MEASUREMENT OF POTENTIAL DIFFERENCE 289 



and in the right-hand branch 



I(a 



e 



Therefore, the change in the P.D. between the terminals -f- X 

 and X due to e will be 



+ e (7 + 5) 



-+ + 7 + 5' 



The maximum value of 7 -f 5 is 14.42 ohms and the value of 

 a + /3 + 7 + 5is 1,089 ohms, so 



= X 0.013 



a + j8 + 7 

 .'. the error introduced is only 1.3 per cent of . 



Fio. 165. Pertaining to effect of thermal e.m.f. in thennokraftfrei 

 potentiometer. 



Similar considerations show that the error introduced by 

 manipulating IV and V is only about 1.2 per cent, of e. These 

 errors having been reduced to negligible amounts, the apparatus 

 is said to be free from thermo-electromotive forces. 



Application of the Potentiometer to Alternating-current 

 Measurements. The principle involved in the potentiometer 

 was applied to alternating-current measurements many years ago, 

 but at that time the necessary adjunct of a convenient phase- 

 shifting device had not been developed by Drysdale, and recourse 

 was had to an arrangement of two small dynamos on the same 

 shaft, one of which could be displaced in phase with reference 

 to the other. The arrangement, while satisfactory in many ways, 

 is much less convenient and more costly than the phase-shaft- 

 ing transformer now used. The development of the alternating- 



19 



