61. This involves the use of the
balanced Wheatstone bridge. The only other thing about this circuit
that needs description, in view of what has previously been said about
it, is that the polarized bell is placed in series with a condenser so
that the two sides of the circuit may be insulated from each other
while the telephone is not in use, and yet permit the passage of
ringing current through the bell.

[Illustration: Fig. 162. Monarch Common-Battery Wall Set]

The use of the so-called direct-current receiver has brought about a
great simplification in the common-battery telephone circuits of
several of the manufacturing companies. By this use the transmitter
and the receiver are placed in series across the line, this path being
normally opened by the hook-switch contacts. The polarized bell and
condenser are placed in another bridge path across the line, this path
not being affected by the hook-switch contacts. All that there is to
such a complete common-battery telephone set, therefore, is a
receiver, transmitter, hook switch, bell, condenser, and cabinet, or
other support.

The extreme simplicity of the circuits of such a set is illustrated in
Fig. 162, which shows how the Monarch Telephone Manufacturing Company
connect up the various parts of their telephone set, using the
direct-current receiver already described in connection with Fig. 54.

[Illustration: VENTILATING PLANT FOR LARGE TELEPHONE OFFICE BUILDING]

CHAPTER XV

NON-SELECTIVE PARTY-LINE SYSTEMS

A party line is a line that is for the joint use of several stations.
It is, therefore, a line that connects a central office with two or
more subscribers' stations, or where no central office is involved, a
line that connects three or more isolated stations with each other.
The distinguishing feature of a party line, therefore, is that it
serves more than two stations, counting the central office, if there
is one, as a station.

Strictly speaking, the term _party_ line should be used in
contradistinction to the term _private_ line. Companies operating
telephone exchanges, however, frequently lease their wires to
individuals for private use, with no central-office switchboard
connections, and such lines are, by common usage, referred to as
"private lines." Such lines may be used to connect two or more
isolated stations. A _private_ line, in the parlance of telephone
exchange working, may, therefore, be a _party_ line, as inconsistent
as this