26 BELL SYSTEM TECIIXICAL JOVRXAL 



Then : 



Lo dii , M di^f , . 

 r-2. dt ro dt 



Integrating from time / = to / = /„, the time at any later instant, 



— I dii -i I diM = — I udt. 



''s Jo ''- X Jo ' 



Now if iM is changed slowly enough 



— I dio 

 '" Jo 

 is negligible and we have: 



- I diM = - 1.(11, 



- Jo ^M) 



r 

 or 



— IM - - Q^f, 



where Qm is the quantity of electricity that has passed through the 

 fluxmeter in time /q. Now let Q.\f = — K8m, where 5i/ is the deflection 

 produced when Qm flows. Then: 



and 



— l,\r — A6.V, 



A- = "■'" 



ri^M ' 



and the quantity of electricity which has passed through the fluxmeter 

 for any other deflection is 



0=-^'s. (1) 



'^20.1/ 



This equation makes it possible to determine B — II, calculated from 

 Q as described below, by observing the deflection h. Relation (1) may 

 be determined once for all as it is a constant of the fluxmeter onh-. 

 The parts of Q passing through R2 and the photo-electric cell will be 

 negligible on account of their high resistances. 



Now suppose a magnetic curve recorded with R(, adjusted until the 

 deflection is due solely to the magnetization of the specimen. Let the 

 resistance of the fluxmeter plus that of the secondary of M be denoted 

 by Rg, and that of S plus R^ be denoted by R,. Then if the field 



