_Telephone Receiver._ The telephone receiver itself serves a useful
purpose as an audible signal. An interrupted or alternating current of
proper frequency and amount will produce in it a musical tone which
can be heard throughout a large room. This fact enables a telephone
central office to signal a subscriber who has left his receiver off
the switch hook, so that normal conditions may be restored.

Visible Signals. _Electromagnetic Signal._ Practical visual signals
are of two general kinds: electromagnetic devices for moving a target
or pointer, and incandescent lamps. The earliest and most widely used
visible signal in telephone practice was the annunciator, having a
shutter adapted to fall when the magnet is energized. Fig. 22 is such
a signal. Shutter _1_ is held by the catch _2_ from dropping to the
right by its own gravity. The name "gravity-drop" is thus obvious.
Current energizing the core attracts the armature _3_, lifts the catch
_2_, and the shutter falls. A simple modification of the gravity-drop
produces the visible signal shown in Fig. 23. Energizing the core
lifts a target so as to render it visible through an opening in the
plate _1_. A contrast of color between the plate and the target
heightens the effect.

[Illustration: Fig. 22. Gravity-Drop]

The gravity-drop is principally adapted to the magneto-bell system of
signaling, where an alternating current is sent over the line to a
central office by the operation of a bell crank at the subscriber's
station, this current, lasting only as long as the crank is turned,
energizes the drop, which may be restored by hand or otherwise and
will remain latched. The visible signal is better adapted to lines in
which the signaling is done by means of direct current, as, for