Wireless Transmission of Photographs.
by Marcus J. Martin.
PREFACE TO SECOND EDITION
Although during the last few years very little, in common with other wireless work, has been possible in connection with the practical side of the wireless transmission of photographs, yet, now that the prospect of experimental work is once again occupying the minds of all wireless workers, advantage has been taken of a reprint of this little volume to amplify a few points that were insufficiently dealt with in the first edition, and also to add some fresh matter.
To Chapter V. has been added a short description of the Nernst lamp, and also some useful information regarding photographic films, and a few notes relating to enlarging included in the Appendix B.
A fresh appendix dealing with the principles of optical lenses has also been added. This is a subject that plays an important part in any system of wireless photography, and to those experimenters whose knowledge of optics is limited this section should prove useful.
To serious workers engaged on the problem of the wireless transmission of photographs, attention {vi} is called to a series of articles which are being published from time to time in the _Wireless World_, on the design and construction of wireless photographic apparatus.
M. J. M.
MAIDSTONE, 1919.
PREFACE
In these progressive times it is only reasonable to expect that some attempt would be made to utilise the ether-waves for other purposes than that of telegraphic communication, and already many clever minds are at work trying to solve the problems of the wireless control of torpedoes and airships, wireless telephony, and, last but not least, the wireless transmission of photographs.
It may seem rather premature to talk about the wireless transmission of photographs at a time when the ordinary systems are not fully developed; but the prospects of wireless photography are of a very encouraging nature, especially for long over-water distances, as there are great difficulties to be overcome in long-distance transmission over ordinary land lines and cables which will be entirely eliminated by wireless methods.
From a perusal of Chapter I. the reader will be able to understand something of the difficulties that are to be encountered in working over long distances, and he will also be able to appreciate something of the advantages that would be derived {viii} from a reliable wireless system.
Apart from the value of such a system for transmitting news pictures, it would also be of great advantage to transmit to ships at sea photographs of criminals for identification purposes. In such a small volume as this it would be impossible to deal with the working of wireless apparatus and the many systems that have been devised for the transmission of photographs over metallic circuits. The Author has taken it for granted that other works have been studied in connection with these subjects, and will therefore only describe such apparatus as is likely to be of use in wireless transmission. At present the transmission of photographs by wireless methods is in a purely experimental stage, and this book will have served its purpose if it helps to put future experimenters on the right track and prevent them from making expensive and fruitless experiments, by showing them the right direction in which investigations are being carried out. As there is no claim to originality in respect of a good many pieces of apparatus, etc., described, I have not thought it necessary to state the various sources from which the information has been obtained.
M. J. M.
ASHFORD, 1916.
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 ill.u.s.trated 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 r.e.t.a.r.d 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 r.e.t.a.r.dation due to capacity is very great. On a cable joining this country with America the current is r.e.t.a.r.ded 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} [Ill.u.s.tration] 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 pa.s.s; 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 r.e.t.a.r.dation 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 circ.u.mstances very quickly supersede land-line telegraphy, while it has proved a formidable commercial compet.i.tor 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.
[Ill.u.s.tration: 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 gla.s.s 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 pa.s.sing through the film (the transmission of which varies in intensity according to the density of that portion of the photograph through which it is pa.s.sing), 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} [Ill.u.s.tration]
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 pa.s.ses 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 gla.s.s 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 gla.s.s 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.
[Ill.u.s.tration: 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 pa.s.ses between the b.a.l.l.s 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 coa.r.s.er, 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.
[Ill.u.s.tration: 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.