(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.
Lamp, Contact.
A lamp depending for its action on loose contact between two carbon
electrodes. At the contact a species of incandescence with incipient
arcs is produced. One of the electrodes is usually flat or nearly so,
and the other one of pencil shape rests upon it.
Lamp, Differential Arc.
An arc lamp, the regulation of the distance between whose carbons
depends on the differential action of two separate electrical coils. The
diagram illustrates the principle. The two carbons are seen in black;
the upper one is movable, The current arrives at A. It divides, and the
greater part goes through the low resistance coil M to a contact roller
r, and thence by the frame to the upper carbon, and through the arc and
lower carbon to B, where it leaves the lamp. A smaller portion of the
current goes through the coil M1 of higher resistance and leaves the
lamp also at B. A double conical iron core is seen, to which the upper
carbon holder is attached. This is attracted in opposite directions by
the two coils. If the arc grows too long its resistance increases and
the coil M1 receiving more current draws it down and thus shortens the
arc. If the arc grows too short, its resistance falls, and the coil M
receives more current and draws the core upwards, thus lengthening the
arc. This differential action of the two cores gives the lamp its name.
R is a pulley over which a cord passes, one end attached to the core and
the other to a counterpoise weight, W.
Fig. 211. DIAGRAM OF THE PILSEN DIFFERENTIAL ARC LAMP.