cut-off relay _12_ is provided
with two windings, that at the right being in the path of the
alternating ringing currents that are supplied to the alternating
current key, and that at the left being in the ground return path for
all of the pulsating ringing currents supplied to the pulsating keys.
With this explanation it is believed that this circuit will be
understood from what has been said in connection with Fig. 371. The
operation of the holding coil _8_ is the same in each case, the holding
magnet in Fig. 372 serving to hold depressed any one of the five ringing
keys that may have been used in calling the subscriber.

[Illustration: AUTOMATIC EQUIPMENT, MAIN OFFICE, BERKELEY, CALIFORNIA A
Feature of Interest Here is That the Cement Floor is Treated with a
Filler and Painted, with No Other Covering.]

[Illustration: Fig. 373. Western Electric Trunk Ringing Key]

The standard four-party line, trunk ringing key of the Western Electric
Company is shown in Fig. 373. In this the various keys operate not by
pressure but rather by being pulled by the finger of the operator in
such a way as to subject the key shaft to a twisting movement. The
holding magnet lies on the side opposite to that shown in the figure and
extends along the full length of the set of keys, each key shaft being
provided with an armature which is held by this magnet until the magnet
is de-energized by the action of the ringing cut-off relay.

[Illustration: Fig. 374. Trunk Relay]

[Illustration: Fig. 375. Trunk Relay]

The standard trunk relays employed by the Western Electric Company in
connection with the circuits just described are shown in Figs. 374 and
375. In each case the dust-cap or shield is also shown. The relay of
Fig. 374 is similar to the regular cut-off relay and is the one used for
relays _9_ and _14_ of Figs. 371 and 372. The relay of Fig. 375 is
somewhat similar to the subscriber's line relay in that it has a tilting
armature, and is the one used at _13_ in Figs. 371 and 372. The trunk
relay _3_ in Figs. 371 and 372 is the same as the _A_-operator's
supervisory relays already discussed.

It has been stated that under certain circumstances _B_-operator's trunk
circuits devoid of ringing keys, and consequently of all keys, may be
employed. This, so far as the practice of the Bell companies is
concerned, is true only in offices where there are no party lines, or
where, as in many of the Chicago offices, the party lines are worked