POWER MEASUREMENT 



335 



K is the dynamometer constant for each of the two elements. 

 Equation (30) gives the correct value of the turning moment. 



If there be interference between the elements, and K' is the 

 constant of the dynamometer formed by the lower fixed coil 

 and the upper movable coil, or by the upper fixed coil and the 

 lower movable coil, the turning moment is modified and becomes 



Af = rF 



K' C T . K C T . K'F. 



Tfr iaindt + 7^ I ^tssCK + -^ I *i 8 



* Jo 1 Jo -'Jo 



(31) 



The electrical connections for 

 pensation are shown in Fig. 198. 



Weston's method of com- 



FIG. 198. Connections for compensating a polyphase wattmeter for 

 interference between elements. 



The compensation is effected by altering the potential coil 

 currents. 



This is done by causing the potential-coil circuits to have the 

 part 7*1 of their resistances in common. The currents through 

 the potential coils then become 



+ VIZ(TI + r) 



and 



i'so 



r(2ri + 



+ r) - 



r) 



