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 shown at the bottom of the dial in Fig. 192.  The dial is then allowed to return by the action of a spring to its normal position, and in doing so it operates a switch within the box to make and break the battery circuit the proper number of times.

Operation. A complete description of the operation may now be had in connection with Fig. 193, which is similar to Fig. 189, but contains the details of the calling arrangement at the central office and also of the talking circuits at the various subscribers’ stations.

[Illustration:  Fig. 191.  K.B.  Lock-Out Station]

Referring to the central-office apparatus the usual ringing key is shown, the inside contacts of which lead to the listening key and to the operator’s telephone set as in ordinary switchboard practice.  Between the outside contact of this ringing key and the ringing generator there is interposed a pair of contact springs 8-8 and another pair 9-9.  The contact springs 8 are adapted to be moved backward and forward by the impulse wheel which is directly controlled by the dial under the manipulation