istening key is
operated--and thence to ground through a retardation coil _14_. No
battery or other source of potential exists in this circuit between
ground and the tip of the calling plug and, therefore, the tip is
normally at ground potential. The sleeve ring of the jack being at
ground potential if the line is idle, no current will flow and no click
will be produced in testing such a line. If, however, the line is busy,
the test ring will be at a higher potential and, therefore, current will
flow from the tip of the calling plug to ground over the path just
traced, and this will cause a rise in potential at the terminal of the
condenser _15_ and a momentary flow of current through the tertiary
winding _16_ of the operator's induction coil; hence the click.

[Illustration: SWITCH ROOM OF CITIZENS' TELEPHONE COMPANY, GRAND RAPIDS,
MICH. One of the Earliest Large Automatic Offices.]

Obviously the testing circuit from the tip of the calling plug to ground
at the operator's set is only useful during the time when the calling
plug is not in a jack, and as the tip strand of the calling plug has to
do double duty in testing and in serving as a part of the talking
circuit, the arrangement is made that the testing circuit will be
automatically broken and the talking circuit through the tip strand
automatically completed when the plug is inserted into a jack in
establishing a connection. This is accomplished by means of the extra
contact on the relay _4_, which relay, it will be remembered, is held
energized when its corresponding plug is inserted in a jack. During the
time when the plug is not inserted, this relay is not energized and the
test circuit is completed through the back contact of its right-hand
armature. When connection is made at the jack, this relay becomes
energized and the tip strand of the cord circuit is made complete by the
right-hand lever being pulled against the front contact of this relay.
The keys shown to the right of the operator's set are order-wire keys.

_Summary of Operation._ We may give a brief summary of the operation of
this system as shown in Fig. 352. The left-hand station calls and the
line relay pulls up, lighting the lamp. The operator inserts an
answering plug in the answering jack, thus energizing the cut-off relay
which operates to cut off the line relay and to complete the connection
between the jacks and the external line. The act of plugging in by the
operator also raises the po