ill be lighted because, as will be shown, it is not yet
shunted out by the operation of its associated supervisory relay.
Tracing the circuit of the calling supervisory relay of the
_A_-operator's circuit, it will be found to pass from the live side of
the battery to the ring side of the trunk circuit through one winding of
the repeating coil of the _B_-operator's cord; beyond this the circuit
is open, since no path exists through the condenser _2_ bridged across
the trunk circuit or through the normally open contacts of the relay _3_
connected in the talking circuit of the trunk. The association of this
relay _3_ with the repeating coil and the battery of the trunk is seen
to be just the same as that of a supervisory relay in the _A_-operator's
cord, and it is clear, therefore, that this relay _3_ will not be
energized until the called subscriber has responded. When it is
energized it will complete the path to ground through the _A_-operator's
calling supervisory relay and operate to shunt out the _A_-operator's
calling supervisory lamp in just the same manner as if the
_A_-operator's calling plug had been connected directly with the line of
the calling subscriber. In other words, the called subscriber in the
second office controls the relay _3_, which, in turn, controls the
calling supervisory relay of the _A_-operator, which, in turn, shunts
out its lamp.

The connection being completed between the two subscribers, the
_B_-operator depresses one or the other of the ringing keys _5_ or _6_,
according to which party on the line is called, assuming that it is a
two-party line. It will be noticed that the springs of these ringing
keys are not serially arranged in the talking circuit, but the cutting
off of the trunk circuit back of the ringing keys is accomplished by the
set of springs shown just at the left of the ringing keys, which set of
springs _7_ is operated whenever either one of the ringing keys is
depressed. An auxiliary pair of contacts, shown just below the group of
springs _7_, is also operated mechanically whenever either one of the
ringing keys is depressed, and this serves to close one of two normally
open points in the circuit of the ringing-key holding magnet _8_. This
holding magnet _8_ is so arranged with respect to the contacts of the
ringing key that whenever any one of them is depressed by the operator,
it will be held depressed as long as the magnet is energized just the
same as if the operator ke