A direct comparison of the different types of incandescent lamps cannot be made but it is desirable at this time to note the following points: The lamps which are 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 respectively. 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 somewhat 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 reflectors, but the Nernst lamp is ordinarily installed without a reflector. Practically all of the other units of high candle-power use reflectors 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

Fig. 26. Distribution Curve of 132 Watt Type Westinghouse Nernst Lamp. Single Glower.

Fig. 26. Distribution Curve of 132-Watt Type Westinghouse Nernst Lamp. Single Glower.

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 voltages. 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.

Fig. 27 Distribution of Light from Multiple Glower Westinghouse Nernst Lamps with

Fig. 27 Distribution of Light from Multiple-Glower Westinghouse Nernst Lamps with 8" Clear Globes, No. 1, 2 Glower; No 2, 3 Glower; No. 3, 4 Glower.