EMISSIVITY AND ENERGY CONSUMPTION. 



125 



It is interesting to note that the point of inflection in the curves oc- 

 curs when the isochromatic wave-length is identical with the wave-length 

 ^max) (E max ). This, of course, is due to the well-known property of spec- 

 tral emission of a complete radiator, or a metal, in which the emissivity 

 in the short wave-lengths increases more rapidly than on the long wave- 

 length side of the maximum emission. 



On the other hand, the graph showing the relation between the emis- 

 sivity of a spectrum line and the energy supplied should intersect the 

 energy axis at a distance from its origin, corresponding to the energy (a 

 finite amount) required for excitation, which is different for different 

 spectral lines. This is a well-known property of spectral lines, being inde- 

 pendent of the wave-length. 



Furthermore, if we follow the common line of reasoning (see Kayser's 

 Spectroscopy, vol. II, pp. 59, 245, and 331) and consider the separate 

 lines as a part of an energy curve, obtained by drawing the envelope 

 through the highest points of the separate emission bands, then the maxi- 

 mum of the envelope must shift 

 toward the short wave-lengths 

 with increase in energy con- 

 sumption, and the slant of the 

 isochromatics must be similar 

 to those of platinum (fig. 93). 

 It is difficult to conceive how 

 this is possible with discrete 

 spectral lines which require the 

 application of a certain amount 

 of energy to excite them. The 

 change of the emission curves 

 of the Nernst glower from a 

 discontinuous into a continuous 

 one has already been noticed. 

 They illustrate this envelope 

 type of energy curve just men- 

 tioned. But it seems more 

 probable that this is due to the 

 rapid growth of the general 

 emission of the intervening fre- 

 quencies, which, with a doubtful broadening of the emission bands, oblit- 

 erates the selective emission at high temperatures. In fig. 94 are shown 

 the isochromatic energy curves of a Nernst glower, the values being 

 taken from fig. 57. In fig. 95 are given a series of isochromatics for a 

 no-volt glower 1.4 cm. long and 1.4 mm. diameter. The current was 

 supplied from a 2000- volt 600- watt transformer on a no- volt circuit. 

 The energy supply was regulated by means of resistances in the primary. 



2 4 6 8/0 iZW&ttS 



Fig. 94. Isochromatic radiation curves of Nernst glower. 



