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FOOTE AND FAIRCHILD I GRAY BODY RADIATION 



As a particular illustration of the possible erroneous conclu- 

 sions which may be drawn from the intersection of isochromatics, 

 viz. that the radiating material is gray and that the temperature 

 corresponding to the intersection is the true temperature of the 

 radiator, the following example may be cited, in which the radia- 

 tion from a black body at various temperatures is compared 

 with that from a non-black and non-gray body. There can be 

 no question that this particular radiator is not gray. We have 

 purposely made it as far from gray as conveniently possible. 



Temperature of non-black radiator = 1400° abs. 



WAVE LENGTH EMI3SIVITY 



The logarithmic isochromatics obtained by (theoretically) com- 

 paring this radiator with a black body at temperatures 1200°, 

 1300°, 1500°, and 1600° absolute are shown in figure 1. A per- 



.0006E 64 



7£ Yq 74 76 



Fig. 1 



feet intersection occurs at the temperature 1170° absolute. Fol- 

 lowing the criterion of Lummer and Pringsheim as applied by 

 Benedict we would conclude that this radiator is gray and is at 



