250 



HEAT. 



Constants Of Radiation. These constants may be taken as having 

 nearly the following values* : 



If R, is the energy emitted per second per square cm. from a full 

 radiator at temperature 6 on the absolute scale, 



where o- = 5'32 x 10~ 5 ergs per second per sq. cm., or 5*32 x 10~ 12 watts 

 per sq. cm. 



In A m = A 



where X m is the wave-length of maximum energy at 6 expressed in terms 

 of //,= 10" 6 metre as unit A 2Q4-0 



In Wien's formula 



0,9* 



= l 



2 =5A=14700.f 

 Cj is of the order 1000 for \ = I/A. 



The Radiation from Surfaces which Absorb Selectively. The 



foregoing results do not apply 

 to surfaces which absorb dif- 

 ferent proportions of different 

 wave - lengths. Even if a 

 surface always absorbed the 

 same proportion of the same 

 wave-length it need not absorb 

 the same fraction of the full 

 radiation at different tempera- 

 tures. Suppose, for instance, 

 that I., Fig. 143 represents the 

 full radiation for one tempera- 

 ture, while II. represents it for 

 another. Suppose that at L a 

 certain surface has an absorp- 

 tion band, L being the wave- 

 length of maximum intensity 

 of II., and suppose this the 

 only absorption band, then in 

 II. a certain amount at the 

 absorbed, while 



* wave length 



FIG. 143. Radiation Curves for Spectrum 

 at Two Different Temperatures. 



maxmum s 



in I. it is not at the maximum and is therefore not such a large fraction 

 of the whole. 



But we cannot assume that a surface does always absorb the same 

 proportion of the same wave-length. It appears possible that the 

 fraction increases in general as the temperature rises, for exceedingly hot 

 surfaces appear to approach fulness in the quality of their radiation even 

 though at low temperatures they are low radiators and only give a small 

 fraction of full radiation. 



* Congrks International de Physiqtte: Lummer, loc. cit. The valus of <r is due to 

 Kurlbuum ; Wied. Ann., Ixv., 1898, p. 748. A simple experiment giving an approxi- 

 mate value of ff is described by Denning: Phil. Mag., x., 1905, p. 270. Uncertainty 

 arises from the fact that the receiving surface, though black, is not a full absorber. 

 See Kurlbaum, Wied. Ann., Ixvii. p. 846, 1899. 



t Holborn and Valentiner (Ann. d. Physik, xxii., 1, 1906, p. 1) give 14200. 



