y stated, a telephone
train-dispatching circuit may be from 25 to 300 miles in length, and
upon this may be as many stations as can be handled by one dispatcher.
The largest known number of stations upon an existing circuit of this
character is 65.

[Illustration: Fig. 488. Dispatcher's Station--Western Electric System]

_Dispatcher's Circuit Arrangement._ The circuits of the dispatcher's
station in the Western Electric system are shown in Fig. 488, the
operation of which is briefly as follows: When the dispatcher wishes to
call any particular station, he gives the key corresponding to that
station a quarter turn. This sends out a series of rapid direct-current
impulses on the telephone line through the contact of a special
telegraph relay which is operated by the key in a local circuit. The
telegraph relay is equipped with spark-eliminating condensers around its
contacts and is of heavy construction throughout in order to carry
properly the sending current.

_Voltage._ The voltage of the sending battery is dependent on the length
of the line and the number of stations upon it. It ranges from 100 to
300 volts in most cases. When higher voltages are required in order
successfully to operate the circuit, it is generally customary to
install a telegraph repeater circuit at the center of the line, in order
to keep the voltage within safe limits. One reason for limiting the
voltage employed is that the condensers used in the circuit will not
stand much higher potentials without danger of burning out. It is also
possible to halve the voltage by placing the dispatcher in the center of
the line, from which position he may signal in two directions instead of
from one end.

_Simultaneous Talking and Signaling._ Retardation coils and condensers
will be noticed in series with the circuit through which the signaling
current must pass before going out on the line. These are for the
purpose of absorbing the noise which is caused by high-voltage battery,
thus enabling the dispatcher to talk and signal simultaneously. The
250-ohm resistance connected across the circuit through one back contact
of the telegraph relay absorbs the discharge of the 6-microfarad
condenser.

[Illustration: Fig. 489. Selector Set--Western Electric System]

=Waystation Circuit.= The complete selector set for the waystations is
shown in Fig. 489, and the wiring diagram of its apparatus in Fig. 490.
The first impulse sent out by the key in the dispatcher's