78 foote: an optical ammeter 



following form where i is the current in amperes and the abso- 

 lute temperature of the filament: 



(1) ^• = 0.09258 -0.000010719 (t? -273) +0.00000018074 (^-273)2 

 whence 



bi _ [—0.000010719 + 0.0000003615 {d - 273)^] § 8t} 



(2) 



i i t^ 



The brightness-temperature relation for this lamp is obtained 

 from Wien's law as follows : 



J \d ^ 



where Cg = 14350, and J = intensity corresponding to X the 

 wave length of the monochromatic light employed. In optical 

 pyrometry this wave length is usually made about X = 0.65^ 

 by the use of a suitable red glass. Combining equations (2) 

 and (3) one obtains: 



, 6i _ [-0.000010719 + 0.0000003615 (^-273)^]^^ X 8J 



Thus for a particular value of such as 1273° absolute we have: 



(5) - = 0.06 — 



i J 



Hence, if the photometric match is made with an accuracy of 

 0.5 per cent the current % is determined with an accuracy of 0.03 

 per cent, or to three parts in ten thousand. If the photometric 

 match is made with an accuracy of only 5 per cent, the current 

 is determined to three parts in one thousand, which is a pre- 

 cision scarcely to be obtained by an ammeter, especially for 

 alternating current. 



One means for adapting this method of photometry to the 

 measurement of current is shown in figure 1, where A' is a red 

 glass screen, A the pupil diaphragm, B the ocular lens, C the 

 electric lamp connected to the electrical apparatus, F a second 

 electric lamp, E a diffusing opal glass screen and D a lens focus- 

 ing upon E and C. The alternating current flowing through C 



