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.

[Illustration: FIG. 65.--The headdress of an operator at a telephone
exchange. The receiver is fastened over one ear, and the transmitter to
the chest.]

If a number on switchboard A calls for a number on switchboard C, the
operator at A connects her subscriber by a jack cord to a trunk line
running to C, where the operator similarly connects the trunk line with
the number asked for, after ringing up the subscriber. The central
exchange of one town is connected with that of another by one or more
trunk lines, so that a subscriber may speak through an indefinite number
of exchanges. So perfect is the modern telephone that the writer
remembers on one occasion hearing the door-bell ring in a house more
than a hundred miles away, with which he was at the moment in telephonic
connection, though three exchanges were in the circuit.

SUBMARINE TELEPHONY.

Though telegraphic messages are transmitted easily through thousands of
miles of cable,[16] submarine telephony is at present restricted to
comparatively short distances. When a current passes through a cable,
electricity of opposite polarity induced on the outside o