CHAPTEE II. 



Induced Electromotive Forces Faraday's Law Induced Currents Len//s Law 

 Self and Mutual Induction Energy of a Magnetic Field due to Electric 

 Currents Currents in Inductive Circuits. 



INDUCED ELECTEOMOTIVE FORCES. 



12. Induced E.M.F.S. Faraday showed experimentally that 

 if a conductor is moved in a magnetic field, or if there is by 

 any means produced a relative motion between the conductor 

 and the lines of magnetic force, so that the conductor cuts the 

 lines of force, an electromotive force is induced in it, and also that 

 the rate of cutting" the lines of force is a measure of the 

 E.M.F. induced. Thus, if dN lines of force are cut in an infinitely 

 small time dt by a moving conducting wire, the E.M.F. c generated 

 is given by 



where 7c is some constant determined by the units employed. If, 

 further, we take as the unit of E.M.F. that which is induced when 

 one centimetre length of wire cuts one line of force per second, 

 & = 1, and the above equation becomes 



. (2) 



dt 



The negative sign is prefixed because the E.M.F. is always 

 induced in such a direction as to oppose the cause which pro- 

 duces it. 



Equation (2) simply means that the E.M.F. induced in the 

 conductor is numerically equal, at each instant, to the rate of 

 which it is cutting lines of magnetic force. 



The C.G.S. unit of electromotive force upon which equation 

 (2) is based is so small that for practical purposes it is convenient 



