319 
has been shown", however, that the relation between temperature and cur- 
rent through the lamp may be represented by the general quadratic equa- 
tion 
I=—a-+ bt-+ ct?, 
so that three known temperatures are sufficient to completely calibrate the 
instrument. Such a pyrometer may be readily calibrated, without the use 
of a black body, by means of a standard pyrometer comparison lamp as 
explained above. If such a lamp is not properly calibrated in terms of 
temperature and current its black body temperature, for any value of 
current, may be determined by means of another calibrated pyrometer. A 
platinum ribbon may be used in the same way. 
Physics Laboratory, 
Purdie University. 
" Bulletin, Bureau of Standards, Vol. I, p. 255. 
