TTERY ROOM AT MONMOUTH,
ILLINOIS]

=Simple Magneto System.= The schematic circuit arrangement of an
excellent form of magneto intercommunicating system is given in Fig.
441. In this, five metallic circuit lines are led to as many stations,
an ordinary two-contact open jack being tapped off of each line at each
station. A magneto bell of the bridging type is permanently bridged
across each line at the station to which that line belongs. The
telephone at each station is an ordinary bridging magneto set except
that its bell is, in each case, connected to the line as just stated.
Each telephone is connected through a flexible cord to a two-contact
plug adapted to fit into any of the jacks at the same station.

The operation is almost obvious. If a person at Station _A_ desires to
call Station _E_, he inserts his plug into the jack of line _E_ at his
station and turns his generator crank. The bell of Station _E_ rings
regardless of where the plug of that station may be. The person at
Station _E_ responds by inserting his own plug in the jack of line _E_,
after which the two parties are enabled to converse over a metallic
circuit. It makes no difference whether the persons, after talking,
leave these plugs in the jacks or take them out, since the position of
the plug does not alter the relation of the bell with the line.

[Illustration: Fig. 441. Magneto Intercommunicating System]

This system has the advantage of great simplicity and of being about as
"fool proof" as possible. It is, however, not quite as convenient to use
as the later common-battery systems which require no turning of a
generator crank.

=Common-Battery Systems.= In the more popular common-battery systems two
general plans of operation are in vogue, one employing a plug and jacks
at each station for switching the "home" instrument into circuit with
any line, and the other employing merely push buttons for doing the
same thing. These may be referred to as the plug type and the
push-button type, respectively.

[Illustration: Fig. 442. Plug Type of Common-Battery Intercommunicating
System]

_Kellogg Plug Type._ The circuits of a plug type of intercommunicating
system, as manufactured by the Kellogg Company, are shown in Fig. 442.
While only three stations are shown, the method of connecting more will
be obvious.

This system requires as many pairs of wires running to all stations as
there are stations, and in addition, two common wires for ringing
p