240 TABLES 250, 251 . 



COOLING BY RADIATION AND CONVECTION. 



TABLE 250. Cooling of Platinum Wire In 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., <rf = 378.8 X 10-*, temperature of enclosure 16 C. 



/= 505 C., et 726. i X io-*, " " 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 asymp- 

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

 enclosure. 



TABLE 251. Effect of Pressure on Loss of Heat at Different Temperatures. 



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



timetre per second. 



SMITHSONIAN TABLES. 



