e will be complete, the
supervisory relay will be energized, and the supervisory lamp will be
extinguished.

_Salient Features of Supervisory Operation._ It will facilitate the
student's understanding of the requirements and mode of operation of
common-battery supervisory signals in manual systems, whether simple or
multiple, if he will firmly fix the following facts in his mind. In
order that the supervisory signal may become operative at all, some act
must be performed by the operator--this being usually the act of
plugging into a jack--and then, until the connection is taken down, the
supervisory signal is under the control of the subscriber, and it is
displayed only when the subscriber's receiver is placed on its hook.

_Cycle of Operations._ We may now trace through the complete cycle of
operations of the simple common-battery switchboard, the circuits of
which are shown in Fig. 314. Assume all apparatus in its normal
condition, and then assume that the subscriber at Station A removes his
receiver from its hook. This pulls up the line relay and lights the line
lamp, the pilot relay also pulling up and lighting the common pilot lamp
which is not shown. In response to this call, the operator inserts the
answering plug and throws her listening key _L.K._ The operator's
talking set is thus bridged across the cord circuit and she is enabled
to converse with the calling subscriber. The answering supervisory lamp
_7_ did not light when the operator inserted the answering plug into the
jack, because, although the contacts in the lamp circuit were closed by
the plug contact _1_ engaging the thimble of the jack, the lamp circuit
was held open by the attraction of the supervisory relay armature, the
subscriber's receiver being off its hook. Learning that the called-for
subscriber is the one at Station B, the operator inserts the calling
plug into the jack at that station and presses the ringing key _R.K._,
in order to ring the bell. The act of plugging in, it will be
remembered, cuts off the line-signaling apparatus from connection with
that line. As the subscriber at Station B was not at his telephone when
called and his receiver was, therefore, on its hook, the insertion of
the calling plug did not energize the supervisory relay coils _5_ and
_6_, and, therefore, that relay did not attract its armature. The
supervisory lamp _8_ was thus lighted, the circuit being from ground
through the right-hand cord-circuit battery, lam