The conductivity of the glower increases with the increase of tem perature—the material has a nega tive temperature coefficient—hence if it were used on a constant poten tial circuit directly, the current and temperature would continue to rise until the glower was de stroyed. To prevent the current from increasing beyond the desired value, a ballast resistance is used in series with the glower. As is well known, the resistance of iron wire increases quite rapidly with increase in temperature, and the resistance of a fine pure iron wire is so adjusted that the resistance of the combined circuit of the glower and the ballast becomes constant at the desired temperature of the glower. The iron wire must be protected from the air to prevent oxidization and too rapid temperature changes, and, for this reason, it is mounted in a glass bulb filled with hydrogen. Hydrogen has been selected for this purpose because it is an inert gas and conducts the heat from the ballast to the walls of the bulb better than other gases which might be used.
All of the parts enumerated, namely, glower, heater, cut-out, and ballast, are mounted in a suitable manner; the smaller lamps have but one glower and are arranged to fit in an incandescent lamp socket, while the larger types are constructed at present with four glowers and are arranged to be supported in special fixtures, or the same as small arc lamps. All parts are mechanically arranged so that renew als may be easily made when necessary and it is not possible to insert a part belonging to one type of lamp into a lamp of a different type.
The advantages claimed for the Nernst lamp are: High effi ciency; a good color of light; a good distribution of light without the use of reflector a long life with low cost of maintenance; and a complete series of sizes of units, thus allowing its adaption to prac tically all classes of illumination.
The lamp is constructed for both direct- and alternating-current service and for 110 and 220 volts. When the alternating-current lamp is used on a 110-volt circuit a small transformer, commonly called a converter coil, Fig. 25, is utilized to raise the voltage at the lamp ter minals to about 220 volts.
Data on the Nernst lamp in its present form are given in Table VIII, and Figs. 26 and 27 show the form of distribution curves.
direct comparison of the different types of incandescent lamps can not be made but it is desirable at this time to note the following points: The lamps which are considered commercial in the United States at the present time are the carbon, gem, tantalum, tungsten, and Nernst lamp. The efficiencies ordinarily accepted run in the order given, approximately 3.1, 2.5, 2, 1.25, and 1.2 watts per candle respec tively. The figure of 1.2 watts per candle for the Nernst lamp is based upon the mean hemispherical candle-power and it should not be compared directly with the other efficiencies. The color of the light in all of the above cases is suitable for the majority of classes of illumination, the light from the higher efficiency units being some what whiter than that from the carbon lamp. All of these lamps are constructed for commercial voltages and for either direct or alternating current. The use of the tantalum lamp on alternating current is not always to be recommended as the service is unsatisfactory in some cases. The minimum size of units for 110 volts is about 4 candle power for the carbon lamp, 20 candle-power for the metallic filament lamp, and 50 candle-power (mean hemispherical) for the Nernst lamp. Some of the metallic filament lamps are constructed for a consumption of as high as 250 watts, while the largest size of the Nernst lamp uses 528 watts. The light distribution of any of the units is subject to considerable variation through the agency of re flectors, but the Nernst lamp is ordinarily installed without a reflec tor. Practically all of the other units of high candle-power use re flectors and only a few of the typical curves of light distribution curves with reflectors have been shown in connection with the description of the lamps. The life of all of the commercial lamps described is considered as satisfactory. The minimum life is seldom less than 500 hours and the useful life is generally between 500 and 1,000 hours. On account of the slender filaments employed in the metallic filament lamps they are not made for low candle-powers at commercial vol tages. The introduction of transformers for the purpose of changing the circuit voltage to one suitable for low candle-power units has not become at all general as yet in this country.