88 INFRA-RED EMISSION SPECTRA. 



RADIATION FROM METAL FILAMENTS. 



The measurement of very high temperatures is based upon an extra- 

 polation of the laws governing the energy emitted by a body with change 

 in temperature. Our knowledge of these laws is confined to the radiation 

 from platinum and from a uniformly heated cavity (so-called black body) 

 which is the nearest approach to a complete radiator. The remarkable 

 progress made in the development of processes requiring an accurate 

 knowledge of the temperatures involved makes it imperative to study the 

 laws of radiation of various substances with variation in temperature. 

 The object of this and of subsequent investigations is to gain an insight 

 into these laws. In order to determine these radiation laws it is generally 

 necessary to study the spectrum energy curves, using for the purpose a 

 prism that is transparent to heat rays, and some sort of very sensitive heat- 

 measuring device, such as, for example, a bolometer or a thermopile. It 

 is also possible to study the total radiation emitted. The chief difficulty 

 in studying these so-called radiation constants of substances lies in the 

 impossibility of determining the temperature of the radiating surface. 



The curves showing the distribution of energy in the normal spectrum 

 of all solid bodies thus far studied are unsymmetrical with respect to the 

 maximum, having the appearance of the probability function, modified by 

 suitable constants. The solids heretofore studied, e.g., platinum, in which 

 it was possible to determine the approximate temperature, have spectrum 

 energy curves, which are represented fairly well by the function, 



(i) = Cl A- a e- C2/;r 



In the case of a complete radiator, 1 or so-called "black body," the 

 exponent 0=5, while for platinum, a=6. 



In order to determine the constants of the above equation from the 

 spectrum energy curves, it is necessary to know the temperature of the 

 radiator. Fortunately the index a may also be obtained from the spectrum 

 energy curve in which the temperature T is constant (E= galvanometer 

 deflections, X = wave-length, are variable), without knowing the actual 

 temperature, for it can be shown from equation (1) that the ratio of the 

 emissivities (the observed bolometer-galvanometer deflections) for any 

 two wave-lengths, X and ^ max , is: 



E \ A,,, * 



(2) 



* ''max 



E 



max 



max e 



from which the value of a may be obtained. It was found by Paschen 



1 Paschen: Ann. der Phys. (3), vol. 58, p. 455; vol. 60, p. 663, 1897; (4), vol. 4, p. 277, 

 1901. Lummer & Pringsheim: Verh. deutsche phys. Gesell., 1, p. 215, 1899. 



