erating selectors are
shown in Fig. 477. There is one key for each waystation on the line and
the dispatcher calls any station by merely giving the corresponding key
a quarter turn to the right. Fig. 478 shows the mechanism of one of
these keys and the means employed for sending out current impulses over
the circuit. The key is adjustable and may be arranged for any station
desired by means of the movable cams shown on the rear in Fig. 478,
these cams, when occupying different positions, serving to cover
different numbers of the teeth of the impulse wheel which operate the
impulse contacts.

[Illustration: Fig. 477. Dispatcher's Keys]

[Illustration: Fig. 478. Dispatcher's Key Mechanism]

_The Gill Selector._ The second type of selector in extensive use
throughout the country today is known as the Gill, after its inventor.
It is manufactured for both local-battery and central-energy types, the
latter being the latest development of this selector. With the
local-battery type, the waystation bell rings until stopped by the
dispatcher. With the central-energy type it rings a definite length of
time and can be held for a longer period as is the case with the Western
Electric selector. The selector is operated by combinations of
direct-current impulses which are sent out over the line by keys in the
dispatcher's office.

[Illustration: Fig. 479. Gill Selector]

The dispatcher has a key cabinet, and calls in the same way as already
described, but these keys instead of sending a series of quick impulses,
send a succession of impulses with intervals between corresponding to
the particular arrangement of teeth in the corresponding waystation
selector wheel. Each key, therefore, belongs definitely with a certain
selector and can be used in connection with no other.

A concrete example may make this clearer. The dispatcher may operate key
No. 1421. This key starts a clockwork mechanism which impresses at
regular intervals, on the telephone line, direct-current impulses, with
intervals between as follows: 1-4-2-1. There is on the line one selector
corresponding to this combination and it alone, of all the selectors on
the circuit, will step its wheel clear around so that contact is made
and the bell is rung. In all the others, the pawls will have slipped out
at some point of the revolution and the wheels will have returned to
their normal positions.

The Gill selector is shown in Fig. 479. It contains a double-wound relay