The cord circuit is of the three-conductor type, the two talking strands
extending to the usual split repeating-coil arrangement, and battery
current for talking purposes being fed through these windings as in the
standard No. 1 board. The supervisory relay is included in the ring
strand of the cord circuit and is shunted by a non-inductive resistance,
so that its impedance will not interfere with the talking currents. The
armature of the supervisory relay closes the lamp contact on its back
stroke, so that the lamp is always held extinguished when the relay is
energized. The supervisory lamp is included in a connection between the
back contact of the supervisory relay and ground, this connection
including the central-office battery. As a result, the illumination of
the supervisory lamp is impossible until a plug has been inserted into a
jack, in which case, assuming the supervisory relay to be de-energized,
the lamp circuit is completed through the wire connecting all of the
test thimbles and the resistance permanently bridged to ground from that
wire.

_Test._ For purposes of the test it is evident that the test rings of an
idle line are always at ground potential, due to their connection to
ground through the resistance coil. It is also evident that the tip of
an unused calling plug will always be at ground potential and,
therefore, that the testing of an idle line will result in no click in
the operator's receiver. When a line is switched, however, the potential
of all the test rings will be raised due to their being connected with
the live pole of the battery through the third strand of the cord. When
the operator in testing touches the test contact of the jack of a busy
line, a current will, therefore, flow from this test contact to the tip
strand of the cord and thence to ground through one of the repeating
coil windings. The potential of the tip side of the cord will,
therefore, be momentarily altered, and this will result in a click in
the operator's receiver bridged across the cord circuit at the time. The
details of the operator's cord circuit and of the pilot lamp and night
alarm circuits will be clear from the diagram.

_Operation._ A brief summary of the operation of this system is as
follows:

The subscriber removes his receiver from its hook, thus drawing up the
armature of the line relay and lighting his line lamp. The operator
answers. The line lamp is extinguished by the falling back of the
line