ELECTRICITY METERS 473 



and lags behind (PD) S by the angle ^' 



\I/ ^ i~sin ^ 



/W2 C + Re 2 + (COL C )2 



tan C = 

 To make C = P , 



The proper value of R s is therefore 



which is independent of the frequency. 



The Induction Watt-hour Meter. It has been shown above 

 that commutating watt-hour meters may be made to register 

 correctly on circuits of all power factors. Formerly, lagged 

 meters of this class were in common use on alternating-current 

 circuits; they have now been superseded by induction watt-hour 

 meters for the following reasons. The moving element of the 

 induction meter may be made very light and at the same time the 

 torque may be kept high. This reduces the wear on the lower 

 pivot and jewel and lessens the chance of errors due to pivot 

 friction. There is no commutator to become rough through wear 

 and sparking, thus increasing the friction, and there are no brushes 

 to keep in order. The net result is a great decrease in the first 

 cost of the meters and in the cost of maintaining them, a decrease 

 in the current necessary to start the meters and an increase in the 

 accuracy of the registration at light loads (see page 468). This 

 last point is of the utmost importance. The losses in the poten- 

 tial coils are less in the induction than in the commutating meter 

 and as the loss goes on continuously 24 hours a day, this fact is 

 of importance. 



Fig. 272 shows in a schematic manner the essential parts of an 

 induction watt-hour meter. The explanation of the creation of an 

 accelerating torque in this instrument is the same as that given 

 for the induction wattmeter, page 448. The accelerating torque 

 is balanced by the retarding torque of a magnetic brake as in 

 direct-current meters. 



