382 Ferrel — Measures of Intensity of Solar Radiation. 



It is readily seen that the relation between I and i depends 

 upon the temperature r, and so is different for different tem- 

 peratures, and consequently I cannot be regarded as a relative 

 measure of i under all circumstances, but only when r remains 

 constant. For instance, the ratio between I and i is not the 

 same when r=0, as in a summer temperature when r=25°. 



If we put I for the value of I when z is equal to any 

 assumed value r , and put 



A=r-r o (6) 



we get from (5) 



I o= I X^.^f.4=IX(l-^)(l+^)^ (7 ) 



in which the last form of expression is only approximate, but 

 sufficiently accurate, unless A is large. Hence, in order to 

 make the observed intensities I of the electric current meas- 

 ures of radiations it is necessary to reduce them to a common 

 temperature r , which, in order to make A small, should be a 

 medium temperature, or at least one not varying much from 

 such a temperature. 



With regard to the values of the constants e, r and jul, that of 

 c differs with different metals forming the circuit, and may be 

 either positive or negative. That of r is always positive, and 

 differs but little in different metals. It was remarked by 

 Clausius that the electrical resistance of all chemically pure 

 metals should be proportional to the absolute temperature, and 

 if so, we should have r='00366. Experiment very nearly 

 verifies this law, but mostly makes the increase of resistance in 

 a ratio a little greater. We shall therefore put 



r = 0-0038 (8) 



The value of /*= 1*0077 given above was obtained from ex- 

 periments made through a range from 80° to 210° C, but it 

 has been shown by the writer* that for lower temperatures it 

 must be much greater, the value at 50° C. being about 1 '0088, 

 and for still lower temperatures it should be considerably 

 greater. 



According to Stefan's law of radiation, we have 



i = KT 6 - 1 ^ (9) 



in which 6=4, K is a constant, and T is the absolute tempera- 

 ture. The value of o from this expression, substituted in (3), 

 gives 



(10) 



1 _ = C(1+CT) 



R o (l+rr)'KT e -i 



* This Journal, vol. xxxviii, July, 1889. 



