ELECTROMOTIVE FORCE AND CURRENT 15 



entering the coil will be Z per second. The total voltage 

 induced in the coil by this change will be 



lines cut per second by each coil x number of coils 



~^l~~ 

 or in symbols 



TT 1, EI Z.N. 



Voltage = E = ~^- 



Z N 

 The fraction ' ' is called the coefficient of self-induction 



lu 



of the coil, and is generally denoted by . 



Definition. The coefficient of self-induction of a coil is 

 numerically equal to the electromotive force induced in it 

 by a change of current of 1 ampere per second. 



or 



The coefficient of self-induction is numerically equal to the 

 number of magnetic lines formed in a coil by a current of 

 1 ampere multiplied by the number of turns of the coil 

 through which these lines are threaded, divided by 10 8 . 



This coefficient is termed the henry. 



The self-induction of a circuit is frequently called the 

 inductance of the circuit. 



A milli-henry = , ^^ henry, and is generally used in 



stating the properties of a circuit, as it is unusual for a circuit 

 to have an inductance greater than a small part of one henry. 



We may thus write the expression for the electromotive 

 force of self-induction when the rate of change of current is 

 1 ampere per second 



E = L. 



If the current changes at the rate of c amperes per second, 

 then the induced voltage will be 



E = c L. 



Since the dotted curve in Fig. 6 shows the rate of change 

 of current per cycle, it would give the rate of change per 

 second if its ordinates were multiplied by n. It would then 

 be the curve of 'c . 



Hence we might obtain a curve showing the fluctuations 

 of the electromotive force of self-induction by drawing 

 a curve in which each ordinate of Curve II. (Fig. 6) is 

 multiplied by nL. This curve must, of course, be plotted 



