66 ALTERNATING CURRENTS 



The driving torque of the meter at unity power-factor is pro- 

 duced as follows: Figure 63 (a) shows a current and a voltage 

 wave in phase. If the meter is properly lagged the potential 

 flux <p p is 90 behind E. The current flux ^ is in phase with 7. 

 Figure 63 (6) shows the magnetic polarities of the meter poles 

 for the various times indicated in (a). At 1 the current is zero 

 so that no flux is produced by the current-coils. The potential- 

 coil flux is a negative maximum so that the potential-pole is S. 

 Therefore the two current lugs must be N poles. At 2 the poten- 

 tial-coil flux is zero, but the current is a maximum. Therefore, 

 the lower poles will be N and S as shown and the potential-lug 

 will have an S on one side and an N on the other. At 3 the 

 upper lug is N and the two lower ones S. Times 4 and 5 are 

 also shown, 5 corresponding to 1. 



In (1), the entire upper lug is an -pole. In (2), this -pole has 

 diminished in magnitude, has moved toward the left-hand side 

 of the lug and an TV-pole appears on the right-hand side of this 

 lug. In (3), an TV-pole occupies the entire upper lug, and in (4) 

 this has diminished and moved toward the left side of the lug. 



A similar cycle takes place on the two lower lugs. In (1), 

 both lugs are TV-poles making one large TV-pole. In (2), this large 

 TV-pole has diminished and moved toward the left, being followed 

 by an $-pole appearing on the right. In (3), the TV-pole has dis- 

 appeared, both lugs becoming $-poles, etc. By following the 

 cycle, it will be observed that an TV-pole moves from right to left 

 on both the upper and lower lugs. Similarly an $-pole does like- 

 wise, following the TV-pole. Therefore, the field "glides" 

 laterally through the gap. In so doing, it cuts the disc and 

 induces eddy currents. These eddy currents induced in the disc 

 react with this gliding field and by Lenz's law the disc tends to 

 follow the field. (See Induction Motor, par. 98, page 225.) 



If the power-factor be zero, <fo, Fig. 63 (a), will be either in 

 time-phase with <f> p if the current lags or will be 180 out of 

 phase with <j> p if the current leads. In either case, if instantane- 

 ous values of flux be taken, as in Fig. 63 (6), it will be found 

 that there is no lateral displacement of the field in the gap but 

 merely a sinusoidal pulsation of flux up and down in the gap. 

 Under these conditions the torque acting on the disc is zero. 



The disc of the induction meter, like that of the direct-current 



