Tables 246, 247. 



EMISSIVITY. 



TABLE 246. — Radiation of Platinum Wire to Copper Envelope. 



Bottomley gives for the radiation of a bright platinum wire to a copper envelope when the space between is at the 

 highest vacuum attainable the following numbers : — 



<=:4o8^ C, ^^ = 378.8 X 10—*, temperature of enclosure i6° C. 

 /= 505'^ C, f^= 726.1 X.o-«, " " 17° C. 



It was found at this degree of exhaustion that considerable relative change of the vacuum produced very small 

 change of the radiating power. The curve of relation between degree of vacuum and radiation becomes asymjv 

 totic for high exhaustions. The following table illustrates the variation of radiation with pressure of air in 

 enclosure. 



TABLE 247. — Effect of Pressure on Radiation at Different Temperatures. 



The temperature of the enclosure was about 15° C. The numbers give the total radiation in therms per square cen- 

 timetre per second. 



Smithsonian Tables. 



236 



