foote: total emissivity and resistivity 3 



The above derivation is essentially that of Aschkinass/ who, 

 in an extensive paper, has discussed the energy emission of metals, 

 the value of \T,n of the displacement law for metals, and the 

 relation of various properties of the radiation from metals to 

 similar properties of black body radiation. He used for the 

 value of £'x the first term only of equation (2), the one term 

 formula having been derived by Planck;^ but inasmuch as Hagen 

 and Rubens^ have since found that the one term relation was 

 insufficient, it appeared of interest to derive the somewhat gen- 

 eral relation (7) for total emissivity from the more accurate 

 relation (2) for monochromatic emissivity. The first term of 

 equation (7) may be obtained directly from Aschkinass' work by 

 dividing his equation (10) by the integral of Planck's spectral 

 equation and correcting the value of C2 in (10) to the present 

 accepted value of 1.445. The second term of equation (7) 

 becomes equal to 11 per cent of the first term at 1700°C. and 

 hence is of considerable importance, especially at the higher 

 temperatures. 



In order to check formula (7) quantitatively, measurements 

 have been made, with Mr. Kellberg's assistance, upon the total 

 emissivity of platinum. The apparent temperatures of thin plati- 

 num strips were measured by three radiation pyrometers of the 

 Fery mirror type and the apparent temperatures for a wave 

 length X = 0.65m were obtained with a Holborn Kurlbaum 

 optical pyrometer. The apparent temperatures measured by 

 the optical pyrometer were converted in the usual manner to 

 true temperatures using the value of Eo.es = 0.33. The emis- 

 sivity of this metal for X = 0.65^ is independent of the tem- 

 perature.-' This is of course in contradiction to the Maxwell 

 relation (2) above. But it must be noted that, for metals, the 

 visible spectrum is usually the region where resonance phenomena 

 are taking place, and hence one would here expect that the gen- 

 eral theoretical deductions might fail to apply, as is experi- 

 mentally found to be the case. This fact does not materially 



^ Aschkinass, Ann. d. Physik., (4) 17: 960. 1905. 



'•> Waidner and Burgess, Bureau of Standards Scientific Paper 55; Mendenhall, 

 Astrophys. J., 33: 91. 1911; Spence, Astrophys. J., 37: 194. 1913. 



