THE NERNST GLOWER. 85 



an accuracy greater than required in the work. In a few cases a correction 

 was made for the reflecting power of the silver mirrors, but it was found 

 negligible except in the visible spectrum. 



With this apparatus, a series of energy curves was obtained, varying 

 the power-consumption from 16 watts (the lowest at which the glower 

 would conduct on no volts) to 123 watts, which is far above the normal. 

 The energy curves, which were continuous, underwent great variations 

 in appearance with rise in temperature. At 2.5 and 3.5 /i depressions 

 would generally appear in the curves, which could not be attributed to 

 experimental errors, and since previous work by others seemed to show 

 that the spectrum is continuous, an attempt was made to locate the disa- 

 greement in the apparatus (in the calibration, or in the slit-width correc- 

 tion curve), but nothing would account for it until the filament was run on 

 a 2000- volt transformer which permitted the heating of the glower at a very 

 low power-consumption. At the lowest temperature the glower was a 

 grayish red. The results are given in fig. 57 (no-volt A. C. glower No. 

 118), and are entirely different from anything hitherto recorded in the 

 emission of solids in the infra-red. At the lowest temperatures (8oo to 

 900 C.) the bands in the region of long wave-lengths are the most in- 

 tense. As the temperature increases, the bands in the region of 2.5 /x in- 

 crease very rapidly in intensity, so that by the time the temperature has 

 increased to 1000 to 1100 the intensity of the group of bands at 2 tt is 

 far in excess of those at 5.5 /x. 



The depression at 3 to 3.5 ;x is to be noticed, for it persists even at 

 normal power-consumption. The region at 2.5 \x is also to be noticed, for 

 the curves at higher temperatures often show a slight depression, not attrib- 

 utable to experimental errors. As the temperature rises (curve e, fig. 57) 

 new emission bands appear, notably at 2.5 and 4 ;x. This shift of the maxi- 

 mum of intensity of the bands, with increase in temperature, is to be ex- 

 pected, if the emission is a purely thermal one, following Kirchhoff's law, 

 and is the most conspicuous illustration yet recorded. 



In fig. 58 are given the emission curves for the glower (200 volts, serial 

 No. 118) at 19.6 and 102.5 watts, respectively. It is to be noticed that the 

 emission curve has already become smooth and continuous, with but two 

 maxima, at 1.4 and 5.5 /, respectively. In this figure curve b is one-tenth 

 the scale of curve a. 



On the whole, from whatever point of view we consider the data at hand, 

 it is evident that even after the emission spectrum has become apparently 

 continuous it does not follow so simple a law as has been established for 

 solids emitting continuous spectra. It is also evident that any estimation 

 of the temperature of the glower based on these laws will lead to erroneous 

 results. From a commercial point of view the efficiency of such a radiator, 

 in which the emissivity is abnormally high at 0.6 to 0.7 /x, while the maxi- 

 mum at 1.2 ti is abnormally low, must be much higher than that of some 



