al. It is carried on
the rod _2_ pivoted on a lever _3_, which, in turn, is pivoted at _4_ in
a stationary portion of the framework. A spring _5_, secured to the
underside of the lever _3_ and projecting to the left beyond the pivot
_4_ of this lever, serves always to press the right-hand portion of the
lever _3_ forward in such direction as to tend to thrust it into the
contact bank. The plunger is normally held out of the contact bank by
means of the latch _6_ carried on the armature _7_ of the trip magnet.
When the trip magnet is energized it pulls the armature _7_ to the left
and thus releases the plunger and allows it to enter the contact bank.

[Illustration: POWER SWITCHBOARD FOR MEDIUM-SIZED OFFICE Mercury Arc
Rectifier Panel and Transformer at Right.]

The master bar is shown at _8_, and a feather on this bar engages a
notch in the segment attached to the rear end of the plunger rod _2_.
This master bar is common to all of the plunger rods and by its
oscillatory movement, under the influence of the master switch, it
always keeps all of the idle plunger bars pointed toward the contacts of
an idle trunk. As soon, however, as the trip magnet is operated to cause
the insertion of a plunger into the contact bank, the feather on the
master bar is disengaged by the notch in the segment of the plunger rod,
and the plunger rod is, therefore, no longer subject to the oscillating
movement of the master bar.

When the release magnet is energized, it attracts its armature _9_ and
this lifts the armature _7_ of the trip magnet so that the latch _6_
rides on top of the left-hand end of the lever _3_. Then, when the
release magnet is de-energized, the spring _5_, which was put under
tension by the latch, moves the entire structure of levers back to its
normal position, withdrawing the plunger from the bank of contacts. The
notch on the edge of the segment of the plunger rod, when thus released,
will probably not strike the feather on the master bar, and the plunger
rod will thus not come under the control of the master bar until the
master bar has moved, in its oscillation, so that the feather registers
with the notch, after which this bar will move with all the others.

If, while the plunger is waiting to be picked up by the master bar, the
same subscriber should call again, his line will be connected with the
same trunk as before. There is no danger in this, however, that the
trunk will be found busy, because the master b