purposes. The relay U is a Brown's telephone relay.

[Illustration: FIG. 40.]

_The Receiver._--The magnified current from the relay U is taken to a
special telephone receiver, the construction of which is given in Fig. 40.
The diaphragm F is about 2-1/2 inches diameter, and should be fairly thin
but very resilient. Only one {84} [Illustration] [Illustration] coil is
provided, and this should be wound with No. 47 S.S.C. copper wire for a
resistance of about 2000 ohms. By using only one coil and therefore only
one core, the movement of the diaphragm is centralised. To the centre of
the diaphragm a light steel point is fastened, about 1/2 inch long, and
provided with a projecting hook H. An enlarged view of this pin is given in
Fig. 41. The movement of the diaphragm and consequently of the steel point
P is communicated to a pivoted rod R, which is of special construction. A
piece of aluminium tube 3-3/4 inches long, and of the section given at B,
is bushed at one end with a piece of brass of the shape shown in Fig. 41a.
A stiff steel wire T about 1 inch long (20 gauge) is screwed into the end
of Z, and carries a counterbalance weight C. A hardened {85} steel spindle,
pointed at both ends, is fastened at D, and runs between two coned
bearings, one of which is adjustable. The underside of Z is flattened, and
a small coned depression is made for the reception of the pointed end of
the pin. By means of the spring J the two pieces, Z and P, are held firmly
together, at the same time allowing perfect freedom of movement. The bridge
G is made from a piece of sheet aluminium placed in a slot cut in the tube
R, the end of the tube being pressed tight upon G, and secured by means of
a small rivet.

The optical arrangements are as follows. By means of the Nernst lamp L, and
the lenses B and B', Figs. 42 and 43, a magnified shadow of G is thrown
upon the screen J. When the shutter G is in its normal position (_i.e._ at
rest), its shadow is just above the small hole in J, and light from L
reaches the photographic film wrapped round the drum V of the machine.

[Illustration: FIG. 42.

J, screen; L, Nernst lamp; G, shutter; B, condensing lens; B_1, focussing
lens.]

When, however, signals are sent out from the transmitting apparatus, the
magnified current from the relay U energises the coil of the special
telephone S, attracting the diaphragm F, and consequently giving movement
to the pivoted rod R. As by means of the optical arra