3'03' 



A body that would absorl) all the radiant vut-ryx niLident upon it is 

 Called a perfectly black body. From a consideration of Prevost's theory 

 of exchange it can be shown that a body inside an inclosure all parts of 

 ^Vbich are at the same temi^erature is a perfectly black body. KirchoQ: has 

 shown that the radiation from a perfectly black body depends only upon 

 its temperature. For this reason the radiant energy emitted by a perfectly 

 black body is taken as the basis for the comparison of high temperature. 

 Radiation and optical pyrometers are calibrated by comparing a series of 

 actual temperatures of a perfectly black body with the amounts of energy 

 radiated at the respective temperatures. Two bodies are at the same black 

 body temperature when they emit eciual amounts of radiant energy. Two' 

 bodies at the same actual temperature, determined by means of a gas ther- 

 mometer, will not be at the same black body temperature unless their sur- 

 faces have the same radiating power. For example, a piece of iron and a 

 piece of porcelain each at an actual temperature of 1200" C, if examined by 

 means of an optical pyrometer calibrated in terms of the red rays emitted by 

 a perfectly black body, would indicate 1140° C. and 1100° C. respectively. 

 If, however, two bodies be placed inside a uniformly heated inclosure they 

 will not only attain the same temperature, but they will also emit radiant 

 energy equally. That is, they will have the same black body temperature. 

 In other words, the actual temperature of a body inside a uniformly heated 

 inclosure equals the black body temperature. 



A pyrometer then, which has been calibrated by comparison with a 

 black body, when sighted upon an incandescent body, reads not its true 

 temperature (thermodynamic temperature), but its black bod.v, which will 

 be somewhat lower than its true temperature. The difference will depend 

 upcn the emission power of the body. If, however, the body sighted upon is 

 a black body, for example a heated inclosure, then the pyrometer indicates 

 its true or thermodynamic temperature. A few substances such as platinum 

 black, carbon and iron oxide radiate approximately as black bodies, but 

 as yet there is no known substance which is absolutely black. In using the 

 term in this sense we must remember that the temperature must be involved 

 as well as the emission and absorption powers. Thus, any body whose 

 radiation is proportional to that of a black body, for all wave lengths, is 

 considered Mack if its temperature is the same as a black body. If its true 

 temperature is higher (it could never be lower) it is considered gray. A 

 carbon lamp fllanient is gray because its spectral distribution is the same as 



