
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
L L'
Fia. 76.
FIG. 77.
inside will be blurred, the reverse being the case when the
cardboard is moved to the point B.
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 thick-
140 WIRELESS PHOTOGRAPHY
ness 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
liVtfths of its thickness. In a plano-convex lens with the
plane side turned towards parallel rays the aberration is
4J times its thickness, but with the convex side turned
towards parallel rays the aberration is only l^VVths 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, L1,
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.
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, 1
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 -tele-
graphy, 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
141
142
WIRELESS PHOTOGRAPHY
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, 63,
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
. ring selenium, 109
photographs for transmitting,
115
sketches on metal foil, 124
. 128
action of, 129
-ss plates, !
Professor Nernflt, 98
INDEX
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
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