320 



ELECTRICAL MEASUREMENTS 



The split dynamometer gives the mean product of the currents 

 in its coils, so 



P = RIK.D, - K,Di]. (17) 



Potier Method for Power Measurement The Electrostatic 

 Wattmeter. 4 The electrostatic wattmeter is an instrument of 

 importance in research work, its particular field of usefulness 

 being the measurement of small amounts of power at high vol- 

 tage and low power factor. It is also used at the National 

 Physical Laboratory, London, in the testing of watt-hour meters. 



This method for the measurement of electrical power was 

 first given by A. Potier. The readings are obtained by using a 

 quadrant electrometer. In the original arrangement, a non- 

 reactive resistance is joined in series with the load and the two 



Load 



A R.L< 



FIG. 186. Connections for Potier method for power measurement. 



pairs of quadrants connected to its terminals. Two readings 

 are then taken, one, with the needle connected to the load ter- 

 minal on the far side of the line, and another, with the needle 

 connected to the other load terminal. The connections are shown 

 in Fig. 186. It has previously been shown that with the instan- 

 taneous potential differences indicated at A, the deflection of the 

 quadrant electrometer may be expressed by 



D = K~ f 



When the connection is at a, if the charging current for the 

 electrometer be neglected, 



D a = K \2R . 1 Pwdf + ~ f Wj = K\2RP + f d*dt] .(18) 

 J Jo -/Jo i Jo 



