y lamps and the one at the
bottom being provided with a large lens for serving as a pilot lamp.

[Illustration: TERMINAL ROOM APPARATUS IN PROCESS OF INSTALLATION
Installed by Dean Electric Company at Detroit, Mich.]

=Mechanical Signals.= As has been stated the so-called mechanical
signals are sometimes used in small common-battery switchboards instead
of lamps. Where this is done the coil of the signal, if it is a line
signal, is substituted in the line circuit in place of the relay coil.
If the signals are used in connection with cord circuits for supervisory
signals, their coils are put in the circuit in place of the supervisory
relay coils. (These signals are referred to in Chapter III in connection
with Fig. 23.) They are so arranged that the attraction of the armature
lifts a target on the end of a lever, and this causes a display of color
or form. The release of the armature allows this target to drop back,
thus obliterating the display. Such signals, often called _visual
signals_ and _electromagnet signals_, should be distinguished from the
drops considered in connection with magneto switchboards in which the
attraction of the armature causes the display of the signal by the
falling of a drop, the signal remaining displayed until restored by some
other means, the restoration depending in no wise on when the armature
is released.

_Western Electric._ The mechanical signal of the Western Electric
Company, shown in Fig. 320, has a target similar to that shown in Fig.
254 but without a latch. It is turned to show a different color by the
attraction of the armature and allowed to resume its normal position
when the armature is released.

[Illustration: Fig. 320. Mechanical Signal]

_Kellogg._ Fig. 321 gives a good idea of a strip of mechanical signals
as manufactured by the Kellogg Company. This is known as the _gridiron_
signal on account of the cross-bar striping of its target. The white
bars on the target normally lie just behind the cross-bars on the shield
in front, but a slight raising of the target--about one-eighth of an
inch--exposes these white bars to view, opposite the rectangular
openings in the front shield.

[Illustration: Fig. 321. Strip of Gridiron Signals]

_Monarch._ In Fig. 322 is shown the visual signal manufactured by the
Monarch Telephone Company.

[Illustration: Fig. 322. Mechanical Signal]

=Relays.= The line relays for common-battery switchboards likewise
assume a great variety o