PRINCIPLES OF ALTERNATING CURRENT 213 



impressed, the amount of current which will flow will be 

 only a few amperes. Now the resistance of the circuit is 

 evidently the same as it was before and therefore the 

 resistance reaction, IR, can be only a few volts. The ques- 

 tion which immediately arises is this: if the sum of the 

 reactions of any circuit is equal to the impressed force what 

 reaction, besides that due to resistance, is there in this coil of 

 wire? 



There must be some other reaction because the IR will 

 be only a few volts (say 10) and the impressed force is 100 

 volts. The other reaction which exists and assists the 

 resistance reaction in balancing the impressed force is that 

 due to the self-induction of the circuit and we call it the 

 inductance reaction. 



Principle of Induced E.M.F. Faraday was the first 

 experimenter to show that when the number of magnetic 

 lines threading a coil of wire was varied, an e.m.f. was 

 induced in the coil. He also discovered that the magni- 

 tude of this induced e.m.f. was proportional to the rate 

 at which the magnetic field through the coil was varying. 

 Now it makes no difference whether the magnetic field 

 is produced by some source apart from the coil or by 

 the coil itself. When the magnetic field (the variation 

 of which is inducing an e.m.f. in the coil) is produced 

 by a current flowing in the coil itself, the e.m.f. is called 

 the e.m.f. of self-induction. 



Direction of induced E.M.F. This e.m.f. of self-induction 

 is always in such a direction that it opposes the change in 

 the current producing it. When the current is increasing 

 in a positive direction (i.e., the rate of change of current is 

 positive) the e.m.f. of self-induction is in the negative 

 direction and when the rate of change of the current is 

 negative the e.m.f. of self-induction is in the positive direc- 

 tion. This e.m.f. of self-induction which opposes any 

 change in the current through a coil we call the inductance 

 reaction. 



