584 



ELECTRICAL MEASUREMENTS 



A variable mutual inductance designed for this measurement is 

 shown in Fig. 205, page 346. 



The arrangement shown in Fig. 358 is an alternating current 

 potentiometer of limited range. The underlying idea may be 

 developed so that an instrument generally applicable to measure- 

 ments with sinusoidal currents results. 



Ratio and Phase Angle of Potential Transformers. The ratio 

 of a potential transformer for moderate voltages may be deter- 

 mined by voltmeter measurements using two similar instruments, 

 one of them being supplied with the proper multiplier, so that 

 the readings of the two voltmeters do not differ greatly. In this 

 case uncalibrated instruments may be used. 



Transformer Under 

 Tett 



ni 



2300 V 



4if 



FIG. 361. Pertaining to determination of ratios of potential transformers by 



voltmeters. 



It is frequently convenient to use a second potential trans- 

 former, as shown in Fig. 361, to raise the ordinary supply vol- 

 tage to that required for the test. The total resistances of the 

 voltmeter circuits are R P and R s . 



Two sets of readings are made, the first with the connections 

 as shown; call the readings, in volts, of the primary and secondary 

 instruments, D P and D s . 



A second set of readings is made after putting the two volt- 

 meters in series and bringing the indications up to D P and D St 

 as nearly as is practicable. If the multiplier is large, this is ac- 

 complished sufficiently well by placing the secondary voltmeter 

 in the primary circuit, in series with the other instrument and its 

 multiplier. During the second set of readings, the same current 

 flows through both instruments. Call the readings, in volts, D' P 



and D's and the current 7; then 

 per scale unit of the two instruments. 



and W, 



are the currents 



