. Carnegie Institution of Washington publication. METAL FILAMENTS. 89 that for carbon, platinum, etc., the value of a was in agreement with that obtained from a knowledge of the temperature of the radiator. With this equation it is possible to obtain some idea of the probable total emissivity of a radiating body, as to whether it is proportional to the 4th power (a—1 = 4 for a black body, a—1 = 5 for platinum), or to some higher power of the absolute temperature. Of course the assumption is made that the emissivity function is similar to that of platinum and of a black body. The appearance of
. Carnegie Institution of Washington publication. METAL FILAMENTS. 89 that for carbon, platinum, etc., the value of a was in agreement with that obtained from a knowledge of the temperature of the radiator. With this equation it is possible to obtain some idea of the probable total emissivity of a radiating body, as to whether it is proportional to the 4th power (a—1 = 4 for a black body, a—1 = 5 for platinum), or to some higher power of the absolute temperature. Of course the assumption is made that the emissivity function is similar to that of platinum and of a black body. The appearance of the energy curves for various tempera- tures will give some clue as to the admissibility of this assumption, which is nothing more than has been made by previous observers. How far this assumption falls short of the observed facts, may be seen in figs. 57 and 58 for the Nernst glower, which has a spectrum composed of numerous emission bands. With substances whose energy spectra undergo no change in contour with change in temperature, it does not seem unreasonable to apply our knowledge gained from the behavior of platinum under similar conditions, especially since the filaments are metals, electrical conductors, which, theoretically,1 should have similar emissive properties. That the. 20 40 60 80 100 Fig. 59. — Radiation constant (a) of Nernst glower. 120 Waits method is open to criticism is admitted, but until a better one is suggested, the present method is the only one available without a knowledge of the temperature of the radiator. The apparatus used in this work consisted of a mirror spectrometer, a fluorite prism, and a bolometer, mentioned in the description of the results on the Nernst glower. The variation of the radiation constant a for the Nernst glower with rise in temperature is shown in fig. 59, from which it will be seen that it decreases from a value of 7 at 18 watts to at 80 to 120 watts. These values are taken from the smooth energy curves, such as
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