current will be promptly cut off by the release
of the ringing key.

The trunk operator's "disconnect lamp" is shown at _4_, and it is to be
remembered that this lamp is lighted only when the _A_-operator takes
down the connection at her end, and also that this lamp is entirely out
of the control of the subscribers, the conditions which determine its
illumination being dependent on the positions of the operators' plugs at
the two ends of the trunk. With both plugs up, the lamp _4_ will receive
current, but will be shunted to prevent its illumination. The path over
which it receives this current may be traced from battery through the
lamp _4_, thence through the coil of the relay _9_ and the cut-off relay
of the called subscriber's line. This current would be sufficient to
illuminate the lamp, but the lamp is shunted by a circuit which may be
traced from the live side of battery through the contact of the relay
_13_, closed at the time, and through the coil of the trunk cut-off
relay coil _14_. The resistance of this coil is so proportioned to the
other parts of the circuit as to prevent the illumination of the lamp
just exactly as in the case of the shunting resistances of the lamps in
the _A_-operator's cord. It will be seen, therefore, that the supply of
current to the trunk disconnect lamp is dependent on the trunk plug
being inserted into the jack of the subscriber's line and that the
shunting out of this lamp is dependent on the energization of the relay
_13_. This relay _13_ is energized as long as the _A_-operator's plug is
inserted into the outgoing trunk jack, the path of the energizing
circuit being traced from the live side of the battery at the second
office through the right-hand winding of this relay, thence over the tip
side of the trunk to ground at the first office. From this it follows
that as long as both plugs are up, the disconnect lamp will receive
current but will be shunted out, and as soon as the _A_-operator pulls
down the connection, the relay _13_ will be de-energized and will thus
remove the shunt from about the lamp, allowing its illumination. The
left-hand winding of the relay _13_ performs no operating function, but
is merely to maintain the balance of the talking circuit, it being
bridged during the connection from the ring side of the trunk to ground
in order to balance the bridge connection of the right-hand coil from
the live side of battery to the tip side of the trunk circuit.

Th