MEASUREMENT OF POTENTIAL DIFFERENCE 291 



The resultant is 



R = \/x 2 + y 2 = H \/sin 2 ut + cos 2 coZ = H, a constant. 



At any instant the tangent of the inclination of this resultant 



x 



to the vertical axis is -; 

 y 



x 

 tan 7 = - = tan co ; 



i/ 



.'. 7 = ut. 



Therefore the resultant field at the center is of constant magni- 

 tude and revolves with a constant angular velocity. 



The flux threading the secondary coil is proportional to cos 

 (co 0) and the induced e.m.f. to sin (ut 0), so the time- 

 phase displacement of the induced e.m.f. is equal to the angular 

 displacement of the secondary from its zero position. 



In the actual construction of the phase shifter the four sta- 

 tionary coils are replaced by stator windings of the induction- 

 motor type; the secondary is wound on an iron core. As built 

 by some makers the apparatus has the fault, serious in some 

 methods of measurement, that change of phase is accompanied 

 by an alteration of secondary e.m.f. This is obviated in Dr 

 Drysdale's own design by a proper arrangement of the windings. 

 If the currents supplied to the phase shifter are not sinusoidal 

 the wave form in the secondary will depend on the value of 0. 

 The device may be wound so as to operate on either two- or 

 three-phase circuits, or it may be arranged to be operated on a 

 single-phase circuit by means of a phase-splitting device. The 

 phase shifter or its equivalent is a necessary adjunct of the 

 alternating-current potentiometer. 



The Drysdale-Tinsley Alternating-current Potentiometer. 

 The Drysdale-Tinsley alternating-current potentiometer is shown 

 diagrammatically in Fig. 167. It consists of a regular Tinsley 

 potentiometer, such as is used for direct-current work (included in 

 the dotted rectangle), supplemented by the electrodynamometer 

 necessary for the measurement of the potentiometer current; 

 a selector switch, for quickly transferring the instrument from 

 one set of terminals to another; a connection board, by which 



