of Thermal Conductivity and Emissivity . 



293 



The third sine term is, in the case of B, too small to give 

 reliable results, but they have been calculated in some cases, 

 and curiously enough the conductivity always comes out 

 lower than that given by the first term. The following have 

 been calculated : — 





EodI. 



Eod II. 



Rod III. 





Set II. 



Set III. 



Set I. 



Set I. 



Set II. 



Set III. 



Conductivity — 1st sine term. 



•251 



•254 



•235 



•237 



•239 



•240 



3rd „ „ 



•229 



•242 



•233 



•222 



•229 



•227 



Emissivity — 1st sine term. 



•00040 



•00040 



•00030 



•00025 



•00024 



•00024 



3rd „ „ 



•00038 



•00025 



•00027 



•00024 



•00026 



•00031 



Taking the mean of the chief results obtained, we get the 

 following table : — 



Rod I. Rod II. Rod III. 



Conductivity . . . -2524 '2344 -2386 



Emissivity : — 



From " a " term. -000324 -000244 -000207 



„ 1st sine term. -000402 -000293 '000242 



"a" 



It hence appears that the emissivity of a body depends to a 

 large extent on the form and dimensions of that body, and 

 that the formula for conduction of heat along a rod, as usually 

 given in text-books, can only be used in a very restricted 

 sense. 



It will be noticed that the two emissivities (deduced from the 

 term and the sine term) are to one another approximately 

 as 1 to 1*2. The ratio appears to be too uniform to be due to 

 accidental error of experiment, but the meaning of the two 

 values is not quite clear. In order to obtain further informa- 

 tion, the | inch rod was heated at one end by steam under 

 approximately the same conditions as before, and the E.M.F. 

 of a german-silver-iron couple at different points on the rod 

 determined by the compensation method in the ordinary way. 

 A preliminary set was taken, which gave about the same 

 results as the following more accurate set (readings were 

 taken going down the rod and then going up): — 



Phil Mag. S. 5. Vol. 39. No. 238. March 1895. X 



