38 On the Temperature of the Carbons of the Electric Arc, $c. 



Note on the Effective Temperature of the Sun. 



In the authors' work on this subject, radiation experiments were 

 made with bare platinum, at temperatures up to 1600 C. approxi- 

 mately, and it was assumed that a formula of the same form as 

 expressed these results wonld also hold for a blacked surface, while 

 the ratio of the emissive powers at high temperatures was taken on 

 Rossetti's authority as abdut 2'9. 



The new work, given above, appears to show that the curve for 

 the black surface does not, however, follow a simple fourth-power law 

 so closely as does that for the bare platinum, and that, taking the 

 law as given on p. 31 of the present paper, a correction must be 

 made to the result obtained by the earlier work. 



The approximate value of this correction may be obtained by taking 

 the figures given as a typical case on p. 386 of last year's pnper, and 

 applying the new law to them. 



In this case, the corrected ratio (i.e., the ratio corrected for atmo- 

 spheric absorption, and for loss by reflection from the glass of the 

 heliostat) of the apparent areas of the bare platinum and the sun was 

 approximately 1295 : 1, and balance was obtained with the platinum 

 at a temperature of 1514 Abs. 



Now by the formula on p. 31, the radiation of bare platinum at 

 this temperature 



= a . 1514 3 + 6 . 1514 4 -O27 = 31177 

 a and 6 having the values given on p. 31. 



The radiation from the sun therefore 



= 1295x311-77=403,450. 



To find the effective temperature of the sun, we have, therefore, 

 to solve the equation 



where a and 6 now have the values corresponding to the curve for the 

 black surface. This gives T = 7800 Abs., approximately, instead of 

 7000, as given by the older method of working. 



That is to say, supposing the new formula to be correct, our esti- 

 mate of the solar temperature would have to be increased by some- 

 thing like 800. 



If, however, the ratio of the emissive powers approaches a constant 

 value, as the figures and curves on p. 32 make possible, the expres- 

 sion for the curve of the black surface would be somewhat altered, 

 in such a direction as to reduce the correction, so that we may say 

 finally that, taking Angstrom's estimate of the atmospheric absorp- 

 tion, which gave in our former work an effective solar temperature of 

 7400 C., its more probable value would now be not very far from 

 8000 C. 



