Fig. 191. K.B. Lock-Out Station
[View full size illustration.]

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 of the operator. When these springs 8 are in their normal position, the ringing circuit is continued through the release-key springs 9 to the ringing generator. These springs 8 occupy their normal position only when the dial is in its normal position, this being due to the notch 10 in the contact wheel. At all other times, i.e., while the impulse wheel is out of its normal position, the springs 8-8 are either depressed so as to engage the lower battery contacts, or else held in an intermediate position so as to engage neither the battery contacts nor the generator contacts.

Fig. 192. Calling Apparatus K.B. System
[View full size illustration.]

When it is desired to call a given station, the operator pulls the subscriber's number on the dial and holds the ringing key closed, allowing the dial to return to normal. This connects the impulse battery to the subscriber's line as many times as is required to move the subscriber's sectors to the proper position, and in such direction as to cause the stepping movement of the various relays. As the impulse wheel comes to its normal position, the springs 8, associated with it, again engage their upper contacts, by virtue of the notch 10 in the impulse wheel, and this establishes the connection between the ringing generator and the subscriber's line, the ringing key being still held closed. The pulling of the transmitter dial and holding the ringing key closed, therefore, not only sends the stepping impulses to line, but also follows it by the ringing current. The sending of five impulses to line moves all of the sectors to the sixth notch, and this corresponds to the position necessary to make the fourth station operative. Such a condition is shown in Fig. 193, it being assumed that the subscriber at Station C originated the call and pressed his own button so as to prevent his sector from being moved out of its normal position. As a result of this, the talking circuit at Station C is left closed, and the talking and the ringing circuit of Station D, the called station, are closed, while both the talking and the ringing circuits of all the other stations are left open. Station D may, therefore, be rung and may communicate with Station C, while all of the other stations on the line are locked out, because of the fact that both their talking and ringing circuits are left open.