POWER MEASUREMENT 



305 



necessary to twist the spring in order to bring the coil back to 

 its initial position. Substituting the above values for the cur- 

 rents in the fixed and movable coils, 



KD 



or 



1 1 C T . 



= p 7p I t'2. 



T Jo 



RKD = P 



idt 



(1) 



If the spring is perfect and the scale is uniform, the power is 

 proportional to the deflection. 



FIG. 176. Showing connections of wattmeter. 



The only assumption involved is that the potential circuit 

 is non-reactive. The influence of reactance in this circuit will 

 be discussed later. 



The relation P = RKD will hold for any wattmeter where the 

 movable coil is always brought back to a definite position with 

 respect to the fixed coil. When the movable coil is allowed to 

 deflect against the action of the spring, K is no longer a constant, 



20 



