METAL FILAMENTS. 



8 9 



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 5.3 at 80 to 120 watts. These 

 values are taken from the smooth energy curves, such as those shown in 



1 Aschkinass: Ann. der Phys. (4), 17, p. 960, 1905. Einstein: Ann. der Phys. (4), I7> P. 

 132, 1905; 22, pp. 191, 569, 800, 1907. 



