THE "TELEPHOGRAPH" 74
Author's System of
Radio-Photography--Requirements--Advantages--Transmitting
machine--Description of Differential Relay--Wireless Receiving
Apparatus--Photo-Telegraphic Receiving Apparatus--Circuit
Breaker--Friction Brake--Magnetic Clutch--Description of
Isochroniser--Method of working--Types of Nernst Lamp--Action of Nernst
Lamp--Comparison of Actinic Value--Inertia of Photographic
Films--Choosing Films--Speed of Films--Standard of Speed--Comparative
Film Speeds--Effects of Minimum Exposure--Effects of Maximum
Exposure--Considerations in working and choosing Films.
APPENDIX A
SELENIUM CELLS 109
Nature of Selenium--Preparation of Selenium--Forms of Selenium
Cells--Action of Selenium Cells--Characteristics of Selenium
Cells--Effects of Inertia in Photo-Telegraphy--Methods of counteracting
Inertia--Sensitiveness of Selenium to Light--Effect of Heat on
Selenium.
APPENDIX B
PREPARING THE METAL PRINTS 115
Outline of Process--Line Screens--Choice of Camera--Fixing Line Screen
in Camera--Lenses and Stops--Taking the Photograph--Copying
Stands--Choice of Photographic Plates--Sources of Illumination--Metal
Prints--Coating the {xi} Metal Sheets--Sensitising Solution--Printing
Operations--Developing--Intensifying--Precautions to be observed in
working--Preparing Sketches on Metal--Apparatus for Reducing or
Enlarging--Improvements to Copying Board--Lenses for Copying--Formula
for Copying.
APPENDIX C
LENSES 126
Action of Light--Law of Refraction--Lenses--Prisms--Action of
Lenses--Focal Length of Lenses--Formation of Images--Apparent Magnitude
of Objects--Real and Virtual Images--Formation of Virtual Images--Power
of Magnification--Defects of Lenses--Aberration.
* * * * *
{xiii}
ILLUSTRATIONS
FIG. PAGE
1. Diagram showing effects of capacity on an intermittent current 5
2. Bernochi's wireless apparatus 7
3. Knudsen's wireless apparatus 10
4. Wireless transmitting station 13
5. Diagram of experiment illustrating principle of line photograph 16
6. Drawing of transmitting machine 17
7. Drawing of transmitting machine 18
8. Drawing of stylus 18
9. Electrical connections of machine 19
10. Photograph of Author's experimental machine _Frontispiece_
10a. End view of Author's experimental machine }
} _facing page_ 21
10b. View of image broken up by a "cross" screen}
11. Connections of complete transmitting apparatus 23
12. Drawing of ordinary type of spark-gap 27
13. Synchronous rotating spark-gap 28
14. Non-synchronous rotating spark-gap 28
15. Connections for complete wireless photographic station 30
16. Connections of Professor Korn's apparatus 31
17. Connections of Poulsen's photographic recorder 33
18. Author's photographic receiver 38
19. Enlarged drawing of cone 39
20. End view of Author's photographic receiver 39
21. Connections of decohering apparatus 41
22. Connections for complete photographic receiver 42
{xiv}
23. Arrangement of Einthoven galvanometer 45
24. Einthoven galvanometer arranged for receiving 46
25. Connection of telephone relay 49
26. Drawing of Author's improved photographic receiver 51
27. Diagram giving ratio of vibrating arm 51
28. Arrangement of polarisation receiver 53
29. Arrangement of kathode-ray receiver 54
30. Connections of electrolytic receiver 56
31. Drawing of improved stylus for receiving 58
32. Drawing of Hughes telegraph governor 66
33. Arrangement of simple speed regulator 68
34. Diagram of connections of simple speed regulator 68
35. Author's arrangement for complete radio-photographic station 77
36. Drawing of transmitting machine and circuit breaker 78
37. Drawing of special transmitting stylus showing adjusting
arrangements 79
37a. End view of transmitting stylus 79
38. Connections of new type of relay designed by the Author 80
39. Arrangement of mercury containers and dipping rods for relay 82
40. Drawing of Author's receiver 84
41. Enlarged drawing of diaphragm and steel point 84
41a. Drawing showing arrangement of bush and counter-weight 84
42. Optical arrangements of receiver 85
43. Optical arrangements of receiver 86
44. Drawing of circuit breaker 88
45. Drawing of friction brake 89
46. Sectional drawing of magnetic clutch 90
47. Plan of magnetic clutch 90
48. Details of Isochroniser 92
49. Connections of Isochroniser 94
50. Dial of Isochroniser 94
51. Diagram of driving mechanism 96
{xv}
52. Diagram showing starting positions of machines 97
52a. Arrangement of small type Nernst lamp 99
52b. Ballasting resistances for Nernst lamps 100
52c. Arrangement of large type Nernst lamp 101
53. Connections of selenium cell elements 110
53a. Form of selenium cell used by Bell and Tainter 110
54. Diagram showing construction of modern cell 111
55. Resistance curve of selenium cell 111
55a. Actual curve of selenium cell 112
56. Diagram of Professor Korn's method for counteracting inertia 113
57. Arrangement of plate sheath and line screen 117
58. Details of clips to hold line screen 118
59. Arrangement of apparatus for copying 119
60. Drawing showing method of arranging camera and copying stand for
adjustment 119
61. Photograph of line screen and metal print }
} _facing page_ 124
62. Photograph of sketch drawn upon metal foil }
63. Method of marking out copying board 124
64. Diagram illustrating law of refraction 127
65. Forms of lenses 128
66. Action of light passed through a prism 129
67. Diagram illustrating action of a lens 130
68. Formation of principal focus of a lens 130
69. Formation of conjugate foci of a lens 131
70. Apparatus illustrating principle of camera 132
71. Formation of an image by a lens 133
72. Diagram illustrating apparent magnitude 134
73. Formation of virtual image by a convex lens 137
74. Formation of virtual image by a concave lens 138
75. Diagram showing spherical aberration 139
76. Combination of plano-convex lenses 139
77. Combination of meniscus and convex lenses 139
* * * * *
{1}
RADIO-PHOTOGRAPHY
CHAPTER I
INTRODUCTORY
Those who desire to experiment on radio-photography, _i.e._ transmitting
photographs, drawings, etc., from one place to another without the aid of
artificial conductors, must cultivate at least an elementary knowledge of
optics, chemistry, mechanics, and electricity; photo-telegraphy calling for
a knowledge of all these sciences. There are, no doubt, many wireless
workers who are interested in this subject, but who are deterred from
experimenting owing to a lack of knowledge regarding the direction
developments are taking, besides which, information on this subject is very
difficult to obtain, the science of photo-telegraphy being, at the present
time, in a purely experimental stage.
The wireless transmission of photographs has, no doubt, a great commercial
value, but for any system to be commercially practicable, it must be
simple, rapid, and reliable, besides being able to work {2} in conjunction
with the apparatus already installed for the purpose of ordinary wireless
telegraphy.
As far back as 1847 experiments were carried out with a view to solving the
problem of transmitting pictures and writing by electrical methods over
artificial conductors, but no great incentive was held forth for
development owing to lack of possible application; but owing to the great
public demand for illustrated newspapers that has recently sprung into
being, a large field has been opened up. During the last ten years,
however, development has been very rapid, and some excellent results are
now being obtained over a considerable length of line.
The wireless transmission of photographs is, on the other hand, of quite
recent growth, the first practicable attempt being made by Mr. Hans Knudsen
in 1908. It may seem rather premature to talk about the wireless
transmission at a time when the systems for transmitting over ordinary
conductors are not perfectly developed, but everything points to the fact
that for long-distance transmission a reliable wireless system will prove
to be both cheaper and quicker than transmission over ordinary land lines
and cables.
The effects of capacity and inductance--properties inherent to all
telegraph systems using metallic conductors--have a distinct bearing upon
the two questions, how far and how quickly can {3} photographs be
transmitted? Owing to the small currents received and to prevent
interference from earth currents it is necessary to use a complete metallic
circuit. If an overhead line could be employed no difficulty would be
experienced in working a distance of over 1000 miles, but a line of this
length is impossible--at least in this country--and if transmission is
attempted with any other country, a certain amount of submarine cable is
essential. It has been found that the electrostatic capacity of one mile of
submarine cable is equal to the capacity of 20 miles of overhead line, and
as the effect of capacity is to retard the current and reduce the speed of
working, it is evident that where there is any great length of cable in the
circuit the distance of possible transmission is enormously reduced.
If we take for an example the London-Paris telephone line with a length of
311 miles and a capacity of 10.62 microfarads, we find that about half this
capacity, or 5.9 microfarads,[1] is contributed by the 23 miles of cable
connecting England with France.
In practice the reduction of speed due to capacity has, to a great extent,
been overcome by means of apparatus known as a line-balancer, which hastens
the slow discharge of the line and {4} allows each current sent out from
the transmitter--the current in several systems being intermittent--to be
recorded separately on the receiver. Photographs suitable for press work
can now be sent over a line which includes only a short length of cable for
a distance of quite 400 miles in about ten minutes, the time, of course,
depending upon the size of the photograph. In extending the working to
other countries where there is need for a great length of cable, as between
England and Ireland, or America, the retardation due to capacity is very
great. On a cable joining this country with America the current is retarded
four-tenths of a second. In submarine telegraphy use is made of only one
cable with an earth return, but special means have had to be adopted to
overcome interference from earth currents, as the enormous cost prohibits
the laying of a second cable to provide a complete metallic circuit. The
current available at the cable ends for receiving is very small, being only
1/200000th part of an ampere, and this necessitates the use of apparatus of
a very sensitive character. One system of photo-telegraphy in use at the
present time, employs what is known as an electrolytic receiver (see
Chapter III.) which can record signals over a length of line in which the
capacity effects are very slight, with the marvellous speed of 12,000 a
minute, but this speed rapidly decreases with an increase of distance
between the {5} [Illustration] two stations. The effect of capacity upon an
intermittent current is clearly shown in Fig. 1. If we were to send twenty
brief currents in rapid succession over a line of moderate capacity in a
given time, we should find that instead of being recorded separately and
distinctly as at _a_, each mark would be pointed at both ends and joined
together as shown at _b_, while only perhaps fifteen could be recorded. If
the capacity be still farther increased as at _c_, only perhaps half the
original number of currents could be recorded in the same time, owing to
the fact that with an increase of resistance, capacity, and inductance of
the line a longer time is required for it to charge up and discharge,
thereby materially lessening the rate at which it will allow separate
signals to pass; the number of signals that can therefore be recorded in a
given time is greatly diminished. If we were to attempt to send the same
number of signals over a line of great capacity, as could be sent, and
recorded separately and distinctly over a line of small capacity--the time
limit being of course the same in both instances--we should find that the
{6} signals would be recorded practically as a continuous line. The two
latter cases _b_, and _c_, Fig. 1, clearly shows the retardation that takes
place at the commencement of a current and the prolongation that takes
place at the finish. If the photo-telegraphic system previously mentioned
could be rendered sensitive enough to work on the Atlantic cables, we
should find that only about 1200 signals a minute could be recorded, and
this would mean that a photograph which could be transmitted over ordinary
land lines in about ten minutes would take at least fifty minutes over the
cable. This would be both costly and impracticable, and time alone will
show whether, for long-distance work, transmission by wireless will be both
cheaper and more rapid than any other method. At present wireless
telegraphy has not superseded the ordinary methods of communicating over
land, but there can be no doubt that wireless telegraphy, if free from
Government restrictions, would in certain circumstances very quickly
supersede land-line telegraphy, while it has proved a formidable commercial
competitor to the cable as a means of connecting this country with America.
Likewise we cannot say that no system of radio-photography will ever come
into general use, but where there is any great distance to be bridged,
especially over water, wireless transmission is really the only practical
solution. From the {7} foregoing remarks, it is evident that a reliable
system of radio-photography would secure a great victory in the matter of
time and cost alone, besides which, the photo-telegraphic apparatus would
be merely an accessory to the already existing wireless installation.
[Illustration: FIG. 2.]
There have been numerous suggestions put forward for the wireless
transmission of photographs, but they are all more or less impracticable.
One of the earliest systems was devised by de' Bernochi of Turin, but his
system can only be regarded interesting from an historical point of view,
and as in all probability it could only have been made to work over a
distance of a few hundred yards it is of no practical value. Fig. 2 will
help to explain the apparatus. A glass cylinder A' is fastened at one end
to a threaded steel shaft, which runs in two bearings, one bearing having
an internal thread corresponding with that on the {8} shaft. Round the
cylinder is wrapped a transparent film upon which a photograph has been
taken and developed. Light from a powerful electric lamp L, is focussed by
means of the lens, N, to a point upon the photographic film. As the
cylinder is revolved by means of a suitable motor, it travels upwards
simultaneously by reason of the threaded shaft and bearing, so that the
spot of light traces a complete spiral over the surface of the film. The
light, on passing through the film (the transmission of which varies in
intensity according to the density of that portion of the photograph
through which it is passing), is refracted by the prism P on to the
selenium cell S which is in series with a battery B and the primary X of a
form of induction coil. As light of different intensities falls upon the
selenium cell,[2] the resistance of which alters in proportion, current is
induced in the secondary Y of the coil and influences the light of an arc
lamp of whose circuit it is shunted. This arc lamp T is placed at the focus
of a parabolic reflector R, from which the light is reflected in a parallel
beam to the receiving station.
The receiver consists of a similar reflector R' with a selenium cell E
placed at its focus, whose resistance is altered by the varying light
falling upon it from the reflector R. The selenium cell {9} E is in series
with a battery F and the mirror galvanometer H. Light falls from a lamp D
and is reflected by the mirror of the galvanometer on to a graduated
aperture J and focussed by means of the aplanatic lens U upon the receiving
drum A^2, which carries a sensitised photographic film. The two cylinders
must be revolved synchronously. The above apparatus is very clever, but
cannot be made to work over a distance of more than 200 yards.
A system based on more practical lines was that invented and demonstrated
by Mr. Hans Knudsen, but the apparatus which he employed for receiving has
been discarded in wireless work, as it is not suitable for working with the
highly-tuned systems in use at the present time.
Knudsen's transmitter, a diagrammatic representation of which is given in
Fig. 3, consists of a flat table to which a horizontal to-and-fro motion is
given by means of a clockwork motor. Upon this table is fastened a
photographic plate which has been prepared in the following manner. The
plate upon which the photograph is to be taken has the gelatine film from
three to four times thicker than that commonly used in photography. In the
camera, between the lens and this plate, a single line screen is
interposed, which has the effect of breaking the picture up into parallel
lines. Upon the plate being developed and before it is {10} [Illustration]
completely dry, it is sprinkled over with fine iron dust. With this type of
plate the transparent parts dry much quicker than the shaded or dark parts,
and on the iron dust being sprinkled over the plate it adheres to the
darker portions of the film to a greater extent than it does to the lighter
portions; a picture partly composed of iron dust is thus obtained. A steel
point attached to a flat spring rests upon this plate and is made to travel
at right angles to the motion of the table. As the picture is partly
composed of iron dust, and as the steel needle is fastened to a delicate
spring it is evident that as the plate passes to and fro under the needle,
both the spring and needle are set in a state of vibration. This vibrating
spring makes {11} and breaks the battery circuit of a spark coil, which in
turn sets up sparking in the spark-gap of the wireless apparatus.
The receiver consists of a similar table to that used for transmitting, and
carries a glass plate that has been smoked upon one side. A similar spring
and needle is placed over this plate, but is actuated by means of a small
electro-magnet in circuit with a battery and a sensitive coherer. As the
coherer makes and breaks the battery circuit by means of the intermittent
waves sent out from the transmitting aerial, the needle is made to vibrate
upon the smoked glass plate in unison with the needle at the transmitting
end. Scratches are made upon the smoked plate, and these reproduce the
picture on the original plate. A print can be taken from this scratched
plate in a similar manner to an ordinary photographic negative.
The two tables are synchronised in the following manner. Every time the
transmitting table is about to start its forward stroke a powerful spark is
produced at the spark-gap. The waves set up by this spark operate an
ordinary metal filings coherer at the receiving end which completes the
circuit of an electro-magnet. The armature of this magnet on being
attracted immediately releases the motor used for driving, allowing it to
operate the table. The time taken to transmit a photograph, quarter-plate
size, is about fifteen minutes. {12} Although very ingenious this system
would not be practicable, as besides speed the quality of the received
pictures is a great factor, especially where they are required for
reproduction purposes. The results from the above apparatus are said to be
very crude, as with the method used to prepare the photographs no very
small detail could be transmitted.
* * * * *
{13}
CHAPTER II
TRANSMITTING APPARATUS
Let us now consider the requirements necessary for transmitting photographs
by means of the wireless apparatus in use at the present time.
[Illustration: FIG. 4.]
The connections for an experimental syntonic wireless transmitting station
are shown in the diagram Fig. 4. A is the aerial; T, the inductance; E,
earth; L, hot-wire ammeter. The closed oscillatory circuit consists of an
inductance F, spark-gap G, and a block condenser C. H is a spark-coil for
supplying the energy, the secondary J being connected to the spark-gap. A
{14} mercury break N and a battery B are placed in the primary circuit of
the coil. The Morse key K is for completing the battery circuit for
signalling purposes. When the key K is depressed, the battery circuit is
completed, and a spark passes between the balls of the spark-gap G
producing oscillations in the closed circuit, which are transposed to the
aerial circuit by induction. For signalling purposes it is only necessary
for the operator by means of the key K to send out a long or short train of
waves in some pre-arranged order, to enable the operator at the receiving
station to understand the message that is being transmitted.
If a photograph could be prepared in such a manner that it would serve the
purpose of the key K, and could so arrange matters that a minute portion of
the photograph could be transmitted separately but in succession, and that
each portion of the photograph having the same density could be given the
same signal, then it would only be necessary to have apparatus at the
receiving station capable of arranging the signals in proper sequence (each
signal recorded being the same size and having the same density as the
transmitted portion of the photograph) in order to receive a facsimile of
the picture transmitted.
The following method of preparing the photograph[3] is one that has been
adopted in several {15} systems of photo-telegraphy, and is the only one at
all suitable for wireless transmission. The photograph or picture which is
to be transmitted is fastened out perfectly flat upon a copying-board. A
strong light is placed on either side of this copying board, and is
concentrated upon the picture by means of reflectors. The camera which is
used for copying has a single line screen interposed between the lens and
sensitised plate, and the effect of this screen is to break the picture up
into parallel lines. Thus a white portion of the photograph would consist
of very narrow lines wide apart, while the dark portion would be made up of
wide lines close together; a black part would appear solid and show no
lines at all. From this line negative it will be necessary to take off a
print upon a specially prepared sheet of metal. This consists of a sheet of
thick lead- or tinfoil, coated upon one side with a thin film of glue to
which bichromate of potash has been added; the bichromate possessing the
property of rendering the glue waterproof when acted upon by light. The
print can be taken off by artificial light (arc lamps being generally
used), but the exact time to allow for printing can only be found by
experiment, as it varies considerably according to the thickness of the
film. The printing finished, the metal print is washed under running water,
when all those parts not acted upon by light, _i.e._ the parts between the
lines, are {16} washed away, leaving the bare metal. We have now an image
composed of numerous bands of insulating material (each band varying in
width according to the density of the photograph at any point from which it
is prepared) attached to a metal base, so that each band of insulating
material is separated by a band of conducting material. It is, of course,
obvious that the lines on the print cannot be wider apart, centre to
centre, than the lines of the screen used in preparing it. A good screen to
use is one having 50 lines to the inch, but one is perhaps more suitable
for experimental work a little coarser, say 35 lines to the inch. To use a
screen having 50 or more lines to the inch, the transmitting apparatus, as
will be evident later on, will require to be very nearly perfect.
[Illustration: FIG. 5.]
Before proceeding further it will perhaps be as well to make an experiment.
If we take one of the metal prints or, more simple, draw a sketch in
insulating ink upon a sheet of metal A, Fig. 5, and connect a battery B and
the galvanometer D as shown, we shall find on drawing the free end of the
wire across the metal plate that all the time the wire is in contact with
the lines of insulating material the needle of the galvanometer will remain
{17} at zero, but where it is in contact with the metal plate the needle is
deflected.
From this experiment it will be seen that we have in our metal line print,
which consists of alternate lines of insulating and conducting material, a
method by which an electric circuit can be very easily made and broken. It
is, of course, necessary to have some arrangement whereby the whole of the
surface of the metal print is utilised for this purpose to the best
advantage. One type of transmitting machine used for this purpose is
represented by the diagram, Fig. 6. The cylinder A is fastened to the steel
shaft B, which runs in the two bearings D and D', the bearing D' having an
internal thread corresponding to that on the shaft. The stylus in this
class of machine is a fixture, the cylinder being given a lateral as well
as a revolving movement. As it is impossible to use a rigid drive, a
flexible coupling F is employed between the shaft B and the motor.
[Illustration: FIG. 6.]
Another type of machine is shown in Fig. 7. The drum in this case is
stationary, the table T moving laterally by reason of the screwed shaft
{18} [Illustration] and half nut F. The table, shown separate in Fig. 8,
carries a stiff brass spring A, to which is attached a holder B made to
take a hardened steel point. The holder is provided with a set screw P for
securing the steel point Z. The spring and needle are insulated from the
rest of the machine, as shown in the drawing. In working, the metal print
is wrapped tightly round the cylinder of the machine, the glue image being,
of course, uppermost. To fasten the print a little seccotine should be
applied to one edge, and the joint carefully smoothed down with the
fingers. [Illustration] If there is any tendency on the part of the print
to slip round on the drum, a couple of small spring clips placed over the
ends of the drum will act as a preventive. It is necessary to place the
print upon the drum in such a manner that the stylus draws away from the
edge of the lap and not towards it, and the metal prints should be of such
a size that when placed round the drum of the {19} machine a lap of about
3/16ths of an inch is allowed.
[Illustration: FIG. 9.]
The steel point Z (ordinary gramophone needles may be used and will be
found to answer the purpose admirably) is made to press lightly upon the
metal print, and while the pressure should be sufficient to make good
electrical contact, it should not be sufficient to cause the needle to
scratch the surface of the foil. The pressure is regulated by means of the
milled nut H. The electrical connections are given in Fig. 9. One wire from
the battery M is taken to the terminal T, and the other wires from M and F
lead to the relay R. The current flows from the battery M through the
spring Y, through the drum and metal print, the stylus Z, spring A, down to
the relay R, and from R back to the battery M. As the drum carrying the
single line half-tone print is revolved, the stylus, by reason of the
lateral movement given to the table or cylinder as the case may be, will
trace a spiral path over the entire surface of the print. As the stylus
traces over a conducting strip the circuit is completed, and the tongue of
the relay R is attracted, making contact with the stop S. {20} On passing
over a strip of insulation the circuit is broken and the tongue of the
relay R returns to its normal position.
As already stated, the conducting and insulating bands on the print vary in
width according to the density of the photograph from which it is prepared,
so that the length of time that the tongue of the relay R is held against
the stop S, is in proportion to the width of the conducting strip which is
passing under the stylus at any instant. The function of the transmitter is
therefore to send to the relay R an intermittent current of varying
duration.
The two photographs Figs. 10 and 10_a_ are of a machine designed and used
by the writer in his experiments. In this machine the drum is 3.5 inches
long and 1.5 inches in diameter. The lead screw has 30 threads to the inch,
and the reduction between it and the drum is 3:1, so that the table has a
movement of 1/90th inch per revolution of the drum.
From the brief description of the various types of machines that have been
given it will be apparent that in the design of the machine proper there is
nothing very complicated, although the addition of the driving and
synchronising apparatus complicates matters rather considerably. The
questions of driving and synchronising the machines at the two stations is
fully dealt with in Chapter IV.
[Illustration: FIG. 10a.]
[Illustration: FIG. 10b. Enlarged view of an image broken up by a cross
screen.]
{21} Although the design of the machines is rather simple great attention
must be paid both to accuracy of construction and accuracy of working, and
this applies, not only to the machines (whether for transmitting or
receiving) but for all the various pieces of apparatus that are used. Too
much care cannot be bestowed upon this point, as in the wireless
transmission of photographs there is a large number of instruments all
requiring careful adjustment, and which have to work together in perfect
unison at a high speed.
The machine shown in Figs. 10 and 10_a_ was designed and used by the writer
solely for experimental work. It will be noticed in the description given
in the appendix of the method of preparing the metal prints that a 5" x 4"
camera is recommended, while the machine, Fig. 10, is designed to take a
print procured from a quarter-plate negative. This size of drum was adopted
for several reasons, and although it will be found quite large enough for
general experimental work the writer has come to the conclusion that for
practical commercial work a drum to take a print 5" x 4" will give better
results.
In making a negative of a picture that is required for reproduction
purposes, the line screen in the camera is replaced by a "cross screen,"
_i.e._ two single line screens placed with their lines at an angle of 90deg
to one another, and this breaks the {22} image up into small squares
instead of lines. By looking at any ordinary newspaper or book illustration
through a powerful magnifying glass the effects of a cross screen will
readily be seen. With a cross screen a certain amount of detail is
necessarily lost, but with a single line screen the amount lost is much
greater. If there is any very small detail in the picture most of this
would be lost in a coarse screen, hence the necessity of employing as fine
a line screen as practicable in order to get as much detail in as possible.
It is mainly on this account that a 5" x 4" print is recommended, as, if
fairly bold subjects are used for copying, the small detail (this is, of
course, a very vague and indefinable term) will not be too fine, and the
time required for transmitting reasonable. For obvious reasons it is a
great advantage to put the print under pressure to cause the glue image to
sink into the soft metal base and leave a perfectly flat and smooth
surface. It is essential that the bands on the print lie along the axis of
the cylinder, so that the stylus traces its path across them, and not with
them.
We have now an arrangement that is capable of taking the place of the key
K, Fig. 4, and the diagram, Fig. 11, gives the connections for the complete
transmitter. A is the aerial, E earth, T inductance, L ammeter. The closed
oscillatory circuit consists of a spark-gap G, inductance F, {23}
[Illustration] and a condenser C. The secondary J of the coil H is
connected to the spark-gap, and the primary P is in circuit with the
mercury break N, the battery B, and the local contacts of the relay R. The
action is as follows. When contact is made between the stylus Z and the
drum V by means of the conducting bands on the line print, the circuit of
the relay R and the battery M is completed. The closing of the local
circuit of the relay R actuates the second relay R', allowing the primary
circuit of the coil H to be closed. As soon as the primary circuit of the
coil is completed sparks pass between the electrodes of the spark-gap G,
causing waves to radiate from the aerial. The duration of the wave-trains
radiated depends upon the duration of contact made by the relays {24} R and
R', and this in turn depends upon the width of the conducting strip that is
passing under the stylus. The battery M should be about 4 volts, and the
battery D about 2 volts. The two-way switch X is connected up so that the
relay R' can be thrown out and the key K switched in for ordinary
signalling purposes. If any sparking takes place at the point of the
stylus, a small condenser C' (about 1 microfarad capacity) should be
connected as shown. In the present instance the condenser should be used
more as a preventive than as a cure, as in all probability the voltage from
M will not be sufficient to cause destructive (if any) sparking; but, as
most wireless workers know, anything in the nature of a spark occurring in
the neighbourhood of a detector (this, of course, only applies when the
receiving apparatus is placed in close proximity to the transmitter) is
liable to destroy the adjustment.
In transmitting over ordinary conductors where the initial voltage is
fairly high and the self-induction of the circuit very great, the use of
the condenser will be found to be absolutely essential. It has also been
noted that the angle which the stylus presents to the drum has a marked
effect upon the sparking, an angle of about 60deg being found to give very
good results.
If the size of the single line print used is 5 inches by 4 inches, and a
screen having 50 lines {25} to the inch is used for preparing it, then the
stylus will have to make 250 contacts during one revolution of the drum.
Assuming the drum to make one revolution in three seconds, then the time
taken to transmit the complete photograph can be found from the equation T
= w x t x s, where w is the width of the print, t the travel of the stylus
during one revolution of the drum, and s the time required for one
revolution of the drum. In the present instance this will be T = 4 x 90 x 3
= 1080 seconds = 18 minutes. The number of contacts made by the stylus per
minute is 5000, and in working at this speed the first difficulty is
encountered in the use of the two relays. The relay R is lightly built, and
capable of working at a fairly high speed, but R' is a heavier pattern, and
consequently works at a slightly lower rate. This relay must necessarily be
heavier, as more substantial contacts are needed in order to pass the heavy
current taken by the spark-coil.
Relays sensitive and accurate enough to work at this speed will in all
probability be beyond the reach of the majority of workers, but there are
several types of relays on the market very reasonable in price that will
answer very well for experimental work, although the speed of working will
no doubt be slower.
For the best results the duration of the wave-trains sent out should be of
the same duration as {26} the contact made by R, and therefore equal to the
time taken by the stylus to trace over a conducting strip; but if the
duration of the contact made by R is t, then that made by R' and
consequently the duration of the groups of wave-trains would be t - v where
v equals the extra time required by R' to complete its local circuit. The
difference in time made by the two relays, although very slight, will be
found to affect very considerably the quality of the received pictures.
Renewing the platinum contacts is also a great expense, as they are soon
burnt out where a heavy current is passed. If the distance experimented
over is short so that the power required to operate the spark-coil is not
very heavy, one relay will be sufficient providing the contacts are massive
enough to carry the current safely. It is useless to expect any of the
ordinary relays in general use to work satisfactorily at such a high speed,
and in order to compensate for this we must either increase the time of
transmitting, or, as already suggested, make use of a coarser line screen
in preparing the photographs.
For reasons already explained, all points of make and break should be
shunted by a condenser. The effective working speed of an ordinary type of
relay may be anything from 1000 to 2500 dots a minute, depending upon
accuracy of design and construction.
In the wireless transmission of photographs it {27} is absolutely essential
to use some form of rotary spark-gap, as where sparks are passed in rapid
succession the ordinary type of gap is worse than useless. When a spark
passes between the electrodes of an ordinary spark-gap, Fig. 12, we find
that for a fraction of a second after the first spark has passed, the
normally high resistance of the gap has been lowered to less than one ohm.
If the column of hot gas which constitutes the spark is not instantly
dispersed, but remains between the electrodes, it will provide an easy path
for any further discharges, and if sparks are passed at all rapidly, what
was at first a disruptive and oscillatory discharge will degenerate into a
hot, non-oscillatory arc.[4]
[Illustration: FIG. 12.]
Two forms of rotating spark-gaps are shown in Figs. 13 and 14, and are
known as "synchronous" and "non-synchronous" gaps respectively. In the
synchronous gap the cog-wheel is mounted on the shaft of the alternator,
and a cog comes opposite the fixed electrode when the maximum of potential
is reached in the condenser, thus ensuring a discharge at every alternation
of current. With this type of gap a spark of pure tone is obtained which
{28} [Illustration] [Illustration] is of great value where the signals are
received by means of a telephone, but where the signals are to be
mechanically recorded the tone of the spark is of little consequence. In a
non-synchronous gap a separate motor is used for driving the toothed wheel,
and can either be mounted on the motor shaft or driven by means of a band,
there being no regard given to synchronism with the alternator. The fixed
electrode is best made long enough to cover about two of the teeth, as this
ensures regular sparking and a uniform sparking distance; the {29} spark
length is double the length of the spark-gap. The toothed wheel should
revolve at a high speed, anything from 5000 to 8000 revolutions per minute,
or even more being required. The shaft of the toothed wheel is preferably
mounted in ball-bearings.
Owing to the large number of sparks that are required per minute in order
to transmit a photograph at even an ordinary speed, it is necessary that
the contact breaker be capable of working at a very high speed indeed. The
best break to use is what is known as a "mercury jet" interrupter, the
frequency of the interruptions being in some cases as high as 70,000 per
second. No description of these breaks will be given, as the working of
them is generally well understood.
In some cases an alternator is used in place of the battery B, Fig. 4, and
when this is done the break M can be dispensed with. In larger stations the
coil H is replaced with a special transformer.
The writer has designed an improved relay which will respond to currents
lasting only 1/100th part of a second, and capable of dealing with rather
large currents in the local circuit.[5] This relay has not yet been tried,
but if it is successful the two relays R and R' can be dispensed with, and
the result will be more accurate and effective transmission.
{30}
[Illustration: FIG. 15.]
The connections for a complete experimental station, transmitting and
receiving apparatus combined, are given in Fig. 15. The terminals W, W are
for connecting to the photo-telegraphic receiving apparatus Q, being a
double pole two-way switch for throwing either the transmitting or
receiving apparatus in circuit. There is another system of transmitting
devised by Professor Korn, which employs an entirely different method from
the foregoing. By using the apparatus just described, the waves generated
are what are known as "damped waves," and by using these damped waves,
tuning, which is so essential to good commercial working, can be made to
reach a fairly high degree of efficiency. {31}
The question of damped _versus_ undamped waves is a somewhat burning one,
and no attempt will be made here to deal with the merits or demerits of the
claims made for the respective systems. A series of articles describing the
production of undamped waves and their efficiency in working compared with
damped waves will be found in the _Wireless World_, Nos. 3 and 4, 1913, and
are well worth reading by any one interested in the subject.
[Illustration: FIG. 16.]
A diagrammatic representation of the apparatus as arranged by Professor
Korn is given in Fig. 16. The undamped or "continuous" waves are generated
by means of a high-frequency alternator or Poulsen arc. In Fig. 16, X is
the generator, F inductance, C condenser; the aerial inductance T is
connected by the aerial A and earth E. By this means the waves are tuned to
a certain period. {32} A metal print, similar to what has already been
described, is wrapped round the drum D of the machine, and when the stylus
Z traces over an insulating strip the waves generated are in tune with the
receiving station, but when it traces over a conducting strip, a portion of
the inductance T is short-circuited, the period of the oscillations is
altered, and the two stations are thrown out of tune.
The receiving station is provided with an aperiodic circuit, which consists
of an inductance F', condenser C', and a thermodetector N. A string
galvanometer H (described in Chapter III.), and the self-induction coils B,
B' are connected as shown, the coils B, B' preventing the high-frequency
currents, which change their direction, from flowing through the
galvanometer. The manner in which the string galvanometer is arranged to
reproduce a transmitted picture is shown in Fig. 24.
The connections adopted by the Poulsen Company for photographically
recording wireless messages are given in Fig. 17, a string galvanometer of
the Einthoven type being used. The two self-induction coils S and S' are in
circuit with the detector D and the galvanometer G. The condenser C'
prevents the continuous current produced by the detector from flowing
through the high frequency circuit; P is the primary of the aerial {33}
inductance and F the secondary. The method of transmitting adopted by
Professor Korn appears to be a simple and reliable arrangement, provided
that an equally reliable method of producing the undamped waves can be
found. Owing to the absence of mechanical inertia it should be capable of
working at a good speed, while the absence of a number of pieces of
delicate apparatus all requiring careful adjustment add greatly to its
reliability.
[Illustration: FIG. 17.]
In any spark system with a properly designed aerial a coil taking ten
amperes is capable of transmitting signals over a distance of thirty to
fifty miles, but where the number of interruptions of the break required
per second is very high, as in radio-photography, it must be remembered
that a much higher voltage is needed to drive the requisite amount of
current through the primary winding of the coil than would be the case if
the interruptions were slower. It is possible to use platinum {34} contacts
for the relays, for currents up to ten amperes, but for heavier currents
than this some arrangement where contact is made with mercury will be found
to be more economical and reliable.
In the transmitter already described and given in Fig. 11, the best results
would be obtained by finding the speed at which the relay R' works best,
and regulating the number of contacts made by the stylus accordingly.
The method employed by De' Bernochi (see Chapter I.) of varying the
intensity of a beam of light by passing it through a photographic film,
which in turn alters the resistance of a selenium cell, has been very
successfully employed in at least one system of photo-telegraphy. Its
application has also been suggested for wireless transmission, and although
with any system using continuous waves this would not be very difficult, it
could hardly be adapted to work with the ordinary spark system. The
apparatus for receiving from this type of transmitter would, on the other
hand, necessarily be more elaborate than the methods that are described in
the next chapter, and as far as the writer's experience goes, experiments
along these lines would not prove very profitable, as simplicity is the
keynote of success in any radio-photographic system.
It has been suggested that in order to decrease the time of transmission a
cylinder capable of {35} taking a print 7 inches by 5 inches be employed,
the print being prepared from rather a coarse line screen--say 35 to the
inch--and a traverse of about 1/50 inch given to the stylus, thus reducing
the time of transmission to about twelve minutes. It is questionable,
however, whether the increase in speed would compensate for the loss of
detail, as only very bold subjects could be transmitted. As already pointed
out, wireless transmission would only be employed for fairly long
distances, and the extra time and expense required to receive a fairly good
detailed picture is negligible when compared with the enormous time it
would take to receive the original photograph by any ordinary means of
transit.
The public much prefer to have passable pictorial illustrations of current
events than wait several days for a more perfect picture--the original, and
the advantage of any newspaper being able to publish photographs several
days before its rivals is obvious. There can also be no doubt but that a
system of radio-photography, if fairly reliable and capable of working over
a distance of say thirty miles, would be of great military use for
transmitting maps and written matter with a great saving of time and even
life. Written matter could be transmitted with even greater safety than
messages which are sent in the ordinary way in Morse Code, as the signals
received in the receiver {36} of an hostile installation would be but a
meaningless jumble of sounds, and even were they possessed of
radio-photographic apparatus the received message would be unintelligible,
unless they knew the exact speed at which the machines were running and
could synchronise accurately.
* * * * *
{37}
CHAPTER III
RECEIVING APPARATUS
There are only two methods available at present for receiving the
photographs, and both have been used in ordinary photo-telegraphic work
with great success. They have disadvantages when applied to wireless work,
however, but these will no doubt be overcome with future improvements. The
two methods are (1) by means of an ordinary photographic process, and (2)
by means of an electrolytic receiver.
In several photo-telegraphic systems the machine used for transmitting has
the cylinder twice the size of the receiving cylinder, thus making the area
of the received picture one-quarter the area of the picture transmitted.
The extra quality of the received picture does not compensate for the
disadvantage of having to provide two machines at each station, and in the
writer's opinion results, quite good enough for all practical purposes, can
be obtained by using a moderate size cylinder so that one machine answers
for both transmitting {38} and receiving, and using as fine a line screen
as possible for preparing the photographs.
[Illustration: FIG. 18.]
The writer, when first experimenting in photo-telegraphy, endeavoured to
make the receiving apparatus "self-contained," and one idea which was
worked out is given in Fig. 18. The electric lamp L is about 8 c.p., and is
placed just within the focus of a lens which has a focal length of 3/4
inch. When a source of light is placed at some point between a lens and its
principal focus, the light rays are not converged, but are transmitted in a
parallel beam the same size as the lens. It has been found that this
arrangement gives a sharper line on the drum than would be the case were
the light focussed direct upon the hole in the cone A. An enlarged drawing
of the cone is given in Fig. 19. The hole in the tip of the cone A is a
bare 1/90 inch in diameter--the size of this hole depends upon the travel
per revolution of the drum or table of the machine used--and in working,
the cone is run as close as possible to the {39} drum without being in
actual contact. The magnet M is wound full with No. 40 S.C.C. wire, and the
armature is made as light as possible. The spring to which the armature is
attached should be of such a length that its natural period of vibration is
equal to the number of contacts made by the transmitting stylus. The spring
must be stiff enough to bring the armature back with a fairly crisp
movement. The spring and armature is shown separate in Fig. 20.
[Illustration: FIG. 19.]
[Illustration: FIG. 20.]
The shutter C is about 1/4 inch square and made from thin aluminium. The
hole in the centre is 1/16 x 1/8 inch, and the movement of the armature is
limited to about 3/32 inch. In all arrangements of this kind there is a
tendency for the armature spring to vibrate, as it were, sinusoidally, if
the coil is magnetised and demagnetised at a higher rate than the natural
period of vibration of the spring. {40} This causes an irregularity in the
rate of the vibrations which affects the received image very considerably.
A photographic film is wrapped round the drum of the machine, being
fastened by means of a little celluloid cement smeared along one edge.
This device, although it will work well over artificial conductors, is not
suitable for wireless work, as it is too coarse in its action; it can be
made sensitive enough to work at a speed of 1000 to 1500 contacts per
minute, with a current of .5 milliampere. It is impossible to obtain a
current of this magnitude from the majority of the detectors in use, so
that if any attempt is made to use this device for radio-photography it
will be necessary to employ a Marconi coherer (filings), as this is
practically the only coherer from which so large a current can be obtained.
There have been many attempts made to receive with an ordinary filings
coherer, but as was pointed out in Chapter I. these have now been discarded
in serious wireless work, being only used in small amateur stations or
experimental sets. As the reasons for this are well known to the majority
of wireless workers there is no need to enumerate them here.
A method whereby a filings coherer can be decohered, the act of decohering
closing a local circuit which contains the photographic {41} receiving
apparatus, is given in the diagram Fig. 21.
[Illustration: FIG. 21.]
In the figure, the coherer C is fixed in rigid supports, one support being
provided with a platinum pin F. To the coherer is connected the sensitive
electro-magnet M, which becomes magnetised as soon as the incoming waves
act upon the coherer. To the armature B is attached a light aluminium arm
S, pivoted at K, and carrying at the other end the striker G, which is
fitted with a platinum contact. When the armature B is attracted the
coherer is decohered by the force of the impact between the contacts F and
G. To prevent damage to the coherer the force of the blow is taken off by
the ability of the striker to work back through a hole in the arm S, the
spring {42} N keeping it normally in a fixed position. T and P are
adjusting screws, and the terminals J are for connecting to the receiving
apparatus. With this arrangement a very short wave-train causes only one
tap of the contacts, so that only one mark is registered on the receiving
drum for every contact made on the transmitter.
[Illustration: FIG. 22.]
The drawing, Fig. 22, gives a diagrammatic representation of apparatus
arranged for another photographic method of receiving. The machine shown in
Fig. 6 is used in this case. A is the aerial, E earth, P primary of
oscillation-transformer, S secondary of transformer, C variable condenser,
C' block condenser, D detector, X two-way switch, T telephone.
A De' Arsonval galvanometer H is also connected to the switch X, so that
either the telephone or the galvanometer can be switched in. The {43}
galvanometer can be made sensitive enough to work with a current as small
as 10^{-7} of an ampere, with a period of about 1/150th of a second. The
screen J has a small hole about 1/8 inch diameter drilled in the centre.
Under the influence of the brief currents which pass through the detector
every time a group of waves is received, the mirror of the galvanometer
swings to-and-fro in front of the screen J, and allows the light reflected
from the source of light M to pass through the aperture in the screen, on
to the lens N.
Round the drum V of the machine is wrapped a sensitive photographic film,
and this records the movements of the mirror which correspond to the
contacts on the half-tone print used in transmitting. Every time current
passes through the galvanometer, the light that is received from M,[6]
passes through the aperture in the screen J, and is focussed by the lens N
to a point upon the revolving film. As soon as the current ceases, the
mirror swings back to its original position, and the film is again in
darkness. Upon being developed a photograph, similar to the negative used
for preparing the metal print is obtained. If desired the apparatus can be
so arranged that the received picture is a positive instead of a negative.
{44}
The detector used should be a Lodge wheel-coherer or a Marconi
valve-receiver, as these are the only detectors that can be used with a
recording instrument. If the swing of the galvanometer mirror is too great,
a small battery with a regulating resistance can be inserted in order to
limit the movement of the mirror to a very short range; the current of
course flowing in an opposite direction to the current flowing through the
coherer.
In this, as in all other methods of receiving, the results obtained depend
upon the fineness of the line screen used in preparing the metal prints;
and as already shown the fineness of the screen that can be used is
dependent upon the mechanical efficiency of the entire apparatus.
Another system, and one that has been tried as a possible means of
recording wireless messages, is as follows. The wireless arrangements
consist of apparatus similar to that shown in Fig. 22, but instead of a
Lodge coherer a Marconi valve is used, and an Einthoven galvanometer is
substituted for the reflecting galvanometer. The Einthoven galvanometer
consists of a very powerful electro-magnet, the pole pieces of which
converge almost to points. A very fine silvered quartz thread is stretched
between the pole pieces, as shown in Fig. 23, the tension being adjustable.
The period of swing is about 1/250th of a second. A hole is bored through
the poles, and one of them is fitted {45} [Illustration] with a sliding
tube which carries a short focus lens N. The light from M passes through
the magnets, and a magnified image of the quartz thread is thrown upon the
ebonite screen J. This screen is provided with a fine slit, and when the
galvanometer is at rest the shadow of the thread just covers the slit in
the screen and prevents any light from M reaching the photographic film.
Upon signals being received the shadow of the thread moves to one side for
a long or short period, uncovering the slit, and allowing light to pass
through. The lens R concentrates the collected light to a point upon the
revolving film. The connections for the complete receiver are given in Fig.
24.
The modified form of the Einthoven galvanometer, as arranged by Professor
Korn for use with his selenium machines for photo-telegraphy over ordinary
land lines, consists of two fine silver wires which are displaced in a
lateral direction between the pole pieces when traversed by a current; the
current passing through both wires in the same {46} direction. A small
shutter of aluminium foil is attached to the wires at the optical centre.
The silver wires used are 1/1000 inch in diameter, with a natural period of
about 1/120th of a second; the length of wires free to swing being usually
about 5 cm.
[Illustration: FIG. 24.]
The period of the wires depends to a great extent upon their length and
diameter, and also upon their tension. By using short fine wires the period
can be made much smaller, but a greater current is required to produce a
similar displacement. Where the current available, as in wireless
telegraphy, is very small, and a definite displacement of the wires is
required, it is at once apparent that with wires of a given diameter there
is a limit to their length and therefore to the period. Finer wires can be
used, but here again there is a practical limit to their fineness, although
galvanometers have been constructed with a single silvered quartz thread
1/12000th of an inch diameter, which, when placed in a powerful field, will
give a good displacement with a current as small as 10^{-8} ampere. {47}
With the apparatus arranged by the Poulsen Company, given in the diagram,
Fig. 17, for photographically recording wireless signals, the current
required to operate the galvanometer for signals transmitted at the rate of
1500 a minute is 1 x 10^{-6} ampere, while for signals up to 2500 a minute
a current about 5 x 10^{-6} ampere is necessary.
Another very sensitive instrument, employed by M. Belin, and known as
Blondel's oscillograph, consists of two fine wires stretched between the
poles of a powerful electro-magnet, a small and very light mirror being
attached to the centre of the wires. The current passes down one wire and
up the other, and the wires, together with the mirror, are twisted to a
degree depending upon the strength of the received current. In order to
render the instrument dead-beat the moving parts are arranged to work in
oil. The light reflected from the mirror is made use of in a manner similar
to that shown in Fig. 22.
In all photographic methods of receiving, the apparatus must be enclosed in
some way to prevent any extraneous light from reaching the film, or better
still placed in a room lighted only by means of a ruby light.
The following method is given more as a suggestion than anything else, as I
do not think it has been tried for wireless receiving, although it is
stated to have given some good results over {48} ordinary land lines. It is
the invention of Charbonelle, a French engineer, and is quite an original
idea. His method consists of placing a sheet of carbon paper between two
sheets of thin white paper, and wrapping the whole tightly round the drum
of the machine. A hardened steel point is fastened to the diaphragm of a
telephone receiver, and this receiver is placed so that the steel point
presses against the sheets of paper. As the diaphragm and steel point
vibrates under the influence of the received currents marks are made by the
carbon sheet on the bottom paper.
Over a line where a fair amount of current is available at the receiver,
the diaphragm would have sufficient movement to mark the paper, but the
movement would be very small with the current received from a detector.
This difficulty could no doubt be overcome to a certain extent by making a
special telephone receiver, with a large and very flexible diaphragm, and
wound for a very high resistance. The movement of an ordinary telephone
diaphragm for a barely audible sound is, measured at the centre, about
10^{-6} of a c.m. With a unit current the movement at the centre is about
1/700th of an inch. Greater movement of the diaphragm could be obtained by
connecting a _Telephone relay_ to the detector, and using the magnified
current from the relay to operate the telephone. {49}
[Illustration: FIG. 25.]
The telephone relay consists of a microphone C, Fig. 25, formed of the two
pieces of osmium iridium alloy. The contact is separated to a minute degree
partly by the action of the local current from F, which flows through it
and also through the winding W of the two magnet coils. The local current
from F assists in forming the microphone by rendering the space between the
contacts conductive. The vibrating reed P is fastened to the metal frame
(not shown) which carries a micrometer screw by which the distance between
the contacts can be accurately regulated. It will be seen from Fig. 25 that
the local circuit consists of a battery F (about 1.5 volts), the microphone
contacts C, the windings W, milliampere meter B, and the terminals T, for
connecting to the galvanometer or telephone, all in {50} series. On the top
of the magnet cores N, S is a smaller magnet D, wound with fine wire for a
resistance of about 4935 ohms, the free ends of the coils being connected
to the detector terminals. The working is as follows. Supposing the current
from the detector flows through D in such a way that its magnetism is
increased, the reed P will be attracted, the contacts opened, and their
resistance increased. It will be seen that the current from F is passed
through the coils W, in such a way as to increase the magnetism of the
permanent magnet, so that any opening of the microphone contact increases
their resistance, causes the current to fall, and weakens the magnets to
such an extent that the reed P can spring back to its normal position. On
the other hand, if the detector current flows through D in such a direction
as to decrease the magnetism in the permanent magnets, the reed P will rise
and make better contact owing to the removal of the force opposing the
stiffness of the reed. Owing to the decrease in the resistance of the
microphone, the strength of the local current will be increased, the
magnets strengthened, and the reed P will be pulled back to its original
position. This relay gives a greatly magnified current when properly
adjusted, the current being easily increased from 10^{-4} to 10^{-2}
amperes. It is also very sensitive, but needs careful adjustment in order
that the best results may {51} be obtained. A greater range of
magnification can be obtained by placing two or more relays in series.
[Illustration: FIG. 26.]
A very sensitive receiver designed by the writer is given in the figures 26
and 27. To the centre of a telephone diaphragm is fastened a light steel
point P, and the movement of this point is communicated to the aluminium
arm D, which is pivoted at C. As will be seen the telephone receiver is of
special construction, it containing only one coil and therefore only one
core; by this means the movement of the diaphragm is centralised. The coil
is wound for a resistance of about 200 ohms, and the diaphragm should be
fairly thin but very resillient.
[Illustration: FIG. 27.]
To the free end of D is fastened the mirror T, made from thin diaphragm
glass about 1-1/2 centimetres diameter, and having a focal length of 40
inches. Light from the lamp L is transmitted by the lens N in a parallel
beam to the mirror which {52} concentrates it to a point upon a hole
1/100th of an inch in diameter in the screen J. As the telephone diaphragm
vibrates under the influence of the received signals the arm, and
consequently the mirror, vibrates also, and the hole in the screen J is
constantly being covered and uncovered by the spot of light. It will be
seen from Fig. 27 that the ratio between the centre of the mirror and the
pivot C, and C and the steel point P is 10:1, so that if a movement of
1/20000th of an inch is obtained at the centre of the diaphragm the mirror
will move 1/2000th of an inch; and as the focal length of the mirror is 40
inches a movement of 1/50th inch is given to the spot of light.
This receiver is capable of working at a fairly high speed, as the inertia
of the moving parts is practically negligible; the weight of the arm and
mirror being less than 20 grains. The hole in the screen is made slightly
less in diameter than the traverse of the revolving cylinder, the slight
distance between the cylinder and the screen allowing the light to disperse
sufficiently to produce a line on the film of about the right thickness.
There are two other possible means of photographically receiving the
picture that upon investigation may yield some results; but it is doubtful
whether the current available, even that obtained from a telephone relay,
will be sufficient to produce the desired magnetic effect, and the {53}
insertion of a second relay would detract greatly from the efficiency by
decreasing the speed of working. If rays of monochromatic light from a lamp
L, Fig. 28, pass through a Nicol prism P (polarising prism), then through a
tube containing CS_2 (carbon bisulphide), afterwards passing through the
second prism P' (analysing prism), and if the two Nicol prisms are set at
the polarising angle, no light from L would reach the photographic film
wrapped round the drum V of the machine. Upon the tube being subjected to a
field produced by a current passing through the coil C, the refractive
index of the liquid will be changed, and light from L will reach the
photographic film.[7]
[Illustration: FIG. 28.]
The second method is rather more complicated, and is based upon the fact
that the kathode rays in a Crookes' tube can be deflected from their course
by means of a magnet. In Fig. 29 the kathode K of the X-ray tube sends a
kathode ray discharge through an aperture in the anode A, through a small
aperture in the ebonite screen J {54} on to the drum V of the machine,
round which is wrapped a photographic film; A and K being connected to
suitable electrical apparatus. Upon the coil M being energised, the
kathode-ray is deflected from its straight-line course, and the drum V is
left in darkness.
[Illustration: FIG. 29.]
The method which is now going to be described is very ingenious, as it
makes use of what is known as an electrolytic receiver. This method of
receiving has proved to be the most practical and simple of all the
photo-telegraphic systems that have been devised.
The application of this system to wireless reception is as follows. The
aerial A, and the earth E, are joined to the primary P of a transformer,
the secondary S being connected to a Marconi valve receiver C. The valve
receiver is connected to the battery B and silvered quartz thread K of an
Einthoven galvanometer (already described). The thread is 1/12000th of an
inch in diameter, and will respond to currents as small as 10^{-8} of {55}
an ampere. The light from M throws an enlarged shadow of the thread over a
slit in the screen J, and as the thread moves to one side under the
influence of a current, the slit in J is uncovered, and the light from M is
thrown upon a small selenium cell R. In the dark the selenium cell has a
very high resistance, and therefore no current can flow from the battery D
to the relay F. When the string of the galvanometer moves to one side and
uncovers the slit in the screen J, a certain amount of light is thrown upon
the selenium cell lowering its resistance, allowing sufficient current to
pass through to operate the relay.
Round the drum of the machine (shown in Fig. 7) is wrapped a sheet of paper
that has been soaked in certain chemicals that are decomposed on the
passage of an electric current through them. As soon as the local circuit
of the relay is closed, the current from the battery Z (about 12 volts)
flows through the paper and produces a coloured mark. The picture,
therefore, is composed of long or short marks which correspond to the
varying strips of conducting material on the single line print. In order to
render the marks short and crisp, a small battery Y, and regulating
resistance L, is placed across the drum and stylus. The diagram, Fig. 30,
gives the connections for the complete receiver. {56}
The paper used is soaked in a solution consisting of
Ferrocyanide of potassium 1/4 oz.
Ammoniac Nitrate 1/2 oz.
Distilled water[8] 4 oz.
[Illustration: FIG. 30.]
The paper has to be very carefully chosen, as besides being absorbent
enough to remain moist during the whole of the receiving, the surface must
also remain fairly smooth, as with a rough paper the grain shows very
distinctly, and if there is an excess of solution the electrolytic marks
are inclined to spread and so cause a blurred image. The writer tried
numerous specimens of paper before one could be found that gave really
satisfactory results. It was also found that when working in a warm room
the paper became nearly {57} dry before the receiving was finished, and the
resistance of the paper being greatly increased (this may be anything up to
1000 ohms), the marking became very faint. A sponge moistened with the
solution and applied to the undecomposed portion of the paper, while still
revolving, was found to help matters considerably.
Another experience which happened during the writer's early experiments,
the cause of which I am still unable to explain, occurred in connection
with the stylus. The stylus used consisted of a sharply pointed steel
needle, and after working for about three minutes it was noticed that the
lines were becoming gradually wider, finally running into each other. Upon
examination it was found that the point of the needle had worn away
considerably, becoming in fact, almost a chisel point. Almost every needle
tried acted in a similar manner, and to overcome this difficulty the stylus
shown in Fig. 31 was devised.
It will be seen that it consists of a holder A, somewhat resembling a drill
chuck, fastened to the flat spring B in such a manner that the angle the
stylus makes to the drum can be altered. The needle consists of a length of
36-gauge steel wire, and as this wears away slowly the jaws of the holder
can be loosened and a fresh length pushed through. The wire should not
project beyond the face of the holder more than 1/8th inch. The gauge {58}
of wire chosen would not suit every machine, the best gauge to use being
found by trial, but in the writer's machine the pitch of the decomposition
marks is much finer than of those made by the commercial machines, and this
gauge, with the slight but unavoidable spreading of the marks, will produce
a mark of just the right thickness. As already mentioned, no explanation of
this peculiarity on the part of the stylus can be given, as there is
nothing very corrosive in the solution used, and the pressure of the stylus
upon the paper is so slight as to be almost negligible.
[Illustration: FIG. 31.]
No special means are required for fastening the paper to the drum, the
moist paper adhering quite firmly. Care should be taken, however, to fasten
the paper--which should be long enough to allow for a lap of about 1/4
inch--in such a manner that when working the stylus draws away from the
edge of the lap and not towards it.
The current required to produce electrolysis is very small, about one
milliampere being sufficient. {59} Providing that the voltage is
sufficiently high, decomposition will take place with practically "no
current," it being possible to decompose the solution with the discharge
from a small induction coil. The quantity of an element liberated is by
weight the product of time, current, and the electro-chemical equivalent of
that element, and is given by the equation W = zct, where
W = quantity of element liberated in grammes.
z = electro-chemical equivalent,
c = current in amperes,
t = time in seconds.
The chemical action that takes place is therefore very small, as the
intermittent current sent out from the transmitter in some cases only lasts
from 1/50th to 1/100th a second.
The decomposed marks on the paper are blue, and, as photographers know,
blue is reproduced in a photograph as a white, so that a photograph taken
of our electrolytic picture, which will of course be a blue image upon a
white ground, will be reproduced almost like a blank sheet of paper. If,
however, a yellow contrast filter is placed in front of the camera lens,
and an orthochromatic plate used, the blue will be reproduced in the
photograph as a dead black.
There is one other point that requires attention. It will be noticed that
the metal print used for {60} transmitting is a positive, since it is
prepared from a negative. The received picture will therefore be a
negative, making the final reproduction, if it is to be used for newspaper
work, a negative also. Obviously this is no good. The final reproduction
must be a positive, therefore the received picture must be also a positive.
To overcome this difficulty matters must be so arranged at the receiving
station that in the cases of Figs. 17, 18, 22, and 24, the film is kept
permanently illuminated while the stylus on the transmitter is tracing over
an insulating strip, and in darkness when tracing over a conducting strip.
In Fig. 30 the relay F should allow a continuous current from Z to flow
through the electrolytic paper, and only broken when the resistance of the
selenium cell is sufficiently reduced to allow the current from D to
operate the relay.
The author has endeavoured to make direct positives on glass of the picture
to be transmitted, so that a negative metal print could be prepared. The
results obtained were not very satisfactory, but the method tried is given,
as it may perhaps be of interest. The plate used in the camera has to be
exposed three or four times longer than is required for an ordinary
negative. The exposed plate is then placed in a solution of protoxalate of
iron (ferrous oxalate) and left until the image shows plainly through the
back of the plate. It {61} is then washed in water and placed in a solution
consisting of
Distilled water 1000 cc.
Nitric acid 2 cc.
Sulphuric acid 3 cc.
Bichromate of potash 105 grammes.
Alum 80 "
After being in this bath for about fifteen minutes the plate is again well
washed in water, and developed in the ordinary way. The first two
operations should be performed in the dark room, but the remaining
operations can be performed in daylight, once the plate has been placed in
the bichromate bath. As already stated, the results obtained were not very
satisfactory, and such a method is not now worth following up, as it is
comparatively easy so to arrange matters at the receiving station that a
positive or negative image can be received at will.
It is necessary to connect the stylus of the receiving machine to the
positive pole of the battery Z, otherwise the marks will be made on the
underside of the paper. The electrolytic receiver, owing to the absence of
mechanical and electro-magnetic inertia, is capable of recording signals at
a very high speed indeed.
"Atmospherics," which are such a serious nuisance in long-distance wireless
telegraphy, will also prove a nuisance in wireless photography, {62} but
their effects will not be so serious in a photographic method of receiving
as they would be in the electrolytic system. In a photographic receiver
where the film is, under normal conditions, constantly illuminated, the
received signals (both the transmitted signals and the atmospheric
disturbances) will be recorded, after development, as transparent marks
upon the film, the remainder of the film being, of course, perfectly
opaque. By careful retouching the marks due to the disturbances can be
eradicated, a print upon sensitised paper having been first obtained to act
as a guide during the process.
* * * * *
{63}
CHAPTER IV
SYNCHRONISING AND DRIVING
Clockwork and electro-motors are the source of driving power that are most
suitable for photo-telegraphic work, and each has its superior claims
depending on the type of machine that is being used. For general
experimental work, however, an electro-motor is perhaps the most
convenient, as the speed can be regulated within very wide limits. For a
constant and accurate drive a falling weight has no equal, but the
apparatus required is very cumbersome and the work of winding both tedious
and heavy. This method of driving was at one time universally employed with
the Hughes printing telegraph, but it has now been discarded in favour of
electro-motors, which are more compact, besides being cheaper to instal in
the first instance.
Synchronising and isochronising the two machines are the most difficult
problems that require solving in connection with wireless photography, and
as previously mentioned, the {64} synchronising of the two stations must be
very nearly perfect in order to obtain intelligible results. The limit of
error in synchronising must be about 1 in 500 in order to obtain results
suitable for publication.
The electrolytic system is perhaps the easiest to isochronise, as the
received picture is visible. On the metal print used for transmitting, and
at the commencing edge a datum line is drawn across in insulating ink. The
reproduction of this line is carefully observed by the operator in charge
of the receiving instrument, and the speed of the motor is regulated until
this line lies close against a line drawn across the electrolytic paper.
Although this may seem an ideal method there are one or two considerations
to be taken into account. Unless the decomposition marks are made the
correct length and are properly spaced, however good the isochronising may
be, the result will be a blurred image. Any one who has worked with a
selenium cell, will know that it cannot change from its state of high
resistance to that of low resistance with infinite rapidity, and the
effects of this inertia, or "fatigue" as it has been called, are more
pronounced when working at a high speed. In working, the effects of this
inertia would be to increase the time of contact of the relay F (Fig. 30)
as the current from D would flow for a slightly longer period through R to
F than the period of {65} illumination allowed by K. This, of course, would
mean a lengthening of the marks on the paper; results would also differ
greatly with different selenium cells. There is a method of compensation by
which the inertia of a cell can almost entirely be overcome, but it would
add greatly to the complicacy of the receiving apparatus.
In using an electro-motor with any optical method of receiving there are
two methods available. The first is an arrangement similar to that used by
Professor Korn in his early experiments with his selenium machines. The
motor used for driving has several coils in the armature connected with
slip rings, from which an alternating current may be tapped off; the motor
acting partially as a generator, besides doing good work as a motor in
driving the machine. This alternating current is conducted to a frequency
meter, which consists of a powerful electro-magnet, over which are placed
magnetised steel springs, having different natural periods of vibration. By
means of a regulating resistance the motor is run until the spring which
has the same period as the desired armature speed vibrates freely. The
speed of the motors at both stations can thus be adjusted with a fair
amount of accuracy. Another method is to make use of a governor similar to
those employed in the Hughes printing telegraph system. A drawing of the
governor is given in Fig. 32. It consists of a
[Illustration] {67} metal frame which supports an upright steel bar S,
whose ends turn on pivots. This bar is rectangular in section. The
gear-wheel G is fastened near the bottom of this rod and gears with a
similar wheel on the shaft of the driving motor (not shown). Suspended from
the broader sides of S are the two flexible arms D, each carrying a brass
ball T. These balls are not fastened to the arms, but can slide up and
down, being held in position by the wire springs M, one end of each spring
being fastened to the screws C. These screws work in a slot cut in the
upper part of S, and are connected to the adjusting screw E. When E is
turned the screws are raised or lowered accordingly, and also the balls on
the arms D.
Fastened to the arms are two brushes of tow B, and these revolve inside but
just clearing the inner surface of the steel ring Z. Upon the motor speed
increasing above the normal the arms D, and consequently the balls T, swing
out, making a larger circle, causing the brushes B to press against the
steel ring Z, setting up friction which, however, is reduced as soon as the
motor regains its ordinary working speed. By careful adjustment the speed
of the motors can be kept perfectly constant. The object of having the
balls T adjustable on D, is to provide a means of altering the motor speed,
as the lower the balls on D the slower the mechanism runs, and _vice
versa_. {68}
[Illustration]
A simple and effective speed regulator devised by the writer is given in
drawings 33 and 34. It comprises two parts, A and B, the part A being
connected to the driving motor, and the part B working independently. The
independent portion B consists of an ordinary clock movement M, a steel
spindle J being geared to one of the slower moving wheels, so that it makes
just one revolution in two seconds. This spindle, which runs in two coned
bearings, carries at its outer end a light [Illustration] pointer D, about
two inches long, to the underside of which is fastened the thin brass
contact spring S, which presses lightly upon the ebonite ring N. {69} The
portion A comprises a spindle, pointer, and contact spring similar to those
employed in B, the spindle J' being geared to the driving motor by means of
F, so that the pointer D' makes a little more than one revolution in two
seconds. By means of a special form of brake on the driving motor, the
speed is reduced, so that both pointers travel at the same rate, viz. one
revolution in two seconds. By careful adjustment the two pointers can be
made to revolve in synchronism,[9] and when this is obtained the contact
springs S, S', pass over the contacts C, C', completing the circuit of the
battery B and lamp L. When working properly the lamp L lights up regularly
once every second. This regulator is an excellent one to use for
experimental work, although it depends a great deal upon the skill of the
operator, but good adjustment should be obtained in about two minutes. It
is a good plan to insert a clutch of some description between the driving
motor and the machine, so that the regulator can be adjusted prior to the
act of receiving or transmitting, the machine being prevented from
revolving by means of a catch. The motor used should be powerful enough to
take up the work of driving the machine without any reduction in speed. The
clocks M can be regulated so that they only gain or lose a few seconds in
{70} twenty-four hours, which gives an accuracy in working sufficient for
all practical purposes.
Connection is made with the contact springs S, S', by means of the springs
T, T', which press against the spindles J, J'.
Another important point is the correct placing of the picture upon the
receiving drum. It is necessary that the two machines besides revolving in
perfect isochronism should synchronise as well, _i.e._ begin to transmit
and record at exactly the same position on the cylinders, viz. at the edge
of the lap, so that the component parts of the received image shall occupy
the same position on the paper or film as they do on the metal print. If
the receiving cylinder had, let us suppose, completed a quarter of a
revolution before it started to reproduce, the reproduction when removed
from the machine and opened out will be found to be incorrectly placed; the
bottom portion of the picture being joined to the top portion, or _vice
versa_, and this means that perhaps an important piece of the picture would
be rendered useless even if the whole is not spoilt. It is evident,
therefore, that some arrangement must be employed whereby synchronism, as
well as isochronism of the two instruments can be maintained.
There are several methods of synchronising that are in constant use in
high-speed telegraphy, in which the limit of error is reduced to a minimum,
{71} and some modification of these methods will perhaps solve the problem,
but it must be remembered that synchronism is far easier to obtain where
the two stations are connected by a length of line than where the two
stations are running independently.
In one system of ordinary photo-telegraphy synchronism is obtained in the
following manner. The receiving cylinder travels at a speed slightly in
excess of the transmitting cylinder, and as its revolution is finished
first is prevented from revolving by a check, and when in this position the
receiving apparatus is thrown out of circuit and an electro-magnet which
operates the check is switched in. When the transmitting cylinder has
completed its revolution (about 1/100th of a second later) the transmitting
apparatus, by means of a special arrangement, is thrown out of circuit for
a period, just long enough for a powerful current to be sent through the
line. This current actuates the electro-magnet. The check is withdrawn and
the receiving cylinder commences a fresh revolution in perfect synchronism
with the transmitting cylinder. As soon as the check is withdrawn the
receiving apparatus is again placed in circuit until another revolution is
completed. As the receiver cannot stop and start abruptly at the end of
each revolution a spring clutch is inserted between the driving motor and
the machine. {72}
Although a method of synchronising similar to this may later on be devised
for wireless photography, the writer, from the result of his own
experiments, is led to believe that results good enough for all practical
purposes can be obtained by fitting a synchronising device whereby the two
machines are started work at the same instant, and relying upon the perfect
regulation of the speed of the motors for correct working.
The method of isochronism must, however, be nearly perfect in its action,
as it is easy to see that with only a very slight difference in the speed
of either machine this error will, when multiplied by 40 or 50 revolutions,
completely destroy the received picture for practical purposes.
From what has been written in this and in the preceding chapters it will be
evident that the successful solution of transmitting photographs by
wireless methods will necessitate the use of a great many pieces of
apparatus all requiring delicate adjustment, and depending largely upon
each other for efficient working. As previously stated, there is at present
no real system of wireless photography, the whole science being in a purely
experimental stage, but already Professor Korn has succeeded in
transmitting photographs between Berlin and Paris, a distance of over 700
miles. If such a distance could be worked over successfully, there is no
reason to doubt that before long {73} we shall be able to receive pictures
from America with as great reliability and precision as we now receive
messages.
In nearly all wireless photographic systems devised up to the present the
chief portion of the receiver consists of a very sensitive galvanometer,
and although very good results have been obtained by their use they are
more or less a nuisance, as the extreme delicacy of their construction
renders them liable to a lot of unnecessary movement caused by external
disturbances. A galvanometer of the De' Arsonval pattern, used by the
writer, was constantly being disturbed by merely walking about the room,
although placed upon a fairly substantial table; and for the same reason it
was impossible to attempt to place the driving motor of the machine on the
same table as the galvanometer. For ship-board work it will be evident that
the use of such a sensitive instrument presents a great difficulty to
successful working, and a good opening exists for some piece of
apparatus--to take the place of the galvanometer--that will be as sensitive
in its action but more robust in its construction.
* * * * *
{74}
CHAPTER V
THE "TELEPHOGRAPH"
In the present chapter it is proposed to give a brief description of a
system of radio-photography devised by the author, and which includes a
greatly improved method of transmitting and receiving, as well as an
ingenious arrangement for synchronising the two stations; the whole being
an attempt to produce a system that would be capable of working
commercially over fairly long distances.
The system about to be described, and which I have designated the
"telephograph," is the outcome of several years' original experimental
work, many difficulties that were manifest in the working of the earlier
systems having been overcome by apparatus that has been expressly designed
for the purpose.
In any practical system of radio-photography the following points are of
great importance: (1) the speed of transmission; (2) the quality of the
received picture; (3) the method of synchronising {75} the two machines so
that transmission and reception begin simultaneously; (4) the correct
regulation of the speed of the driving motors; (5) the simplicity and
reliability of the entire arrangement. Points 1 and 2 are dependent upon
several factors; the number of contacts made by the stylus per minute; the
size of the metal print used; the number of lines per inch on the screen
used in preparing the print; and the accurate and harmonious working of the
various pieces of apparatus employed.
In the system under discussion the size of the metal print used is 5 inches
by 7 inches, and a screen having 50 lines to the inch is used for preparing
it. With the drum of the machine making one revolution in four seconds, the
stylus makes 87 contacts per second, or 5220 a minute, the time for
complete transmission being twenty-five minutes. By the use of ordinary
relays not more than 2000 contacts a minute can be obtained, and in the
present system it is only by means of a specially designed relay that such
a high rate of working has been made possible. Similarly, too, with the
receiving of such a large number of signals transmitted at such a high
speed, a special instrument has been devised that can record this number of
signals without any trouble, and could even record up to 8000 signals a
minute, provided that a suitable transmitter could be designed. {76}
In the present system the writer does not claim to have completely solved
the problem of the wireless transmission of photographs, but it is a great
advance on any system previously described, and the following advantages
are put forward for recognition: (1) a greatly improved method of
transmitting and receiving; (2) a simple method of regulating the speed of
the driving motors and maintaining isochronism with a limit of error of
less than 1 in 800; (3) an arrangement for synchronising the two machines
whereby transmitting and receiving begin simultaneously; (4) the use of one
machine only at each station.
TRANSMITTING APPARATUS
A diagrammatic representation of the apparatus required for a complete
station, transmitting and receiving combined, is given in Fig. 35, the
usual wireless equipment having been omitted from the diagram to avoid
confusion.
_The Machine._--This, as will be seen from Fig. 36, consists of a
base-plate M, to which are attached the two bearings B and B'. The bearing
B' is fitted with an internal thread to correspond with the threaded
portion of the shaft D. The drum V is a brass casting, being fastened to
the shaft by set screws. The shaft is threaded 75 to the inch. The bearings
are preferably of the concentric type. The circuit breaker C is so arranged
that when {77} the drum has traversed the required distance, the end of the
shaft pushes back the spring M, breaking the circuit of the driving gear
and stopping the machine. The machine is connected to the driving gear by
the flexible coupling A.
[Illustration: FIG. 35.
M, motor; Y, isochroniser; F, clutch; A, machine; R, stylus; S, relay; X,
gearing; O, circuit breaker; T, receiver; C, condenser; U, telephone relay;
K, polarised relay; L, contact breaker; D, D^1, D^2, D^3, batteries; P,
friction brake; B, B^1, double-pole two-way switches; N, N^1, N^2, single
switches; W, key; E, electric clock; J, telephones.]
The drum measures 5 inches long by 2-1/8 inches diameter, and this takes a
metal print 5 inches by 7 inches, which allows for a lap of about 1/4 inch.
In working, the print is wrapped tightly round the drum, being secured by
means of a little seccotine smeared along one edge. Care must be taken that
the edge of the lap draws away from the point of {78} the stylus and not
towards it. A margin of bare foil, about 1/8 inch wide, should be left on
the print at the commencing edge, the purpose of which will be explained
later.
[Illustration: FIG. 36.]
_The Stylus._--As the drum of the machine travels laterally, by reason of
the threaded shaft and bearing, the stylus must necessarily be a fixture.
It consists of a holder B, drilled to take a hardened steel point S,
attached to the spring M. The spring is arranged to work in the guide F,
which is provided with an adjusting screw W for regulating the pressure of
the stylus upon the print; the pressure being sufficient to enable good
contact to be made, but must not be heavy enough to scratch the soft foil.
The needle should present an angle of about 60deg to the surface of the
print, as this angle has been found to give the best results in working.
To eliminate any sparking that may take place at the point of make and
break, due to the self-induction of the relay coils, a condenser C, about 1
microfarad capacity, should be connected across {79} the drum and stylus.
The complete stylus is given in the drawings, Figs. 37, 37_a_, and also in
the diagrams Figs. 8 and 9.
[Illustration: FIG. 37.
Showing the arrangement for sliding the stylus to or from the machine.]
[Illustration: FIG. 37a.]
_The Relay._--As will be seen from the diagram, Fig. 38, this consists of
two electro-magnets having very soft iron cores, the magnet M being wound
in the usual manner, while the magnet N is wound differentially. The
armature A is made as light as possible, and is pivoted at P, and when
there is no current flowing through any of the coils, is held midway
between the magnet cores by the two spiral springs S and T, which are under
slight but equal tension. The connections are as follows. The wires from
the winding on M are connected directly to the relay terminals F and H, as
are also the wires from one winding on N. The other winding on N is
connected in series with the battery C, ammeter B, and regulating
resistance R. {80}
[Illustration: FIG. 38.]
When the circuit of the battery C is completed, the coil of N, to which it
is connected, is energised, and the armature A is attracted against the
stop V. When in this position the tension of the spring S is released,
while the tension of the spring T is increased. As soon as the circuit of
the battery D is completed by means of the metal line print on the
transmitting machine, the current divides at the terminals F and H, a
portion flowing through the magnet coil M, and a portion through the
remaining winding on N. The current which flows through the winding on N
produces a magnetising effect equal to that caused by the other winding on
N, but since the two windings are of equal length and resistance, and since
the current flowing through the two windings is of equal strength but in
opposite directions, the result is to neutralise {81} the magnetising
effects produced by each winding, and consequently no magnetism is produced
in the cores.
The other portion of the current from D flows through the coil M, and it
becomes magnetised at the same time that the coil N becomes demagnetised.
The armature A is attracted by M against the stop X, and this attraction is
assisted by the spring T, which was under increased tension. The conditions
of the springs are now reversed, the spring S being under increased
tension, while the tension of the spring T is released.
As soon as the current from D is broken, the magnetism disappears from M,
the neutralising current in N ceases, and N once more becomes magnetised,
owing to the current which still flows through one winding from C; the
armature is therefore again attracted by N, assisted by the spring S. The
current flowing through the two windings of N must be perfectly equal, and
the regulating resistance R, and ammeters B and B', are inserted for
purposes of adjustment. The current from C must flow in a direction
opposite to that which flows from D.
[Illustration: FIG. 39.
H, H', containers; M, mercury; E, paraffin oil; T, T', terminals; C,
suspending rod; D, base; F, F', dipping rods.]
The local circuit of the relay is completed by means of a copper dipper in
mercury, somewhat resembling an ordinary mercury break, but modified to
suit the present requirements. The arrangement will be seen from Fig. 39.
The whole of the {82} moving parts are made as light as possible, and for
this reason the rod C and the dippers F, F' should be made as short as
convenient. The containers H, H' are separate, of cast iron, and
rectangular in shape. The dipper is of very thin copper tube--an advantage
where alternating current is to be used--and is made adjustable for height
on the suspending rod C. The leg F is of such a length that permanent
contact is made with the mercury in the container H, while the leg F'
clears the surface of the mercury by about 1/4 inch, when the armature of
the relay is in its normal position. To prevent undue churning of the
mercury, which would necessarily take place if the dipper entered and left
the mercury at each movement of the armature, a pointed ebonite plug is
inserted in the end of the tube. This will be found to give good results at
a high speed, the mercury being practically undisturbed, and the production
of "sludge" reduced to a minimum. To prevent oxidation of the mercury, and
to prevent arcing, the surface is covered with paraffin oil. If this is not
sufficient to prevent arcing a condenser should be shunted across the {83}
containers. The volume of mercury, and the area of the dippers, should be
sufficient to carry the current used for a considerable period without
heating up to any extent. An adjustable weight J is provided in order to
balance the armature and dipping rod.