Wireless Transmission of Photographs / Second Edition, Revised and Enlarged 1919

by Marcus J. Martin

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

be obtained. A greater range of magnification can be obtained by placing two or more relays in series.

A very sensitive receiver designed by the writer is given in the figures 26 and 27. To the centre of a telephone diaphragm is fastened a light steel point P, and the movement of this point is 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.

To the free end of D is fastened the mirror T, made from thin diaphragm glass about 1¹/₂ 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

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

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 CS₂ (carbon bisulphide), afterwards passing through the second prism P' (analysing prism), and if the two Nicol prisms are set at the polarising angle, no light from L would reach the photographic film wrapped round the drum V of the machine. Upon the tube being subjected to a field produced 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]]