do so. If, on making the test with one of the multiple jacks
of the line leading to Station _B_, that line is idle and free to be
connected with, its test rings will all be at zero potential because of
the fact that they are connected with ground through the cut-off relay
winding with no source of current connected with them. The tip of the
calling plug will also be at zero potential in making this test, because
it is connected to ground through the tip side of the calling-plug
circuit and one winding of the cord-circuit repeating coil. As a result
no flow of current will occur, the operator will receive no click, and
she will know that she is free to connect with the line. As soon as she
does so, by inserting the plug, the third strand of the cord will be
connected with the test thimble of the calling line and the resulting
flow of current will bring about three results, two of which are the
same, and one of which is slightly different from those described as
resulting from the insertion of the answering plug into the jack of the
calling line. First, the cut-off relay will be operated and cut off the
line signaling apparatus from the called line; second, a flow of current
will result through the calling supervisory lamp _5_, which in this case
will be sufficient to illuminate that lamp for the reason that the
called subscriber has not yet responded, the calling supervisory relay
_6_ has, therefore, not yet been energized, and the lamp has not,
therefore, been shunted by its associated resistance _7_; third, the
test thimbles of the called line will be raised to a potential above
that of the earth, and thus the line will be guarded against connection
at another section of the switchboard. As soon as the called subscriber
responds to the ringing current sent out by the operator, current will
flow over the cord circuit and over his line through his transmitter.
This will cause the calling supervisory relay to be energized and the
calling lamp to be extinguished. Both lamps _1_ and _5_ remain
extinguished as long as the connected subscribers are in conversation,
but as soon as either one of them hangs up his receiver the
corresponding lamp will be lighted, due to the de-energization of the
supervisory relay and the breaking of the shunt around the lamp. The
lighting of both lamps associated with a cord circuit is a signal to the
operator for disconnection.

[Illustration: TERMINAL ROOM IN MEDIUM-SIZED MANUAL OFFICE Relay R