THE NERNST GLOWER. 



8l 



the height of the vessel of liquid air was regulated. The temperature 

 changed so rapidly that it was necessary to begin taking readings with the 

 liquid air just touching the bottom of the vessel, when fairly consistent 

 results were obtained. The observations are plotted in fig. 56. Curve a 

 is plotted through the mean values of the observations, while curve b is 

 drawn through the highest observed values. The two curves are fairly 

 consistent, considering the difficulties in making the observations. In the 

 region of 6 // atmospheric absorption reduced the deflections. 



3456789/0// 

 Fig. 56. Emission spectrum of Ruliens thermopile. 



12 



13, 



Y 



Curves a' and b' are the two lower curves after correcting for variation 

 in slit-width, i.e., reducing them to a normal spectrum. The maximum 

 of these radiation curves lies at about 9.6 /. The temperature of the 

 thermopile was 21 C, or 294 abs. Substituting these values in the 

 "displacement" formula, the constant is .4 = 2822, which is about 3 per 

 cent smaller than that of a complete radiator. For the latter, the maxi- 

 mum would occur at about 9.95 jx. It appears that the thermopile ap- 

 proaches very closely to that of a Kirchhoff radiator. 



SELECTIVE RADIATION FROM THE NERNST GLOWER.' 



The study of the radiation from metal filaments of incandescent lamps 

 is of interest in connection with the speculations as to whether the great 

 light emissivity (high luminous efficiency) is due to an abnormal emission 

 in the visible spectrum, with a corresponding suppression of the radiation 

 in the infra-red, or whether the effect is due to the high temperature at 

 which the lamp is burning. In the latter case, the distribution of energy 

 in the spectrum may be uniform (no discontinuities), but a great deal of 

 it will lie in the visible spectrum. From a theoretical consideration 2 of 



1 A detailed description of this investigation is given in the Bulletin of the Bureau of 

 Standards, vol. 4, p. 533, 1908. 



2 Aschkinass, Ann. der Phys. (4), 17, p. 960, 1905; Einstein, Ann. der Phys. (4), 17, p- 

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



