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, the
operating current through the line and signal in series with a fixed
voltage at the central office being not harmfully great when the
entire line resistance is shunted out at or near the central office.
The increase of current may be as great as ten times without damage to
the winding of such a signal. In lamps, the safe margin is much less.
The current which just gives a sufficient lighting of the signal may
be about doubled with safety to the filament of the lamp. Consequently
it is not feasible to place the lamp directly in series with long
exposed lines. A short circuit of such a line near the central office
will burn it out.