4 foote: total emissivity and resistivity 



alter the validity of the expression (7) for the total emissivity, 

 since by far the greater part of the radiant energy of glowing 

 metals at temperature below 1500°C. is confined to a spectral 

 region where resonance phenomena do not exist. 



The relation between the true absolute temperature T of a 

 material, the apparent absolute temperature, S, measured by a 

 total radiation pyrometer, and the total emissivity is given by 

 the following expression: 



E = I^E^l (^) 



where To is the temperature of the receiver. The temperature 

 coefficient of mass resistivity of pure platinum from to 1100°C. 

 is very accurately given by the Callendar*' parabolic equation 

 where 5 = 1.49 and C the fundamental coefficient is 0.0039. 



It does not appear likely that serious error is introduced by 

 the extrapolation of this equation to the melting point of plati- 

 num. Observations by Pirani,^ Holborn and Wien,^ and Lang- 

 muir^ above 1100°C. do not deviate seriously from the parabolic 

 relation. Langmuir concluded from his determinations that 

 the relation was linear above 1200° but he uses the value 1710° 

 instead of 1755° for the melting point of platinum. If his data 

 are corrected for the error in the temperature scale the obser- 

 vations follow the Callendar formula quite satisfactorily. The 

 mass resistivity of a 0.6 mm. X 100 cm. wire of Heraeus plati- 

 num, presumably of the same degree of purity as that of the 

 strips used in the emissivity determinations, was very kindly 

 measured by Mr. C. F. Hanson of this Bureau, with an accuracy 

 of about 0.1 per cent. Reduction to the volume resistivity at 

 0°C. gave the value of 9.77 X 10 -« ohms cm, with possibly an 

 error as great as 1 per cent. The ratio r</ro, determined by re- 

 sistances at constant mass, is equal to the ratio determined at 

 constant volume within the accuracy of the present work. 



* Waidner and Burgess, Bureau of Standards Scientific Paper 124: 151. 



7 Pirani. Verh. d. Phys. Ges., 12: 315. 1910. 



8 Holborn and Wien, Ann. d. Physik, 59: 360. 1896. 



9 Langmuir, J. Am. Chem. Soc, 28: 1357. 1906. 



