rrents 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.

{44}

The detector used should be a Lodge wheel-coherer or a Marconi
valve-receiver, as these are the only detectors that can be used with a
recording instrument. If the swing of the galvanometer mirror is too great,
a small battery with a regulating resistance can be inserted in order to
limit the movement of the mirror to a very short range; the current of
course flowing in an opposite direction to the current flowing through the
coherer.

In this, as in all other methods of receiving, the results obtained depend
upon the fineness of the line screen used in preparing the metal prints;
and as already shown the fineness of the screen that can be used is
dependent upon the mechanical efficiency of the entire apparatus.

Another system, and one that has been tried as a possible means of
recording wireless messages, is as follows. The wireless arrangements
consist of apparatus similar to that shown in Fig. 22, but instead of a
Lodge coherer a Marconi valve is used, and an Einthoven galvanometer is
substituted for the reflecting galvanometer. The Einthoven galvanometer
consists of a very powerful electro-magnet, the pole pieces of which
converge almost to points. A very fine silvered quartz thread is stretched
between the pole pieces, as shown in Fig. 23, the tension being adjustable.
The period of swing is about 1/250th of a second. A hole is bored through
the poles, and one of them is fitted {45} [Illustration] with a sliding
tube w