A method whereby a filings coherer can be decohered, the act of decohering closing a local circuit which contains the photographic
receiving apparatus, is given in the diagram 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
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.
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
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.