Temperature of the Carbons of the Electric Arc, $-c. 31 



temperature of its surroundings, while a and 6 are constants 

 determined in any case from two experimental points at some 

 distance apart. When these have been calculated, the expression 

 may be simplified by writing aT 3 + 6T * = a third constant, c say, 

 so that we have 



c being very small and unimportant when any but very low values 

 of T are concerned. 



The curves (fig. 3) and figures (Table I and Table II) already 

 given were obtained from expressions of this form ; for bare platinum 

 the constants were calculated from 



T = 804 abs., q = 22-5 1 

 and T = 1133 q = 94'5 / 



from this, the values obtained are 



logai = 9-92855 

 log 6, = 11-81280 



a being, however, , and b +. 



TO was always about 288 (i.e., the temperature of the room was 

 15 C.), and in this case, c comes out = 0*24 (i.e., 0*24 mm. on the 

 scale of the radio-micrometer), which is practically negligible. 



For blacked platinum the constants were calculated from the 

 experimental points 



T= 683 abs., q= 67-01 

 T = 1107 q = 349-0 J 

 whence we obtain 



log a, = _7-22166 



log 6 2 = 11-92915, 



both a and b being +, while c = 4'6, so that for calculating the 

 radiation, in our arbitrary units, at any temperature, we have 



q = a,T 3 + b 2 T 4 -4'6 for blacked platinum, 

 and q = c^T'+fe/T* 0'2 for bare platinum, 



the constants being those given above for blacked and bare platinum 

 respectively. 



From the two radiation curves, for bare and blacked platinum 

 respectively, we may obtain the relative values of the emissive powers 

 at different temperatures. That the ratio is not constant has been 

 known for some time ;* the table given below will show the nature 

 and extent of the variation. 



* Schleiermacher, ' Wied. Ann.,' TO!. 26,' 1885, p. 287. Also Wilson and Gray's 

 paper already quoted, p. 380, in which several references will be found, relating to 

 experiments on this point, and the law of radiation, &c. 



