mount of current than carbon filaments.

[Illustration: FIG. 52a.]

Nernst lamps are made in two sizes, the larger {99} being intended for the
same work as usually done by arc lamps, and the smaller to replace
incandescent lamps; the smaller type being made to fit into the ordinary
bayonet lampholders. The principal parts of a Nernst lamp consist of the
filament, the heater, the automatic cut-out, and the resistance, and their
arrangement in the smaller type of lamp is given in the diagram, Fig. 52a.
The current enters at the positive terminal, passes through the heater M,
and out through the negative terminal. The filament B, which consists of a
short length of an infusible earth made of the oxides of several rare
minerals, of which zirconia is one, is a non-conductor at first, but
becomes a conductor upon being raised to a high temperature by means of the
heater M. As soon as the filament becomes conductive the current then
passes through the automatic cut-out H, and the armature D is attracted,
thus breaking the heater circuit. The current then flows from the positive
terminal {100} [Illustration] through the cut-out H, resistance J, and
filament B, and from thence out of the lamp. Since the resistance of the
filament decreases the hotter it gets, it is necessary to insert a
ballasting resistance in series with it which has the opposite property of
increasing its resistance as it gets hotter, to prevent the filament taking
too much current and destroying itself. Such a resistance, J, consists of a
filament of fine iron wire, which, to prevent oxidation from exposure to
the air, is enclosed in a glass bulb filled with hydrogen gas. Fig. 52_b_
shows the form of ballast resistance used in the small and large type of
lamp respectively.

Either direct or alternating current can be used with these lamps, and with
direct current the polarity must be strictly observed, and that the
positive wire is connected to the positive and the {101} negative wire to
the negative terminal. With the smaller type of lamp once it has been
correctly placed in its holder it is essential that it should not be
turned, as a change in the direction of the current will rapidly destroy
the filament.

[Illustration: FIG. 52c.]

The arrangement of the larger type of Nernst lamp can be readily seen from
the drawing, Fig. 52c.

Care must be taken to see that the voltage required by the burner and
resistance equals the voltage of the supply circui