4 8 



ELEMENTS OF ELECTRICAL ENGINEERING. 



coils causes coil A to move and carry a pointer over a divided 

 scale. The compensated wattmeter is described in Vol. I., 

 Art. 91. 



The wattmeter, when calibrated with direct current and direct 

 electromotive force, indicates power accurately when used with 

 alternating current, provided the inductance of the voltage coil, A, 

 Fig. 38, is negligibly small. 



Proof. Consider a wattmeter which gives a certain deflection 

 with a steady electromotive force 5 across its voltage coil and a 

 steady current C flowing through its coarse wire coil. The 

 deflecting force is proportional to the product of the currents, 

 S/R and C, in the two coils, that is, it is equal to x S/Rx C. 



Supply main 



Supply main 



Fig. 38. 



Suppose that the instrument gives the same deflection as above 

 when an alternating current i flows through its current coil and 

 an alternating electromotive force e acts across its voltage coil, 

 i and e both referring to the instantaneous values ; then the 

 current in the voltage coil is ejR, if the circuit AR, Fig. 38, 

 is non-inductive, the instantaneous force action between the coils 

 is kx e/Rx i or kj Rx ei, and the average force action is equal 

 to kjR x average ci ; but, since the deflection is assumed to be 

 the same as before, this average force action is equal to the con- 

 stant force action kxSJRxC, so that average ei = &C. Now* 

 average ei is the power delivered by the alternating current, and 

 SC is the power delivered by the direct current, and since aver- 

 age ei is equal to SC, it is evident that, if the instrument indi- 



