. Western Electric Selector]

[Illustration: Fig. 476. Western Electric Selector]

_The Western Electric Selector._ This selector is the latest and perhaps
the simplest. Fig. 475 shows it with its glass dust-proof cover on, and
Fig. 476 shows it with the cover removed. This selector is adapted for
operating at high speed, stations being called at the rate of ten per
second.

The operating mechanism, which is mounted on the front of the selector
so as to be readily accessible, works on the central-energy
principle--the battery for its operation, as well as for the operation
of the bell used in connection with it, both being located at the
dispatcher's office. The bell battery may, however, be placed at the
waystation if this is desired.

The selector consists of two electromagnets which are bridged in series
across the telephone circuit and are of very high impedance. It is
possible to place as many of these selectors as may be desired across a
circuit without seriously affecting the telephonic transmission.
Direct-current impulses sent out by the dispatcher operate these
magnets, one of which is slow and the other quick-acting. The first
impulse sent out is a long impulse and pulls up both armatures, thereby
causing the pawls above and below the small ratchet wheel, shown in Fig.
476, to engage with this wheel. The remaining impulses operate the
quick-acting magnet and step the wheel around the proper number of
teeth, but do not affect the slow-acting magnet which remains held up by
them. The pawl connected to the slow-acting magnet merely serves to
prevent the ratchet wheel from turning back. Attached to the ratchet
wheel is a contact whose position can be varied in relation to the
stationary contact on the left of the selector with which this engages.
This contact is set so that when the wheel has been rotated the desired
number of teeth, the two contacts will make and the bell be rung. Any
selector may thus be adjusted for any station, and the selectors are
thus interchangeable. When the current is removed from the line at the
dispatcher's office, the armatures fall back and everything is restored
to normal. An "answer-back" signal is provided with this selector
dependent upon the operation of the bell. When the selector at a station
operates, the bell normally rings for a few seconds. The dispatcher,
however, can hold this ring for any length of time desired.

The keys employed at the dispatcher's office for op