MEASUREMENT OF POTENTIAL DIFFERENCE 281 



5[P.D.], the slides remaining fixed, a current will flow through 

 the galvanometer 



T 



, p n(r 2 + flA) 



"G + ri + r2 + ^ 



The total potential difference is, therefore, n times the sum of the 

 reading of the slides and the reading of the galvanometer in 

 volts, provided the coils be so arranged that 



/R(n 1) , rj(r 2 + Rh) \ 



\ n*~ r 4- r + Rh) 



is a constant for all settings of the slides, and the galvanometer 

 has been calibrated as a voltmeter with this resistance. The 

 expression in parenthesis is the total resistance of the galva- 

 nometer circuit when e and the volt box are short-circuited. 



CASE III. CURRENT MEASUREMENT 



FIG. 161. Diagram for Brooks deflectional potentiometer Case III. 



The mesh equations are: 



Ia(S + Ro + ri) - ?/ri - IS = 



is- eri 



i 737 



. T T\ -r r 2 ~r Kn 



. . IG = ~ 



. 



If the current be increased by a small amount d[I] from that 



