474 



ELECTRICAL MEASUREMENTS 



The terminals T 3 and T 4 are connected to the supply while the 

 load is connected between T\ and TV 



PC is the coil of the highly inductive potential circuit; it is 

 connected across the line. Most of the flux through this coil 

 passes down the central core and returns via BA and CE. Some 

 of it, however, goes to the potential coil lug PL and thus magnet- 

 izes it. The flux which proceeds outward from PL cuts the 

 pivoted disc D which forms the movable element of the meter. 



FIG. 272. Showing electric and magnetic circuits of induction watt-hour 



meter. 



The line current flows through the oppositely wound series coils 

 FF. On the passage of currents in all the coils, the flux from 

 PL induces currents in the ' disc which are acted upon by the 

 flux due to F, and the flux from FF induces currents in the disc 

 which are acted on by the flux due to PL. With sinusoidal cur- 

 rents a driving torque is thus generated whose value is 



T = K'Vl sin(/3 2 - ft) - k, [^f + ^Ja/. See page 448. 



The retarding torque is due to the movement of the disc through 

 the air gaps of the drag magnets, which in this meter are placed 

 diametrically opposite PL. 



For steady motion the driving torque must equal the retarding 



