; all
of the other segments are, however, stepped up until the ringing and
talking circuits of the desired station are in proper position, at
which time ringing current is sent over the line. The segments in Fig.
189, except at Station C, are shown as having been stepped up to the
sixth position, which corresponds to the ringing position of the
fourth station, or Station D. The condition shown in this figure
corresponds to that in which the subscriber at Station C originated
the call and pressed his button, thus retaining his own segment in its
normal position so that the talking circuits would be established with
Station D.

When the line is in normal position any subscriber may call central by
his magneto generator, not shown in Fig. 189, which will operate the
drop at central, but will not operate any of the subscribers' bells,
because all bell circuits are normally open. When a subscriber desires
connection with another line, the operator sends an impulse back on
the line which steps up and locks out all instruments except that of
the calling subscriber.

[Illustration: Fig. 190. K.B. Lock-Out Station]

A complete K.B. lock-out telephone is shown in Fig. 190. This is the
type of instrument that is usually furnished when new equipment is
ordered. If, however, it is desired to use the K.B. system in
connection with telephones of the ordinary bridging type that are
already in service, the lock-out and selective mechanism, which is
shown on the upper inner face of the door in Fig. 190, is furnished
separately in a box that may be mounted close to the regular telephone
and connected thereto by suitable wires, as shown in Fig. 191. It is
seen that this instrument employs a local battery for talking and also
a magneto generator for calling the central office.

The central-office equipment consists of a dial connected with an
impulse wheel, together with suitable keys by which the various
circuits may be manipulated. This dial and its associated mechanism
may be mounted in the regular switchboard cabinet, or it may be
furnished in a separate box and mounted alongside of the cabinet in
either of the positions shown at _1_ or _2_ of Fig. 192.

In order to send the proper number of impulses to the line to call a
given party, the operator places her finger in the hole in the dial
that bears the number corresponding to the station wanted and rotates
the dial until the finger is brought into engagement with the fixed
stop