272 



ELECTRICAL MEASUREMENTS 



adjusted. Let n be the resistance from 6 to c when the galva- 

 nometer deflection is zero arid the standard cell is in circuit, and 

 7*2 the reading of the slider when the galvanometer deflection is 

 zero and the switch is on P.D., the current in ab being as before. 

 It is necessary that the current lab be constant. To ascertain if 

 this is so necessitates the throwing of the switch to E and the 

 resetting of the slider. The potentiometer current will be 



E 



and 



P.D.= 



An obvious improvement is to tap in the standard cell at a 

 fixed point on ab', if the e.m.fs. of all cells which are commonly 



Rh. 



b c 



Std.Oell 



Std.Cell 



FIG. 154. Illustrating the 

 principle of the potentiom- 

 eter. 



FIG. 155. Illustrating 

 principle of potentiometer 

 with adjustment for standard 

 cells of different e.m.fs. 



employed at E were the same this would fix the value of lab 

 at which the galvanometer would be in balance, but some forms 

 of standard cells have temperature coefficients and the Weston 

 cell in its commercial form, which is the one most frequently 

 used, is a secondary standard, the e.m.f. of which varies slightly 

 with different cells. The resistance between the cell terminals 

 must therefore have a slight adjustment if I a b is always to be 

 brought to a definite value. The arrangement then becomes 

 that shown in Fig. 155. 



Each step of the resistance bb f is marked with the standard- 

 cell e.m.f. to which it corresponds. 



I a b is always adjusted to some predetermined value. There- 

 fore E = I ab TI where r\ is the resistance between c' and g. The 



