calling key at one
station and the home key at the other, no current can flow even though
both receivers are off their hooks, because in that case no connection
will exist with the positive side of the battery. This relation is shown
in Fig. 449, which gives a simplified circuit arrangement for two
connected stations.

[Illustration: Fig. 449. Monarch Intercommunicating System]

Referring to Fig. 449, when the station called depresses the home button
the talking circuit is then completed after the hook switch is raised.
This is because the talking battery is controlled by the home key.
Conductors from both the negative and the positive sides of the battery
enter this key. In the normal position of the springs, the negative side
of the battery is in contact with the master spring in the home key and
through these springs the negative battery is applied to all the calling
keys, and from there on to the hook switch. When, however, the home
button is operated, the spring which carries the negative battery to
the home key is opened, and the spring which carries the positive
battery is closed. This puts the positive battery on at the hook switch
instead of the negative battery, as in its normal condition.

In this system it is seen that a separate pair of line wires is used for
each station, and in addition to these, two common pairs are run to all
stations, one for ringing and one for talking battery connections.

=For Private Branch Exchanges.= So far the intercommunicating system has
been discussed only with respect to its use in small isolated plants. It
has a field of usefulness in connection with city exchange work, as it
may be made to serve admirably as a private branch exchange. Where this
is done, one or more trunk lines leading to an office of the city
exchange are run through the intercommunicating system exactly as a
local line in that system, being tapped to a jack or push button at
every station. A person at any one of the stations may originate a call
to the main office by inserting his plug in the trunk jack, or pushing
his trunk push button. Also any station, within hearing or sight of the
trunk-line signal from the main office, may answer a main-office call in
the same way. In order that the convenience of a private branch exchange
may be fully realized, however, it is customary to provide an
attendant's station at which is placed the drop or bell on which the
incoming trunk signal is received. The duty