When the bell is rung, the first stroke it makes unlatches the springs, which assume the position shown in the right-hand cut of Fig. 199, and this, it will be seen from Fig. 200, establishes proper conditions for enabling the subscriber to transmit and to receive speech.

The hook switch breaks both transmitter and receiver circuits when down and in raising it establishes a momentary circuit between the ground and the limb L of the line, both upper and lower hook contacts engaging the hook lever simultaneously during the rising of the hook.

The mechanism at the central office by which selection of the proper station is made in a rapid manner is shown in Fig. 201.  It has already been stated that the selection of the proper subscriber is brought about by the sending of a predetermined number of impulses from the central office, these impulses passing in one direction only and over the metallic circuit.  After the proper party has been reached, the ringing current is put on in the reverse direction.

[Illustration:  Fig. 201.  Central-Office Impulse Transmitter]

The operator establishes the number of impulses to be sent by placing the pointer opposite the number on the dial corresponding to the station wanted.  The ratchet wheel is stepped around automatically by each impulse of current from an ordinary pole changer such as is employed in ringing biased bells.  When the required number of impulses has been sent, a projection, carried on a group of springs, drops into a notch on the drum of the selector shaft, which operation instantly stops the selecting current impulses and at the same time throws on the ringing current which consists of impulses in the reverse direction.  So rapidly does this device operate that it will readily follow the impulses of an ordinary pole changer, even when this is adjusted to its maximum rate of vibration.

[Illustration:  VIEW OF A LARGE FOREIGN MULTIPLE SWITCHBOARD]

Operation. Space will not permit a full discussion of the details of the central-office selective apparatus, but a general resume of the operation of the system may now be given, with the aid of Fig. 202, which shows a four-station line with the circuits of three of the stations somewhat simplified.  In this figure Station A, Station B, and Station D are shown in their locked-out positions, A and B having been passed by the selection and ringing of Station C, while Station D is inoperative because it was not reached in the selection and the line is still broken at Station C. Station C, therefore, has possession of the line.

When the subscriber at Station C raised his receiver in order to call central, a “flash” contact was made as the hook moved up, which momentarily grounded the limb L of the line. (See Fig. 200.) This “flash” contact is produced by the arrangement of the hook which assures that the lower contact shall, by virtue of its flexibility, follow up the hook lever until the hook lever engages the upper contact, after which the lower contact breaks.  This results in the momentary connection of both the upper and the lower contacts of the hook with the lever, and, therefore, the momentary grounding of the limb L of the line.  This limb always being continuous serves, when this “flash” contact is made, to actuate the line signal at the central office.