gle, 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]

[Illustration: 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.

[Illustration: 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
pass 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
passage of an electric current through