113 



TABLE 105.— 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 : 



t = 408° C, et = 378.8 X 10~\ temperature of enclosure 16° C. 



r = 505° C, ct = 726.1 X 10"*, " " 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 asymptotic for high exhaustions. The following table 

 illustrates the variation of radiation with pressure of air in enclosure. 



TABLE 106.— 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 calories per square centimeter per second. 



Note.— An interesting feature (because of its practical importance in electric lighting) 

 is the effect of difference of surface condition on the radiation of heat. The energy 

 required to keep up a certain degree of incandescence in a lamp when the filament is 

 dull black and when it is "flashed" with coating of hard carbon, was found to be as follows : 



Dull black filament, 57.9 watts. 

 Bright " " 39.8 watts. 



SMITHSONIAN PHYSICAL TABLES 



