d when this is substituted for the high-resistance winding of
the relay _2_, the tip relay on the calling side of the _A_-operator's
cord is energized, resulting in the extinguishing of the calling
supervisory lamp. The illumination of the _A_-operator's calling
supervisory lamp depends, therefore, on whether the high-resistance
relay _2_, or the low-resistance relay _4_, is bridged across the trunk,
and this depends on whether the relay _1_ is energized or not. The relay
_1_, being bridged from the tip side of the trunk circuit to ground and
serving as the means of supply of battery current to the called
subscriber, is operated whenever the called subscriber's receiver is
removed from its hook. Therefore, the called subscriber's hook controls
the operation of this relay _1_, which, in turn, controls the conditions
which cause the illumination or darkness of the calling supervisory lamp
at the distant office.

Assuming that the _A_-operator has received and answered a call, and has
communicated with the _B_-operator, telling her the number of the
called subscriber, and has received, in turn, the number of the trunk to
be used, and that both operators have put up the connection, then it
will be clear from what has been said that the calling supervisory lamp
of the _A_-operator will be lighted until the called subscriber removes
his receiver from its hook, because the tip relay in the _A_-operator's
cord circuit will not pull up through the 15,000-ohm resistance winding
of the relay _2_. As soon as the subscriber responds, however, the relay
_1_ will be operated by the current which supplies his transmitter. This
will substitute the low-resistance winding of the relay _4_ for the
high-resistance winding of the relay _2_, and this will permit the
energizing of the tip supervisory relay of the _A_-operator and put out
the calling supervisory lamp at her position. As in the Western Electric
circuit, therefore, the control of the _A_-operator's calling
supervisory lamp is from the called subscriber's station and is relayed
back over the trunk to the originating office.

In this circuit, manual instead of automatic ringing is employed,
therefore, unlike the Western Electric circuit, means must be provided
for notifying the B-operator when the calling subscriber has answered.
This is done by placing at the _B_-operator's position a ringing lamp
associated with each trunk cord, which is illuminated when the
_B_-operator places