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A body that would absorb all the radiant energy incident upon it is 
called a perfectly black body. From a consideration ot Prevost’s theory 
of exchange it can be shown that a body inside an inclosure all parts of 
which are at the same temperature is a perfectly black body. Wirchoff 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 
actnal 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 equal 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 femperature, but they will also emit radiant 
energy equally. That is, they will have the same black body temperature. 
Tn 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 body, which will 
be somewhat lower than its true temperature. The difference will depend 
upon 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 black 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 filament is gray because its spectral distribution is the same as 
