54 



ELEMENTS OF ELECTRICAL ENGINEERING. 



electromotive force in phase, it is called a lagging current ; and 

 when a current is ahead of an electromotive force in phase, it is 

 called a leading current. 



The phase difference between a harmonic electromotive force 



Axis of time 



Fig. 43. 



and a harmonic current is, properly speaking, a time interval, 

 namely, the time interval / in Fig. 43 ; but it is most convenient 

 to express phase difference as an angle. Thus, Fig. 44 is a 

 clock diagram in which the lines E and I represent the har- 

 monic electromotive force e and 

 the harmonic current i which are 

 shown in Fig. 43, and the phase 

 difference is represented by the 

 angle 6. 



The electromotive force and cur- 

 Fig. 44. ^. ,. fr 

 rent represented in Fig. 42 differ 



in phase, and the algebraic expressions for the vertical projections 

 of the lines E and I are 



and 



e = E sin o>t 

 z = I sin (w/ 



