Licht-im-Terrarium: Literaturdatenbank

In a previous paper on the radiation constants of metals the Wien equation was tentatively assumed for a working basis. The results obtained indicated quite conclusively that the distribution of energy in the emission spectrum of a metal can not be represented by an equation which is as simple as the Wien formula.

In a subsequent investigation of the reflecting power of tungsten and other metals it was shown that a spectral-radiation formula of metals must contain factors which take into consideration the the variation in emissivity (i—reflecting power) which is a function of the wave length and the temperature. For it is well established that in the infra-red the variation (decrease) in reflecting power 4 with rise in temperature becomes perceptible at a wave length of about 2/x and is very marked for the region of the spectrum beyond 4/z. On the other hand, in the visible and ultraviolet region the most recent work 5 seems to indicate that the emissivity decreases (reflecting power increases) with rise in temperature and decrease in wave length.

The reflecting-power curve obtained by the writer (loc. cit.) does not show elevations in the infra-red at 1.5/1 and 2/x. Hence, it is highly improbable that the depressions in the spectral-energy curves, at these two points in the spectrum, observed by Nyswander,6 are due to selective emission, but are to be attributed to atmospheric absorption bands and to improper factors for reduction to the normal spectrum. Similarly, the unusual results of McCauley 7 are open to question. For example, he found that at high temperatures the maximum emissivity of tantalum occurs "at a longer wave length than does a black body" (at the same temperature), which from a consideration of the reflecting-power data does not appear to be possible. He found that the metals investigated (tantalum, platinum, and palladium) acquired a minimum reflecting power in the region of i.2ju, which become more marked with rise in temperature. In view of the fact that these data were obtained indirectly by computation, it seems more probable that the reflection minimum at i.2ju is to be attributed to incomplete knowledge of the radiation constants used in the calculations. Such reflection minima would give rise to emission maxima in the spectral-energy curve, but no maxima have yet been observed. The point of interest in this investigation was the verification of the previous work just mentioned indicating that the Wien equation can not be applied to the spectral radiation from metals.

In a paper on the reflecting power of tungsten 8 physical data were given showing why tungsten (and in fact all metals which have a lower reflecting power in the blue than in the red) radiates selectively in the visible portion of the spectrum in such a way as to make the amount of radiation, relative to that of a black body, greater in the blue than in the red.