G. C has direct contact with line
terminal T, which screws into it; D with T^1 through the brass casing,
screw S, and a small plate at the back of the transmitter. Voice
vibrations compress G G, and allow current to pass more freely from D
to C. This form of microphone is very delicate, and unequalled for
long-distance transmission.

[Illustration: FIG. 64.--A diagrammatic representation of a telephonic
circuit.]

GENERAL ARRANGEMENT OF A TELEPHONE CIRCUIT.

In many forms of subscriber's instruments both receiver and transmitter
are mounted on a single handle in such a way as to be conveniently
placed for ear and mouth. For the sake of clearness the diagrammatic
sketch of a complete installation (Fig. 64) shows them separated. The
transmitters, it will be noticed, are located in battery circuits,
including the primary windings P P_2 of induction coils. The
transmitters are in the line circuit, which includes the secondary
windings S S_2 of the coils.

We will assume that the transmitters are, in the first instance, both
hung on the hooks of the metallic switches, which their weight depresses
to the position indicated by the dotted lines. The handle of the
magneto-generator at the left-end station is turned, and current passes
through the closed circuit:--Line A, E B_2, contact 10, the switch 9;
line B, 4, the other switch, contact 5, and E B. Both bells ring. Both
parties now lift their receivers from the switch hooks. The switches
rise against contacts 1, 2, 3 and 6, 7, 8 respectively. Both primary and
both secondary circuits are now completed, while the bells are
disconnected from the line wires. The pulsations set up by transmitter T
in primary coil P are magnified by secondary coil S for transmission
through the line circuit, and affect both receivers. The same thing
happens when T_2 is used. At the end of the conversation the receivers
are hung on their hooks again, and the bell circuit is remade, ready for
the next call.

[Illustration: A TELEPHONE EXCHANGE.]

DOUBLE-LINE CIRCUITS.

The currents used in telephones pulsate very rapidly, but are very
feeble. Electric disturbances caused by the proximity of telegraph or
tram wires would much interfere with them if the earth were used for the
return circuit. It has been found that a complete metallic circuit (two
wires) is practically free from interference, though where a number of
wires are hung on the same poles, speech-sounds may be faintly induced