317 



Radiation Pyrwneters. — Eqiia. (1) may be written in the form 



1 

 logio J = Ki — K2 — , (4) 



T 



where 



log e 



K, = C. — — . 



Co for a black body temperature equals 14,500 when /, is given in terms 



of jJ.^ 



Equa. (4) may be applied to any pyrometer, using monochromatic 

 light, in which the luminous intensity can be varied in a continuous and 

 determinate manner as in the Wanner and Le Chatelier. Either of the 

 instruments will, therefore, indicate temperature indefinitely high, but the 

 limit of accuracy is reached at about 2,000° C, so that at higher tempera- 

 tures the incident radiation is usually cut down by means of one or more 

 absorption glasses. The amount by which it is cut down is determined as 

 follows : 



Let J' equal the luminous intensity of the incident radiation and Ji 

 the value as indicated by the instrument when one glass is used, then 



J' = JR, 

 where R is the absorption factor. For two absorption glasses 



J' = ( JR) R = JR=, 

 and for n glasses 



J' = JR« (5) 



also 



R = J'/J. (6) 



The general expression, then, for the relation between energy and abso- 

 lute temperature, is from (4) 



1 



From (5) 



whence 



log J' = Ki — K.,— 

 T 



1 

 log J -f n log R = Kj — K2 — , 



T 



— 273, 



Kj — log J — n log R 

 where t is temperature in degrees C. 



