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.

[Illustration:  Fig. 25.  Lamp Signal Controlled by Relay]

The qualities of electromagnets and lamps in these respects are used to advantage by the lamp signal arrangement shown in Fig. 25.  A relay is in series with the line and provides a large range of sensibility.  It is able to carry any current the central-office current source can pass through it.  The local circuit of the relay includes the lamp.  Energizing the relay lights the lamp, and the reverse; the lamp is thus isolated from danger and receives the current best adapted to its needs.

All lines are not long and when enclosed in cable or in well-insulated interior wire, may be only remotely in danger of being short-circuited.  Such conditions exist in private-branch exchanges, which are groups of telephones, usually local to limited premises, connected to a switchboard on those premises.  Such a situation permits the omission of the line relay, the lamp being directly in the line.  Fig. 26 shows the extreme simplicity of the arrangement, containing no moving parts or costly elements.  Lamps for such service have improved greatly since the demand began to grow.  The small bulk permitted by the need of compactness, the high filament resistance required for simplicity of the general power scheme of the system, and the need of considerable sturdiness in the completed thing have made the task a hard one.  The practical result, however, is a signal lamp which is highly satisfactory.