ut its supervisory lamp. The
arrangement and proportion of these resistances is such that when a plug
is inserted into the jack of a line the lamp will receive current from a
circuit traced from the negative pole of the battery in the center of
the cord circuit through the lamp and the 80-ohm series resistance,
through the third strand of the cord to the test thimble of the jack,
and thence to the positive or grounded pole of the battery through the
third conductor in the multiple and the winding of the cut-off relay.
This current always flows as long as the plug is inserted, and it is
just sufficient to illuminate the lamp when the supervisory relay
armature is not attracted. When, however, the supervisory relay armature
is attracted, the shunting of the lamp by the 40-ohm resistance cuts
down the current to such a degree as to prevent the illumination of the
lamp, although some current still flows through it.

The usual ringing and listening key is associated with the calling plug,
and in some cases a ring-back key is associated with the answering plug,
but this is not standard practice.

_Operation._ The operation of this cord circuit in conjunction with the
line circuit of Fig. 345 may best be understood by reference to Fig.
347. This figure employs a little different arrangement of the line
circuit in order more clearly to indicate how the two lines may be
connected by a cord; a study of the two line circuits, however, will
show that they are identical in actual connections. It is to be
remembered that all of the battery symbols shown in this figure
represent in reality the same battery, separate symbols being shown for
greater simplicity in circuit connections.

We will assume the subscriber at Station _A_ calls for the subscriber at
Station _B_. The operation of the line relay and the consequent lighting
of the line lamp, and also the operation of the pilot relay will be
obvious from what has been stated. The response of the operator by
inserting the answering plug into the answering jack, and the throwing
of her listening key so as to bridge her talking circuit across the cord
in order to place herself in communication with the subscriber, is also
obvious. The insertion of the answering plug into the answering jack
completed the circuit through the third strand of the cord and the
winding of the cut-off relay of the calling line, and this accomplishes
three desirable results. The circuit so completed may be tr