482 ELECTRICAL MEASUREMENTS 



same as that of the voltage applied at the potential terminals un- 

 less the last be sinusoidal. The currents induced in the disc by 

 the current coils depend on the time rate of change of the flux 

 due to the current coils and will differ from the current wave in 

 form unless this form be sinusoidal. 



When the circuit conditions are such that the wave form is 

 greatly distorted, this source of error may give rise to inaccura- 

 cies in metering, the reasons for which become apparent only 

 when the wave form has been determined by an oscillograph or 

 other wave-tracing device. 



In a certain case of this kind where induction meters persist- 

 ently refused to operate correctly, in spite of the fact that fac- 

 tory tests showed them to be commercially correct, it was found 

 that the e.m.f. wave form was as shown in Fig. 2755. 



FIG. 275B. PD. wave. When the generator was connected to a trans- 

 mission line the wave form was so badly distorted that induction meters 

 could not be relied upon. 



Effect of Voltage Variation. Departure of the voltage from its 

 normal value may influence the accuracy of the meter because of 

 change in the resistance of the potential coil winding and of 

 saturation effects in the iron core of the potential coil. For the 

 ordinary range of voltages on a circuit which is nominally operated 

 at a constant potential, these effects will not be large. This is 

 illustrated by Fig. 276. 



Polyphase Watt-hour Meters. For metering in polyphase 

 circuits where the two-wattmeter method is applicable, a special 

 form of induction watt-hour meter has been developed. An 

 example of this is shown in Fig. 277. 



The instrument consists of two complete induction watt-hour 

 meters mounted in the same case and with the two discs rigidly 



