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 in one circuit from another. This defect is, however, minimized by crossing the wires about among themselves, so that any one line does not pass round the corresponding insulator on every pole.
TELEPHONE EXCHANGES.
In a district where a number of telephones are used the subscribers are put into connection with one another through an "exchange," to which all the wires lead. One wire of each subscriber runs to a common "earth;" the other terminates at a switchboard presided over by an operator. In an exchange used by many subscribers the terminals are distributed over a number of switchboards, each containing 80 to 100 terminals, and attended to by an operator, usually a girl.
When a subscriber wishes to be connected to another subscriber, he either turns the handle of a magneto generator, which causes a shutter to fall and expose his number at the exchange, or simply depresses a key which works a relay at the exchange and lights a tiny electric lamp. The operator, seeing the signal, connects her telephone with the subscriber's circuit and asks the number wanted. This given, she rings up the other subscriber, and connects the two circuits by means of an insulated wire cord having a spike at each end to fit the "jack" sockets of the switchboard terminals. The two subscribers are now in communication.