which is bridged across the telephone circuit and operates the selector.
This relay has a resistance of 4,500 ohms and a high impedance, and
operates the selector mechanism which is a special modification of the
ratchet and pawl principle. The essential features of this selector are
the "step-up" selector wheel and a time wheel, normally held at the
bottom of an inclined track.

The operation of the selector magnet pushes the time wheel up the track
and allows it to roll down. If the magnet is operated rapidly, the wheel
does not get clear down before being pushed back again. A small pin on
the side of the pawl, engaging the selector wheel normally, opposes the
selector wheel teeth near their outer points. When the time wheel rolls
to the bottom of the track, however, the pawl is allowed to drop to the
bottom of the tooth. Some of the teeth on the selector wheel are formed
so that they will effectually engage with the pawl only when the latter
is in normal position, while others will engage only while the pawl is
at the bottom position; thus innumerable combinations can be made which
will respond to certain combinations of rapid impulses with intervals
between. The correct combination of impulses and intervals steps the
selector wheel clear around so that a contact is made. The selector
wheels at all other stations fail to reach their contact position
because at some point or points in their revolution the pawls have
slipped out, allowing the selector wheels to return "home."

The "answer-back" is provided in this selector by means of a few
inductive turns of the bell circuit which are wound on the selector
relay. The operation of the bell through these turns induces an
alternating current in the selector winding which flows out on the line
and is heard as a distinctive buzzing noise by the dispatcher.

[Illustration: Fig. 480. Cummings-Wray Dispatcher's Sender]

_The Cummings-Wray Selector._ Both of the selectors already described
are of a type known as the _individual-call_ selectors, meaning that
only one station at a time can be called. If a plurality of calls is
desired, the dispatcher calls one station after another. The third type
of selector in use today is of a type known as the _multiple-call_, in
which the dispatcher can call simultaneously as many stations as he
desires.

The Cummings-Wray selector and that of the Kellogg Switchboard and
Supply Company are of this type and operate on the principle of
sy