d as this wears away slowly the jaws of the holder
can be loosened and a fresh length pushed through. The wire should not
project beyond the face of the holder more than 1/8th inch. The gauge {58}
of wire chosen would not suit every machine, the best gauge to use being
found by trial, but in the writer's machine the pitch of the decomposition
marks is much finer than of those made by the commercial machines, and this
gauge, with the slight but unavoidable spreading of the marks, will produce
a mark of just the right thickness. As already mentioned, no explanation of
this peculiarity on the part of the stylus can be given, as there is
nothing very corrosive in the solution used, and the pressure of the stylus
upon the paper is so slight as to be almost negligible.

[Illustration: FIG. 31.]

No special means are required for fastening the paper to the drum, the
moist paper adhering quite firmly. Care should be taken, however, to fasten
the paper--which should be long enough to allow for a lap of about 1/4
inch--in such a manner that when working the stylus draws away from the
edge of the lap and not towards it.

The current required to produce electrolysis is very small, about one
milliampere being sufficient. {59} 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 notice