[Ill.u.s.tration: 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 princ.i.p.al 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.
[Ill.u.s.tration: FIG. 19.]
[Ill.u.s.tration: FIG. 20.]
The shutter C is about 1/4 inch square and made from thin aluminium. The hole in the centre is 1/16 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 coa.r.s.e 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.
[Ill.u.s.tration: 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.
[Ill.u.s.tration: 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 pa.s.s 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 pa.s.s 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 pa.s.ses through the galvanometer, the light that is received from M,[6]
pa.s.ses 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 subst.i.tuted 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} [Ill.u.s.tration] with a sliding tube which carries a short focus lens N. The light from M pa.s.ses 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 pa.s.s 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 pa.s.sing 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.
[Ill.u.s.tration: 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 10^{-6} ampere, while for signals up to 2500 a minute a current about 5 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 pa.s.ses 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}
[Ill.u.s.tration: 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 a.s.sists in forming the microphone by rendering the s.p.a.ce 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 pa.s.sed 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.
[Ill.u.s.tration: 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.
[Ill.u.s.tration: FIG. 27.]
To the free end of D is fastened the mirror T, made from thin diaphragm gla.s.s 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, pa.s.s through a Nicol prism P (polarising prism), then through a tube containing CS_2 (carbon bisulphide), afterwards pa.s.sing through the second prism P" (a.n.a.lysing 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 pa.s.sing through the coil C, the refractive index of the liquid will be changed, and light from L will reach the photographic film.[7]
[Ill.u.s.tration: 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.
[Ill.u.s.tration: 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 pa.s.s 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 pa.s.sage 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 pota.s.sium 1/4 oz.
Ammoniac Nitrate 1/2 oz.
Distilled water[8] 4 oz.
[Ill.u.s.tration: 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.