280 



ELECTRICAL MEASUREMENTS 



the adjusting rheostat, and a D' Arson val galvanometer be used, 

 the galvanometer scale may be graduated to read 6[P.D.] directly 

 in volts. In order that this graduation, as a voltmeter, may be 

 correct for all three of the cases mentioned above, the resistance 

 of the galvanometer circuit must always be kept the same, 

 irrespective of the use to which the potentiometer is put. This 

 is the cardinal point in the design of the deflection potenti- 

 ometer, and is attained by an ingenious arrangement of coils 

 and switches. 



CASE II. POTENTIAL DIFFERENCE BY USE OF VOLT Box 



FIG. 160. Diagram for Brooks deflectional potentiometer Case II. 

 The mesh equations are: 



xR - IG~ - P.D. = 0; 



Io(~ + R G + ri }-x~-yr l = 0; 

 \T* / 71 



y(ri + r 2 + Rh) - I ri + 6 = 0. 



P.D. _ &n 



n ~r, H-rj+lto 



R(n - 



RG 



Rh) 



If the slides be set so that I G is zero, then 

 eri P.D. 



If now the potential difference be increased by a small amount, 



