tery in the dispatcher's office.

[Illustration: Fig. 493. Gill Selector--Central Energy]

The sending keys of these two types of circuits differ, in that with the
local-battery selector the key contact is open after the selector has
operated, and the ringing of the bell must be stopped by the dispatcher
pressing a button or calling another station. Either of these operations
sends out a new current impulse which releases the selector and opens
its circuit.

With the central-energy selector, however, the contacts of the sending
key at the dispatcher's office remain closed after operation for a
definite length of time. This is obviously necessary in order that
battery may be kept on the line for the operation of the bell. In this
case the contacts remain closed during a certain portion of the
revolution of the key, and the bell stops ringing when that portion of
the revolution is completed. If, however, the dispatcher desires to give
any station a longer ring, he may do so by keeping the key contacts
closed through an auxiliary strap key as soon as he hears the
"answer-back" signal from the called station.

=Cummings-Wray Circuits.= The Cummings-Wray system, as previously
stated, is of the multiple-call type, operating with synchronous clocks.
Instead of operating one key after another in order to call a number of
stations, all the keys are operated at once and a starting key sets the
mechanism in motion which calls all these stations with one operation.
Fig. 494 shows the circuit arrangement of this system.

[Illustration: Fig. 494. Cummings-Wray System]

In order to ring one or more stations, the dispatcher presses the
corresponding key or keys and then operates the starting key. This
starting key maintains its contact for an appreciable length of time to
allow the clock mechanism to get under way and get clear of the
releasing magnet clutch. Closing the starting key operates the
clock-releasing magnet and also operates the two telegraph-line relays.
These send out an impulse of battery on the line operating the bridged
2,500-ohm line relays and, in turn, the selector releasing magnets;
thus, all the waystation clocks start in unison with the master clock.
The second hand arbor of each clock carries an arm, which at each
waystation is set at a different angle with the normal position than
that at any other station. Each of these arms makes contact precisely at
the moment the master-clock arm is passing over the c