305 
platinum ‘and a piece of carbon would appear equally bright within B, if 
viewed through a small hole, but if quickly removed the platinum would 
appear less bright than the carbon, for it gives out less light that is proper 
to itself since it is a good reflector but a poor absorbent and consequently 
a poor radiator, while C gives out more light that is proper to itself since 
it is a poor reflector but a good absorbent and a good radiator. 
Now if the wall D were partly removed or were cooler than the rest 
‘of the walls it could hot radiate to A as much as © does because it receives 
less from D. In this case we would have a slight departure from black 
‘body conditions. Hence the general statement : 
The true temperature as indicated by a thermo-couple, of all substances 
heated in an inclosure, is the same as the black body temperature, as indi- 
cated by a pyrometer, which has been calibrated against a black body. If, 
however, the walls of the inclosure, wholly or in part, are cooler than the 
radiating object, its true temperature will in general be higher than the 
black body temperature. However, if the walls are reflecting, but at the 
same time cold, the difference in the two temperatures is less. This dif- 
ference will be still less if the objects considered are of carbon or platinum 
black, ete. 
a. Be 
An experimental black body should therefore be as uniformly heated 
as possible and the aperture should be small, or if one end is entirely re- 
moved, as in Fig. 3, the length should be large compared with its cross- 
section. 
It thus appears that in order to attain actual temperature by radiation 
methods the body whose temperature is desired must be made as nearly ¢ 
black body as possible. In many cases this can be done with little diffi- 
culty. For example, if the temperature of an annealing oven is required, 
one could insert into the oven the closed end of a long metal or porcelain 
tube. The radiant energy coming from the bottom of this tube will be a 
close approximation to that of a perfectly black body. If black body 
conditions are not realized as an incandescent sheet of metal the tempera- 
[20—26988 ] 
