the methods by which current
is supplied to the subscribers' transmitters in common-battery systems
as discussed in Chapter XIII. While the arrangements in this respect of
Figs. 311, 312, and 313 illustrate only three of the methods, these
three are the ones that have been most widely and successfully used.

_Supervisory Signals._ The signals that are associated with the cord
circuits are termed supervisory signals because of the fact that by
their means the operator is enabled to supervise the condition of the
lines during times when they are connected for conversation. The
operation of these supervisory signals may be best understood by
considering the complete circuits of a simple switchboard and must be
studied in conjunction with the circuits of the lines as well as those
of the cords.

[Illustration: Fig. 314. Simple Common-Battery Switchboard]

_Complete Circuit._ Such complete circuits are shown in Fig. 314. The
particular arrangement indicated is that employed by the Kellogg
Company, and except for minor details may be considered as typical of
other makes also. Two subscribers' lines are shown extending from
Station A and Station B, respectively, to the central office. The line
wires are shown terminating in jacks in the same manner as indicated in
Figs. 307, 308, and 309, and their circuits are normally continued from
these jacks to the ground on one side and to the line relay and battery
on the other. The jack in this case has three contacts adapted to
register with three corresponding contacts in each of the plugs. The
thimble of the jack in this case forms no part of the talking circuit
and is distinct from the two jack springs which form the line terminals.
It and the auxiliary contact _1_ in each of the plugs with which it
registers, are solely for the purpose of co-operating in the control of
the supervisory signals.

The tip and sleeve strands of the cord are continuous from one plug to
the other except for the condensers. The two batteries indicated in
connection with the cord circuit are separate batteries, a
characteristic of the Kellogg system. One of these batteries serves to
supply current to the tip and sleeve strand of the cord circuit through
the two windings _3_ and _4_, respectively, of the supervisory relay
connected with the answering side of the cord circuit, while the other
battery similarly supplies current through the windings _5_ and _6_ of
the supervisory relay associated with the