Read Ebook: Wireless Transmission of Photographs Second Edition Revised and Enlarged 1919 by Martin Marcus J

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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 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 as the current from D would flow for a slightly longer period through R to F than the period of 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.

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 . 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.

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

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, 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 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.

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 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.

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: the speed of transmission; the quality of the received picture; the method of synchronising the two machines so that transmission and reception begin simultaneously; the correct regulation of the speed of the driving motors; 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 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: a greatly improved method of transmitting and receiving; 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; an arrangement for synchronising the two machines whereby transmitting and receiving begin simultaneously; 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 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 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.

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.

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 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 containers. The volume of mercury, and the area of the dippers, should be sufficient to carry the current used for a considerable period without heating up to any extent. An adjustable weight J is provided in order to balance the armature and dipping rod.

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

RECEIVING APPARATUS

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

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

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

DRIVING APPARATUS

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

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

The bushes S are bored a running fit for the steel rod W , which is coned at both ends, and runs between two countersunk bearings, the bottom bearing E being fixed while the top bearing is adjustable. A needle K is fastened near the end of the rod W, and attached to this needle is the spring Z, which presses lightly but firmly upon the contact block Q. To provide a level surface for Z to work over, the spaces between the contact pieces are filled in with an insulating material, and the whole surface finished off perfectly smooth. The spring Z is 1/8 inch wide for portion of its length, but at the point where it presses upon Q it is reduced in width to 1/64th of an inch . The driving arrangements are as follows. A counter-shaft Q, Fig. 51, fitted with a grooved pulley, is run in bearings parallel with the shaft W, and is connected by suitable gearing to the shaft of the driving motor, so that the needle K makes one revolution in about 2-1/2 seconds. A belt passing over the pulleys connects the two shafts, and the tension of the belt is regulated by means of an adjustable jockey pulley.

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

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

METHOD OF WORKING

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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