26 TREATISE ON ALTERNATING CURRENTS. 



which shows that the E.M.F. of self-iuductiou is also a sine 



FIG. G. 



function of the time, and lags behind the current in time by an 

 amount given by 



f , * 

 ~2p 



Now, since 



sm(pt - 9) = sin (pi - - 2?r) 

 the Periodic Time is given by 



P 



therefore the E.M.F. of self-induction lag's behind 

 the current by a quarter of a period. 



The P.D., current, and E.M.F. of self-induction are graphically 

 represented in Fig. 6. 



We have seen that the maximum value of the current in an 

 inductive circuit is given by 



/= 



The quantity V?- 2 + p*L 2 is called the Impedance of the 

 circuit. 



In a non-inductive circuit we have 



(Ohm's law) 



resistance 

 and in an inductive circuit 



E 



impedance 

 The value of pt -f 9 at any instant is called the Phase of 



