nsitive instrument, employed by M. Belin, and known as
Blondel's oscillograph, consists of two fine wires stretched between the
poles of a powerful electro-magnet, a small and very light mirror being
attached to the centre of the wires. The current passes down one wire and
up the other, and the wires, together with the mirror, are twisted to a
degree depending upon the strength of the received current. In order to
render the instrument dead-beat the moving parts are arranged to work in
oil. The light reflected from the mirror is made use of in a manner similar
to that shown in Fig. 22.

In all photographic methods of receiving, the apparatus must be enclosed in
some way to prevent any extraneous light from reaching the film, or better
still placed in a room lighted only by means of a ruby light.

The following method is given more as a suggestion than anything else, as I
do not think it has been tried for wireless receiving, although it is
stated to have given some good results over {48} ordinary land lines. It is
the invention of Charbonelle, a French engineer, and is quite an original
idea. His method consists of placing a sheet of carbon paper between two
sheets of thin white paper, and wrapping the whole tightly round the drum
of the machine. A hardened steel point is fastened to the diaphragm of a
telephone receiver, and this receiver is placed so that the steel point
presses against the sheets of paper. As the diaphragm and steel point
vibrates under the influence of the received currents marks are made by the
carbon sheet on the bottom paper.

Over a line where a fair amount of current is available at the receiver,
the diaphragm would have sufficient movement to mark the paper, but the
movement would be very small with the current received from a detector.
This difficulty could no doubt be overcome to a certain extent by making a
special telephone receiver, with a large and very flexible diaphragm, and
wound for a very high resistance. The movement of an ordinary telephone
diaphragm for a barely audible sound is, measured at the centre, about
10^{-6} of a c.m. With a unit current the movement at the centre is about
1/700th of an inch. Greater movement of the diaphragm could be obtained by
connecting a _Telephone relay_ to the detector, and using the magnified
current from the relay to operate the telephone. {49}

[Illustration: FIG. 25.]

The telephone relay consists of a microphone C, Fig. 25, formed of the t