the filament. If then, by raising the e.m.f. of the supply more current passes through the lamp, the filament gets hotter, its resistance decreases, and still more current is permitted to pass. The result of this interaction is that an increase of voltage does not produce a proportional, but an exaggerated increase of current and vice versa, with a corresponding exaggerated variation in the light given. Such lamps are sensitive to changes in voltage, more so than the metal filament lamps which, by reason of their positive temperature coefficient, burn with greater stability.
The most sensitive of all filaments is, however, the pencil of a "Nernst" lamp. This, when cold, is not a conductor at all; to make it conducting it must be heated to a dull red heat by a platinum spiral placed near it in the lamp. When sufficiently hot the pencil becomes a conductor of considerable resistance, so that a much shorter length than the filament of a metal or carbon lamp offers sufficient resistance for a working e.m.f. of 200 or 220 volts. By the passage of the current the pencil is maintained at white heat, and a very brilliant light is emitted. The pencil is, however, very sensitive to changes in voltage. It
