INDUCED CURRENTS AND E.M.F.S. 



11 



to take as the unit of E.M.F. that which is induced in a conductor 

 which cuts 10 8 C.G.S. lines of force per second. This practical 

 unit is called a Volt. 



The direction of the induced E.M.F. depends not only on the 

 direction of motion of the conductor, but also on the direction of 

 the magnetic field. The following is an easily remembered rule 

 for finding the direction of the E.M.F. when the directions of 

 motion and of the field are given. 



Suppose that the directions of motion 

 and of the field are given by the lines OM, 

 OF respectively (Fig. 1), then the induced 

 E.M.F. is given in direction by the line OE 

 drawn at right angles to both OM and OF 

 in such a way that beginning at E and 

 going round in a counter-clockwise direction, 

 the order is EMF. 



FIG. l. 



INDUCED CURRENTS. 



13. If the conductor which is moving in a magnetic field 

 forms a closed circuit, the induced E.M.F. will produce an electric 

 current, and, neglecting self-induction, the value of the current at 

 any instant is given by 



dN 



..-* 

 r 



where r is the total resistance of the circuit, and JV is the number of 

 magnetic lines threading through the circuit at that instant. Here 

 we suppose that the current i is due solely to the induced E.M.F. 



E.M.F. of Self-induetion. If a closed circuit carries a 

 current i the number of lines of force of self-induction is Li 



( 6). If, by any means, i is varied, an E.M.F. equal to -5 

 is induced, or, if L is considered constant. 



T di 



e - Li- 

 cit 



(3) 



This is called the E.M.F. of self-induction, and is that E.M.F. 

 which is induced in a conductor when the magnetic field due to 

 the current flowing in it varies. 



