Nernst lamps are made in two sizes, the larger
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
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
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.
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 circuit, and that only parts of the same amperage are used together on the same lamp. No advantage is obtained by over-running a Nernst lamp, this only shortening its life without increasing the light. Under normal conditions the average life of the burner is about 700 hours.
The efficiency of the Nernst lamp is fairly high, being only 1.45 to 1.75 watts per c.p. The light given is remarkably steady, and the lamps are adaptable for all voltages from 100 to 300. In one of the large type of lamps for use on a 235-volt