example, in systems where the removal of the receiver from the hook at
the subscriber's station closes the line circuit, causing current to
flow through the winding of the visible signal and so displaying it
until the receiver has been hung upon the hook or the circuit opened
by some operation at the central office. Visible signals of the
magnetic type of Fig. 23 have been widely used in connection with
common-battery systems, both for line signals and for supervisory
purposes, indicating the state and the progress of the connection and
conversation.

[Illustration: Fig. 23. Electromagnetic Visible Signal]

[Illustration: Fig. 24. Lamp Signal and Lens]

_Electric-Lamp Signal._ Incandescent electric lamps appeared in
telephony as a considerable element about 1890. They are better than
either form of mechanical visible signals because of three principal
qualities: simplicity and ease of restoring them to normal as compared
with drops; their compactness; and their greater prominence when
displayed. Of the latter quality, one may say that they are more
_insistent_, as they give out light instead of reflecting it, as do
all other visible signals. In its best form, the lamp signal is
mounted behind a hemispherical lens, either slightly clouded or cut in
facets. This lens serves to distribute the rays of light from the
lamp, with the result that the signal may be seen from a wide angle
with the axis of the lens, as shown in Fig. 24. This is of particular
advantage in connection with manual-switchboard connecting cords, as
it enables the signals to be mounted close to and even among the
cords, their great visible prominence when shining saving them from
being hidden.