MAGNETISM AND ELECTROMAGNETICS 49 



the closed coil, or if the second coil is fixed in position and the 

 current in it is varied. The induced current only exists while 

 the magnet or inducing coil is moving with respect to the fixed 

 coil or while the current in the inducing coil is varying. 



The induced current is due to the fact that a difference of 

 potential or electromotive force is produced in the circuit by 

 changing the number of lines of magnetic flux threading through 

 it or by causing lines of magnetic flux to cut across it. 



40. Laws of Induction. First Law. A change in the num- 

 ber of lines which pass through a closed circuit induces a current 

 around the circuit in such a direction as to oppose the change in 

 the flux threading the circuit. 



Second Law. The electromotive force induced around a closed 

 circuit is equal to the rate of change of the flux which passes 

 through the circuit; or the electromotive force induced in a 

 conductor is equal to the rate at which it cuts across lines of 

 magnetic flux. 



41. Unit of Electromotive Force. The absolute unit of elec- 

 tromotive force (e.m.f.) is the electromotive force induced in a 

 coil of one turn when the flux threading the coil is changing at 

 the rate of one line per second; or it is the electromotive force 

 induced in a conductor when it is cutting one line per second. 



The practical unit is the electromotive force produced by cut- 

 ting 10 8 lines per second and is called the volt. Electromotive 

 force is commonly called voltage. 



To change from absolute units of electromotive force to volts 

 divide by 10 8 . 



If a coil of wire has n turns and the flux through it is changing 



at the rate -^ lines per second the e.m.f. induced in the coil is 



e = n ~ absolute units (79) 



The negative sign is used because when the flux is decreasing 

 the induced e.m.f. is in the positive direction, that is, it tends to 

 prevent the decrease of the flux. 



42. Force Exerted by a Magnetic Field on an Electric Circuit. 

 Every part of an electric circuit situated in a magnetic field is 

 acted upon by a force tending to move it into the position where 

 it will include the greatest possible flux. 



