SIMPLE TELEPHONE CONNECTION]

COMPLETE INSTALLATION.--To install a more complete system requires, at
each end, a switch, a battery and an electro-magneto bell. You may use,
for this purpose, a bell, made as shown in the chapter on bells.

Fig. 84 shows such a circuit. We now dispense with one of the line
wires, because it has been found that the ground between the two
stations serves as a conductor, so that only one line wire (A) is
necessary to connect directly with the telephones of the two stations.
The telephones (B, B', respectively) have wires (C, C') running to the
pivots of double-throw switches (D, D'), one terminal of the switches
having wires (E, E'), which go to electric bells (F, F'), and from the
bells are other wires (G, G'), which go to the ground. The ground wires
also have wires (H, H'), which go to the other terminals of the switch
(D, D'). The double-throw switch (D, D'), in the two stations, is thrown
over so the current, if any should pass through, will go through the
bell to the ground, through the wires (E, G or E', G').

[Illustration: _Fig. 84._ TELEPHONE STATIONS IN CIRCUIT]

Now, supposing the switch (D'), in station 2, should be thrown over so
it contacts with the wire (H'). It is obvious that the current will then
flow from the battery (I') through wires (H', C') and line (A) to
station 1; then through wire C, switch D, wire E to the bell F, to the
ground through wire G. From wire G the current returns through the
ground to station 2, where it flows up wire G' to the battery, thereby
completing the circuit.

[Illustration: _Fig. 85._ ILLUSTRATING LIGHT CONTACT POINTS]

The operator at station 2, having given the signal, again throws his
switch (D') back to the position shown in Fig. 84, and the operator at
station 1 throws on his switch (D), so as to ring the bell in station 2,
thereby answering the signal, which means that both switches are again
to be thrown over so they contact with the battery wires (H and H'),
respectively. When both are thus thrown over, the bells (G, G') are cut
out of the circuit, and the batteries are both thrown in, so that the
telephones are now ready for talking purposes.

MICROPHONE.--Originally this form of telephone system was generally
employed, but it was found that for long distances a more sensitive
instrument was necessary.

LIGHT CONTACT POINTS.--In 1877 Professor Hughes discovered,
accidentally, that a light contact point in an electric circui