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.

The influence of the lamp signal was one of the potent ones in the
development of the type of multiple switchboard which is now universal
as the mechanism of large manual exchanges. The first large trial of
such an equipment was in 1896 in Worcester, Mass. No large and
successful multiple switchboard with any other type of signal has been
built since that time.

Any electric signal has upper and lower limits of current between
which it is to be actuated. It must receive current enough to operate
but not enough to become damaged by overheating. The magnetic types of
visible signals have a wider range between these limits than have lamp
signals. If current in a lamp is too little, its filament either will
not glow at all or merely at a dull red, insufficient for a proper
signal. If the current is too great, the filament is heated beyond its
strength and parts at the weakest place.

This range between current limits in magnetic visible signals is great
enough to enable them to be used direct in telephone lines,