A lamp in which the light is produced by a voltaic arc. Carbon
electrodes are almost universally employed. Special mechanism, operating
partly by spring or gravity and partly by electricity, is employed to
regulate the distance apart of the carbons, to let them touch when no
current passes, and to separate them when current is first turned on.

The most varied constructions have been employed, examples of which will
be found in their places. Lamps may in general be divided into classes
as follows, according to their regulating mechanism and other features:

(a) Single light regulators or monophotes. Lamps through whose
regulating mechanism the whole current passes. These are only adapted to
work singly; if several are placed in series on the same circuit, the
action of one regulator interferes with that of the next one.

(b) Multiple light regulators or polyphotes. In these the regulating
mechanism and the carbons with their arc are in parallel; the regulating
device may be a single magnet or solenoid constituting a derived or
shunt-circuit lamp, or it may include two magnets working differentially
against or in opposition to each other constituting a differential lamp.

(c) Lamps with fixed parallel carbons termed candles (q. v., of various
types).

(d) Lamps without regulating mechanism. These include lamps with
converging carbons, whose object was to dispense with the regulating
mechanism, but which in some cases have about as much regulating
mechanism as any of the ordinary arc lamps.