Messrs. W. E. Wilson and P. L. Gray. On the 



The following are the temperatures at which the radiation from 

 the bare platinum, balanced that from the hottest part of the 

 -f pole : 



Position B. 



715 C. 



717 

 737 

 720 

 732 



722 

 722 



Mean = 724 C. = 997 C 



Position A. 862 C. 

 930 

 924 

 922 

 902 

 933 

 985 

 945 



Mean = 925 C. = 1198 abs. 



The results in position A are not so concordant as tliose in 

 position B, but the arc was not quite so steady ; the first low reading 

 (862) was probably taken when the growth of condensed carbon on 

 the pole was partly shading the receiving surface from the heat of 

 the crater, while the high reading (935 C.) probably corresponds 

 to one of those sudden " bursts " of high temperature which we have 

 frequently observed to take place, although we cannot offer any 

 explanation of them. 



The current in the above experiments was about 14 amperes ; a 

 few observations were subsequently made with less resistance in 

 circuit, and a current of about 25 amperes ; the temperature then 

 appeared to be a little higher than with the smaller current, but the 

 arc in this case was so unsteady as to prevent the observations being 

 made very carefully. 



[Later experiments with a higher voltage (110 volts) and a current 

 varying from 10 to 40 amperes indicate an exact equality of tempe- 

 rature, which confirms the usually-accepted view. April 8th, 1895.] 



Working out the results of the two positions separately, we have, 

 for the ratio of emissive powers, at 



nn^o T, hlack 



997 abs., - = 4'4. 



bare 



1198 



3-85. 



From the formula (p. 31), for blacked platinum, 



3 = aT 3 +&T 4 -4-56, 

 we have for position B, q = 244'48, 

 and for A, q = 456'87. 



