vidual to the line are shown in Fig. 385, the line and
trunk contact bank being shown in the lower portion of this figure and
also in a separate view in the detached figure at the right. A detailed
group of several such line switches with the oscillating master bar is
shown in Fig. 386. This figure shows quite clearly the relative
arrangement of the line and trunk contact banks, the plungers for each
bank, and the master bar.

[Illustration: Fig. 385. Line Switch]

In practice, four groups of twenty-five line switches each are mounted
on a single framework and the group of one hundred line switches,
together with certain other portions of the apparatus that will be
referred to later, form a line-switch unit. A front view of such a unit
is shown in Fig. 387. In order to give access to all portions of the
wiring and apparatus, the framework supporting each column of fifty line
switches is hinged so as to open up the interior of the device as a
whole. A line-switch unit thus opened out is shown in Fig. 388.

[Illustration: Fig. 386. Portion of Line-Switch Unit]

_Circuit Operation._ The mode of operation of the line switch may be
best understood in connection with Fig. 389, which shows in a schematic
way the parts of a line switch that are individual to a subscriber's
line, and also those that are common to a group of fifty or one hundred
lines. Those portions of Fig. 389 which are individual to the line are
shown below the dotted line extending across the page. The task of
understanding the line switch will be made somewhat easier if Figs. 385
and 389 are considered together. The individual parts of the line switch
are shown in the same relation to each other in these two figures with
the exception that the bank of line and trunk springs in the lower
right-hand corner of Fig. 389 have been turned around edgewise so as to
make an understanding of their circuit connections possible.

[Illustration: Fig. 387. Line-Switch Unit]

[Illustration: Fig. 388. Line-Switch Unit]

[Illustration: Fig. 389. Circuits of Line-Switch Unit]

The vertical and rotary sides of the subscriber's line are shown
entering at the lower left-hand corner of this figure, and they pass to
the springs of the contact bank. Immediately adjacent to these springs
are the trunk contacts from which the vertical and the rotary limbs of
the first selector trunk proceed. The plunger is indicated at _1_, it
being in the form of a wheel of insulating materi