he closure of contacts _19_ and _20_ by the plunger causes
the operation of the bridge cut-off relay which opens the circuit of the
trip magnet, rendering it inoperative; and also establishes ground
potential on all the private wire contacts of that line in the banks of
the connectors, so as to guard the line and its associated apparatus
against intrusion by others. The line is cut through, therefore, to a
first selector and all of the line-switch apparatus is completely cut
off from the talking circuit.

It must be remembered that all of the actions of the line switch, which
it has taken so long to describe, occur practically instantaneously and
as a result of the first part of the first movement of the subscriber's
dial. The line switch has done its work and "gone out of business"
before the selective impulses of the first digit begin to take place.

=Selecting Switches.= The first selector is now in control of the
calling subscriber. The circuits and elements of the first selector
switch are shown in Fig. 390. The general mechanical structure of the
first selectors, second selectors, and connectors, is the same and may
be referred to briefly here. Fig. 391 shows a rear view of a first
selector; Fig. 392, a side view of a second selector; and Fig. 393, a
front view of a connector. The arrangement of the vertical and rotary
magnets, of the selector shafts, and of the contact banks are identical
in all three of these pieces of apparatus and all these switches work on
the "up-and-around principle" referred to in connection with Fig. 380.
It is thought that with the general structure shown in Figs. 391, 392,
and 393 in mind, the actual operation may be understood much more
readily from Fig. 390.

Four magnets--the vertical, the rotary, the private, and the
release--produce the switching movements of the machine. These magnets
are controlled by various combinations brought upon the circuits by
three relays--the vertical, the rotary, and the back release. The fourth
relay shown, called the _off-normal_, is purely for signaling purposes,
as will be described.

_Side Switch._ Another important element of the selecting switches is
the so-called side switch which might better be called a pilot
switch--but we are not responsible for its name. This side switch has
for its function the changing of the control of the subscriber's line to
successive portions of the selector mechanism, rendering inoperative
those portions that ha