Dimensions of a Body on its Thermal Emission. 275 



Radius in 

 centim. 



Experimental 



value of e for 



0° excess. 



Calculated 



value from 



above formula. 



Calculated by 



Peclet's own 



formula for 



air-effect and 



adding radiation. 



7-465 

 10-58 

 15-38 



•0001051 

 •0000898 

 •0000801 



•0001054 

 899 

 796 



•0001048 

 906 

 800 



It will be observed that the radiation-constant is practically 

 the same as that derived from his cylinder results *, while the 

 value of c is rather less. 



Experiments on spheres (both of 2 centim. radius) have 

 also been made by M.acFarlanef and extended by Bottomley J, 

 both using the same enclosure, which was of " large dimen- 

 sions " (Bottomley). 



Extrapolation from Peclet's results for a sphere of 2 centim. 

 radius in his enclosure gives : — 



Using his formula. 



•000237. 



Using formula given above. 



•000256 



MacFarlane's results for a blackened sphere (Peclet's were dull) 

 give '000238 for 0° excess. Bottomley's results give about 

 •000260 for 0° excess, showing that the formula derived from 

 Peclet's results is roughly applicable here. 



Bottomley also experimented on the same sphere in a 

 spherical enclosure of 5 centim. radius. If the formulae 

 hitherto given in this paper were applicable to this case, the 

 emissivity ought to increase with decrease of radius of en- 

 closure. Bottomley' s results show a marked decrease. The 

 value at 17° for 0° excess is not more than *00011 after 

 correcting approximately for temperature. 



Since the true radiation was determined separately it is 

 possible to find the value of (l c" for these experiments, 



* Pe"clet endeavoured to estimate the radiation independently by means 

 of a thermopile, and deducted it from the total emission to obtain the air- 

 effect. The value he obtained was only '0000072 for 0° excess, a value 

 so much smaller than received values that I have ignored it entirely, and 

 dealt only with the total effect. 



t D. MacFarlane, Proc. Roy. Soc. 1892. 



X J. T. Bottomley " On Thermal Radiation in Absolute Measure," 

 Trans. Roy. Soc. 1892. 



