16 ELECTROMOTIVE FORCE AND CURRENT. 



to a scale of volts vertically, the horizontal scale being the 

 same as for the other curves. 



Since the maximum rate of change of current per second 

 = maximum value of c = 2 v n C, where C = maximum 

 value of current, we may write as the maximum value of 

 the electromotive force of self-induction 



From what has just been stated, it appears that when 

 an alternating current flows in a circuit, it will give rise to 

 an alternating electromotive force opposing the change of 

 current at every moment. Also, if the curve I in Fig. 6 is 

 taken to represent the values of such a current, a curve 

 similar to curve II. (but drawn to a scale n times as great) 

 will show the values of the voltage which must be applied 

 to the circuit in order to overcome the back electromotive 

 force of self-induction caused by the changing field which the 

 change of current produces. 



The curve of voltage is J period in advance of the current 

 curve in phase, as shown by the fact that the voltage curve 

 reaches its maximum and minimum values always 90 

 earlier than the curve of current. 



We are now in a position to apply the results shown in the 

 curves in Fig. 6 to explain the conditions governing the 

 production of a current. 



According to Ohm's Law for an electric circuit, in order 

 to maintain a current, an electromotive force must be 

 applied to the circuit whose value at any instant is equal to 

 the product of the current multiplied by the resistance 

 of the circuit through which it flows. This will evidently 

 be an electromotive force which varies with the current, 

 and is in phase with it, i.e., passes through its maximum and 

 minimum values at the same time as the current. 



In Fig. 7 is represented a current having a maximum 

 value of 25 amperes (Curve C). This current is supposed to 

 be flowing in a circuit having a resistance of 1-2 ohm. The 

 dotted curve E r represents the electromotive force which has 

 to be overcome owing to this resistance, and is obtained by 

 multiplying each ordinate of the current curve by 1-2. The 

 maximum valufe of this electromotive force is 25x1-2=30 

 volts. In this case current and volts can be represented to 

 the same scale, although this would not be convenient if 



