1,1 • RADIATING CHARACTERISTICS OF SURFACES 



oxidation. The following generalizations may be made concerning the 

 emissivity of surfaces: (1) The emissivities of metallic conductors are 

 quite low and substantially proportional to the absolute temperature; 

 and the proportionality constant for different metals varies as the square 

 root of the electrical resistance at a standard temperature [1,2]. Unless 



Per cent of total energy found below A, as a 

 function of AT 



85 90 



1000 2000 aOOO SOOO 10,000 



AT, micron °K 



Fig. I, la. Distribution of energy in the spectrum of a black body. 



25,000 



extraordinary pains are taken to prevent any possibility of oxidation or 

 imperfection of polish, however, a specimen may exhibit several times 

 this theoretical minimum emissivity. (2) The emissivities of nonconduc- 

 tors are much higher, depend on the refractive index, and, in contrast to 

 metals, generally decrease with increase in temperature. Refractory ma- 

 terials may be expected to decrease in emissivity one-fourth to one-third 

 as the temperature increases from 1850 to 2850°F; their grain structure 



( 503 > 



