ALTERNATING-CURRENT CIRCUITS 27 



ire 25 shows a current wave I. Starting at (a) the current 

 is cha?i(jin<j at its maximum rate in a positive direction. There- 

 fore, at this instant tin* elect mmotive force of self-induction must 

 be at its negative maximum value. At point 6, the top of the 

 current wave is horizontal and, therefore, at this instant the 

 current is not changing at all. Hence the electromotive force of 

 self-induction is zero. At c the current is changin.ir at its maxi- 

 mum rate negatively and the electromotive force of self-induct ion 

 must he maximum positive, because of the negative sign in the 



line Vcltnre 



Fio. 25. Current and volt ting in an alternating-current circuit con- 



taining inductance only. 



formula. Continuing in this way the voltage curve a'b'c' is 

 obtained. It will be observed that this wave is a >ine wave and 

 is laL r L r in<: the current by 90. 



Thi.- is the only voltage in the circuit which opposes the change 

 of current. It corresponds to the back electromotive force of a 

 motor. The line, in the case of the motor, must supply a voltage 

 opposite and equal to the back electromotive force before any 

 current can How into the armature. This same condition exists 

 in the alternating-current circuit. 1'n-fon- any current can How 

 into a circuit containing inductance, but no reastanoe, : voltage 

 'rid equal to the elect romot ive force of self-induct ion 

 mu-i be Hipplied by the line. 



Therefore, in I-'ig. 'J."i the VOltagQ / . which is the line vol 1 



is Opposite ;md e<|Ual to the . -I.-eJ r< Uliot i Ve force nf -elf-illdlictinn. 



It will be noted that tlie imj)ressed voltage /MN/.S- tiie current by 

 r the current /m/x this volt: With inductance 



only in the circuit , the current /</</- the imprr rd voltai 



