JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. VI APRIL 19, 1916 No. 8 



PHYSICS. — A misconception of the criterion for gray body radia- 

 tion. Paul D. Foote and C. O. Fairchild, Bureau of 

 Standards. 



Some 15 years ago Lummer and Pringsheim 1 investigated 

 spectrophotometrically the radiation of carbon and platinum by 

 comparison with a black body at various known temperatures. 

 It was found, when the carbon was maintained at a constant 

 temperature and the temperature of the black body was altered, 

 that the graph of the logarithm of the ratio of the intensities of 

 the two sources at any given wave length plotted against the 

 reciprocal of the absolute temperature of the black body was a 

 straight line, and further that these linear graphs for various 

 wave lengths in the visible spectrum intersected at one common 

 point. The fact that such a common point of intersection existed 

 was casually suggested as a possible proof of the "grayness" of 

 carbon where the term gray is understood to denote that the 

 material has an emissivity independent of the wave length. 

 Consequently the value of the temperature coordinate corre- 

 sponding to this point of intersection would be the true tempera- 

 ture of the gray radiating material. 



Recently an extensive paper on this subject has been published 

 by Elisabeth Benedict 2 working under the direction of Lummer 

 and Pringsheim in which the following questions among others 

 proposed by Dr. Lummer are considered. 



1 Lummer and Pringsheim. Verh. d. Deut. Phys. Ges. 3: 36-42. 1901. 

 s Benedict. Ann. d. Phys. 47: 641-678. 1915. 



193 



