pring _19_--which button may be seen projecting from the instrument
shown in Fig. 382, and from the base of the one shown in Fig. 383--is
pressed, the continuity of the talking circuit is interrupted and the
vertical side of the line is connected with the ground. It is by this
operation, after the connection has been made with the desired
subscriber's line, that the central-office apparatus acts to send
ringing current out on that line.

Release Springs:--The fifth set of springs is the one shown at the
left-hand side of Fig. 384, embracing springs _21_, _22_, and _23_. The
long curved spring _21_ is engaged by the projecting lug on the switch
hook when it rises so as to press this spring away from the other two.
On the return movement of the hook, however, this spring is pressed to
the left so as to bring all three of them into contact, and this, it
will be seen, grounds both limbs of the line at the subscriber's
station. This combination cannot be effected by any of the other springs
at any stage of their operation, and it is the one which results in the
energization of such a combination of relays and magnets at the central
office as will release all parts involved in the connection and allow
them to return to their normal positions ready for another call.

_Salient Points._ If the following things are borne in mind about the
operation of the subscriber's station apparatus, an understanding of the
central-office operations will be facilitated. First, the selective
impulses always flow over the vertical side of the line; they are always
preceded and always followed by a single impulse over the rotary side of
the line. The ringing button grounds the vertical side of the line and
the release springs ground both sides of the line simultaneously.

=The Line Switch.= The first thing to be considered in connection with
the central-office apparatus is the line switch. This, it will be
remembered, is the device introduced into each subscriber's line at the
central office for the purpose of effecting a reduction of the number of
first selectors required at the central office, and also for bringing
about certain important functional results in connection with trunking
between central and sub-offices. The function of the line switch in
connection with the subscriber's line, however, is purely that of
reducing the number of first selectors.

The line switches of one hundred lines are all associated to form a
single unit of app