aratus, which, besides the individual line switches,
includes certain other apparatus common to those lines. Such a group of
one hundred line switches and associated common apparatus is called a
_line-switch unit_, or frequently, a _Keith unit_. Confusion is likely
to arise in the mind of the reader between the individual line switch
and the line-switch unit, and to avoid this we will refer to the piece
of apparatus individual to the line as the line switch, and to the
complete unit formed of one hundred of these devices as a line-switch
unit.

_Line and Trunk Contacts._ Each line switch has its own bank of contacts
arranged in the arc of a circle, and in this same arc are also placed
the contacts of each of the ten individual trunks which it is possible
for that line to appropriate. The contacts individual to the
subscriber's line in the line switch are all multipled together, the
arrangement being such that if a wedge or plunger is inserted at any
point, the line contacts will be squeezed out of their normal position
so as to engage the contacts of the trunk corresponding to the
particular position in the arc at which the wedge or plunger is
inserted. A small plunger individual to each line is so arranged that it
may be thrust in between the contact springs in the line-switch bank in
such manner as to connect any one of the trunks with the line terminals
represented in that row, the particular trunk so connected depending on
the portion of the arc toward which the plunger is pointed at the time
it is thrust in the contacts.

These banks of lines and trunk contacts are horizontally arranged, and
piled in vertical columns of twenty-five line switches each. The ten
trunk contacts are multipled vertically through the line-switch banks,
so that the same ten trunks are available to each of the twenty-five
lines. We thus have, in effect, an old style, Western Union, cross-bar
switchboard, the line contacts being represented in horizontal rows and
the trunk contacts in vertical rows, the connection between any line and
any trunk being completed by inserting a plunger at the point of
intersection of the horizontal and the vertical rows corresponding to
that line and trunk.

_Trunk Selection._ The plungers by which the lines and trunks are
connected are, as has been said, individual to the line, and all of the
twenty-five plungers in a vertical row are mounted in such manner as to
be normally held in the same vertical plane,