that current is fed to the subscriber's station, and, therefore, the
armature of this relay is responsive to the movements of the
subscriber's hook. As the relay _3_ holds the supervisory lamp circuit
closed as long as a plug is inserted in a jack of the line, it follows
that during a connection the relay _4_ will have entire control of the
supervisory lamp.

_Listening Key._ The listening key, as usual, serves to connect the
operator's set across the talking strands of the cord circuit, and the
action of this in connection with the operator's set needs no further
explanation.

_Ringing Keys._ The ringing-key arrangement illustrated is adapted for
use with harmonic ringing, the single springs _5_, _6_, _7_, and _8_
each being controlled by a separate button and serving to select the
particular frequency that is to be sent to line. The two springs _9_ and
_10_ always act to open the cord circuit back of the ringing keys,
whenever any one of the selective buttons is depressed, in order to
prevent interference by ringing current with the other operations of the
circuit.

Two views of these ringing keys are shown in Figs. 355 and 356. Fig. 356
is an end view of the entire set. In Fig. 355 the listening key is shown
at the extreme right and the four selective buttons at the left. When a
button is released it rises far enough to cause the disengagement of the
contacts, but remains partially depressed to serve as an indication that
it was last used. The group of springs at the extreme left of Fig. 355
are the ones represented at _9_ and _10_ in Fig. 354 and by the anvils
with which those springs co-operate.

[Illustration: Fig. 354. Dean Multiple Board Circuits]

_Test._ The test in this Dean system is simple, and, like the Western
Electric and Kellogg systems, it depends on the raising of the
potential of the test thimbles of all the line jacks of a line when a
connection is made with that line by a plug at any position. When an
operator makes a test by applying the tip of the calling plug to the
test thimble of a busy line, current passes from the test thimble
through the tip strand of the cord to ground through the left-hand
winding of the calling supervisory relay _4_. The drop of potential
through this winding causes the tip strand of the cord to be raised to a
higher potential than it was before, and as a result the upper plate of
the condenser _11_ is thus altered in potential and this change in
potential across t