calling side of the cord
circuit. The windings of these relays, therefore, act as impedance coils
and the arrangement by which battery current is supplied to the cord
circuits and, therefore, to the lines of the connected subscribers, is
seen to be the combined impedance coil and condenser arrangement
discussed in Chapter XIII.

As soon as a plug is inserted into the jack of a line, the line relay
will be removed from the control of the line, and since the two strands
of the cord circuit now form continuations of the two line conductors,
the supervisory relay will be substituted for the line relay and will be
under control of the line. Since all of the current which passes to the
line after a plug is inserted must pass through the cord-circuit
connection and through the relay windings, and since current can only
flow through the line when the subscriber's receiver is off its hook, it
follows that the supervisory relays will only be energized after the
corresponding plug has been inserted into a jack of the line and after
the subscriber has removed his receiver. Unlike the line relays, the
supervisory relays open their contacts to break the local circuits of
the supervisory lamps _7_ and _8_ when the relay coils are energized,
and to close them when de-energized; but the armatures of the
supervisory relays do alone control the circuits of the supervisory
lamps. These circuits are normally held open in another place, that is,
between the plug contacts _1_ and the jack thimbles. It is only,
therefore, when a plug is inserted into a jack and when the supervisory
relay is de-energized, that the supervisory lamp may be lighted. When a
plug is inserted into a jack and when the corresponding supervisory
relay is de-energized, the circuit may be traced from ground at the
cord-circuit batteries through the left-hand battery, for instance,
through lamp _7_, thence through the contacts of the supervisory relay
to the contact _1_ of the plug, thence through the thimble of the jack
to ground. When a plug is inserted into the jack, therefore, the
necessary arrangements are completed for the supervisory lamp to be
under the control of the subscriber. Under this condition, whenever the
subscriber's receiver is on its hook, the circuit of the line will be
broken, the supervisory relay will be de-energized, and the supervisory
lamp will be lighted. When, on the other hand, the subscriber's receiver
is off its hook, the circuit of the lin