

^ I L I • « A R V 



JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. V JANUARY 4, 1915 No. 1 



PHYSICS. — The total emissivity of platinum and the relation 

 between total emissivity and resistivity. Paul D. Foote. 

 Communicated by C. W. Waidner. 



On the basis of the Maxwell theory, Planck^ has derived the 

 following equation representing the relation between the reflec- 

 tion coefficient of a metallic surface, the volume resistivity of 

 the metal, and the wave length of the incident radiation: 



„ V3600 \yr^ + 1 + 1 - V2(V3600 \yr^ +1-1) .-.x 



•^x = — =iz^ 1 • ■ \^^ 



VseOO X-'/r- + 1 + 1 + V2(V3600 XVr^ +1-1) 



where Ex is the ratio of the intensities of the reflected to the 

 incident radiation, r the volume resistivity of the metal in ohms 

 cm, and X the wave length of the radiation in cm. Defining 

 the monochromatic emissivity as Ex = 1 — Rx and expanding 

 the above expression into a series of ascending powers of Vr/X, 

 Hagen and Rubens- have obtained the following equation in 

 which the terms of higher order than those indicated are neg- 

 ligible : 



^x = 0.365 ^~ - 0.0667 (^^\ + ...... (2) 



The spectral distribution of the energy radiated by a black 

 body may be expressed by the Planck equation: 



1 Planck, Sitz. Konigl. Ak. Wiss. Berlin, p. 278. 1903. 



^ Hagen and Rubens, Konigl. Ak. Wiss. Berlin, p. 468. 1910. 



1 



