office is a
long direct-current impulse, the first tooth being three or four times
as wide as the other teeth. This impulse operates both magnets of the
selector and attracts their armatures, which, in turn, cause two pawls
to engage with the ratchet wheel, while the remaining quick impulses
operate the "stepping-up" pawl and rotate the wheel the requisite number
of teeth. Retardation coils are placed in series with the selector in
order to choke back any lightning discharges which might come in over
the line. The selector contact, when operated, closes a bell circuit,
and it will be noted that both the selector and the bell are operated
from battery current coming over the main line through variable
resistances. There are, of course, a number of selectors bridged across
the circuit, and the variable resistance at each station is so adjusted
as to give each approximately 10 milliamperes, which allows a large
factor of safety for line leakage in wet weather. The drop across the
coils at 10 milliamperes is 38 volts. If these coils were not employed,
it is clear that the selectors nearer the dispatcher would get most of
the current and those further away very little.

[Illustration: Fig. 490. Selector Set--Western Electric System]

A time-signal contact is also indicated on the selector-circuit diagram
of Fig. 490. This is common to all offices and may be operated by a
special key in the dispatcher's office, thereby enabling him to send out
time signals over the telephone circuit.

[Illustration: Fig. 491. Gill Dispatcher's Station]

=Gill Circuits.= The circuit arrangement for the dispatcher's outfit of
the Gill system is shown in Fig. 491. This is similar to that of the
Western Electric system just described. The method of operation also is
similar, the mechanical means of accomplishing the selection being the
main point of difference. In Fig. 492 the wiring of the Gill selector at
a waystation for local-battery service is shown. The selector contact
closes the bell circuit in the station and a few windings of this
circuit are located on the selector magnets, as shown. These provide the
"answer-back" by inductive means.

[Illustration: Fig. 492. Gill Selector--Local Battery]

Fig. 493 shows the wiring of the waystation, central-energy Gill
selector. In this case, the local battery for the operation of the bell
is omitted and the bell is rung, as is the case of the Western Electric
selector, by the main sending bat