A tungsten comparison lamp in the photometry of carbon lamps

Abstract

In the photometry of incandescent lamps the substitution method is commonly used. That is, the lamp measured is not compared directly with a standard, but with a "comparison lamp" which is carefully calibrated against a candlepower standard or standards at the beginning of a run, and at frequent intervals. This method is adopted not only to obtain the greater accuracy of a substitution method, but to avoid operating a valuable standard continuously, with consequent comparatively rapid change in its value. After the standard check, the results of the measurement depend entirely upon the correctness and constancy of the comparison lamp. Since, however, the comparison lamp is subject to the changes inherent in the type of filament, it is necessary with carbon lamps to use only seasoned ones, and to keep close watch on their candlepower by checking with standards. Some time ago. Professor Rosa, who is in charge of the photometric work of the Bureau, requested us to try a Tungsten lamp, operated at voltages to give carbon lamp colors, as a comparison lamp in the testing of carbon filament lamps. The object in view was to obtain a long-lived and constant comparison light which at the same time would give perfect color match with all types of carbon lamps. It was expected that this would be found the case with the Tungsten lamp, because, first, the candlepower changes with this filament are small during life, even at rated voltage; secondly, the life of all glow lamps is greatly increased at high watts per candle, and the Tungsten lamp matches the carbon lamps in color at efficiencies near 3 watts per