Providing that the voltage is sufficiently high, decomposition will take place with practically "no current," it being possible to decompose the solution with the discharge from a small induction coil. The quantity of an element liberated is by weight the product of time, current, and the electro-chemical equivalent of that element, and is given by the equation W = zct, where
W = quantity of element liberated in grammes.
z = electro-chemical equivalent,
c = current in amperes,
t = time in seconds.
The chemical action that takes place is therefore very small, as the intermittent current sent out from the transmitter in some cases only lasts from 1/50th to 1/100th a second.
The decomposed marks on the paper are blue, and, as photographers know, blue is reproduced in a photograph as a white, so that a photograph taken of our electrolytic picture, which will of course be a blue image upon a white ground, will be reproduced almost like a blank sheet of paper. If, however, a yellow contrast filter is placed in front of the camera lens, and an orthochromatic plate used, the blue will be reproduced in the photograph as a dead black.
There is one other point that requires attention. It will be noticed that the metal print used for
transmitting is a positive, since it is prepared from a negative. The received picture will therefore be a negative, making the final reproduction, if it is to be used for newspaper work, a negative also. Obviously this is no good. The final reproduction must be a positive, therefore the received picture must be also a positive. To overcome this difficulty matters must be so arranged at the receiving station that in the cases of Figs. 17, 18, 22, and 24, the film is kept permanently illuminated while the stylus on the transmitter is tracing over an insulating strip, and in darkness when tracing over a conducting strip. In Fig. 30 the relay F should allow a continuous current from Z to flow through the electrolytic paper, and only broken when the resistance of the selenium cell is sufficiently reduced to allow the current from D to operate the relay.
The author has endeavoured to make direct positives on glass of the picture to be transmitted, so that a negative metal print could be prepared. The results obtained were not very satisfactory, but the method tried is given, as it may perhaps be of interest. The plate used in the camera has to be exposed three or four times longer than is required for an ordinary negative. The exposed plate is then placed in a solution of protoxalate of iron (ferrous oxalate) and left until the image shows plainly through the back of the plate. It