548 Prof. F. Rosetti's Experimental Researches 



X. Determination of the effective Temperature of the Sun. 



We have seen that the thermal effect which the solar radia- 

 tion, falling perpendicularly on the blackened face of pile No. 1, 

 would produce if there were no atmosphere, or if the pile 

 were situated at the higher limit of the atmosphere, is expressed 



a = 323 divisions. 



The solar observations were made with sixteen Siemens resist- 

 ance units introduced into the circuit, whilst those on the 

 radiation from artificial sources were made with no other resist- 

 ance in the circuit than that offered by the pile itself, the 

 rheophores, and the wire of the galvanometer. In short, in 

 order to compare the solar observations with the others and 

 to apply to them the formula, it is necessary to convert the 

 value of a = 323 into the value y, which would have given the 

 same solar radiation if the sixteen Siemens units had been 

 omitted. For this purpose it is necessary to find the resist- 

 ance offered by the pile, the rheophores, and the wire of the 

 galvanometer. The mean value from a dozen experiments was 

 3*408 Siemens units. The values furthest from the mean were 

 3*374 and 3*496. Nine experiments made another day gave 

 a mean value of 3*411. I therefore took 11=3*41 Siemens 

 units to express the total resistance. Separately, the resist- 

 ances were : — 



The wire of the galvanometer . . r = 1*522 



The rheophores ?\ = 0*552 



The pile No. 1 r 2 = 1*336 



The value of y can now be easily calculated : 

 R + 16 19-41 KfiQ01 

 * R 3*41 



Since a = 323, 



?/ = 1838*5 divisions. 



The effective temperature of the sun may be defined as that 

 temperature which an incandescent body of the same size 

 placed at the same distance ought to have in order to produce 

 the same thermal effect y if it had the maximum emissive 

 power, i. e. E = l. In this case we could apply the formula 

 y = mT-(T-6)-7i(T-6); 



and if we consider the surrounding temperature during the 

 observations to have been about 24°, giving = 297, we obtain 



T = 10238°*4; 



so that the effective temperature of the sun, represented in 



