290 



ELECTRICAL MEASUREMENTS 



current potentiometer as a distinct instrument is due to 

 C. V. Drysdale, whose instrument is shown diagrammatical!] 

 in Fig. 167. 



In applying the potentiometer principle to alternating-current 

 measurements it is obvious that to balance two potential differ- 

 ences at every instant, they must be of the same frequency, the 

 same wave form, and in the same time phase. The first two 

 conditions demand that the potentiometer current be derived, 

 through a suitable transformer, from the same source as the cur- 

 rent to be measured. The third implies the use of some form of 



phase-shifting device. In addition 

 there must be some means of insur- 

 ing that the potentiometer current, 

 when it is alternating, is of such a 

 magnitude that the r.m.s. value of 

 the potential difference between the 

 terminals of each of the coils of the 

 instrument is given by the poten- 

 tiometer scale. As the coils are 

 wound non-inductively, this may be 

 accomplished if the potentiometer 

 current be measured by an electro- 

 dynamometer of the astatic form. Such an instrument gives 

 r.m.s. values and is equally accurate on direct and alternating 

 current circuits; therefore it is very readily calibrated. 



The Drysdale Phase Shifter. The principle involved in the 

 Drysdale phase-shifting transformer may be illustrated by the 

 following ideal arrangement of the apparatus (Fig. 166). The 

 two sets of coils are of equal magnetic strength and may have 

 their axes at right angles, in which case they are energized by 

 currents in quadrature, as from the two phases of a two-phase 

 circuit. The secondary is so mounted that it may be turned by 

 hand and clamped in position; thus may be given any desired 

 value. Let the coils in phase 2 be traversed by a current / sin cotf, 

 and the coils in phase 1 by a current 90 out of phase with the 

 first, or / cos ut. The rectangular components of the resultant 

 field at the center are 



x = H sin co; 

 y = H cos coZ. 



