INDUCED ELECTROMOTIVE FORCE. 



121 



short-circuited winding of wire is magnetized, the current induced 

 in the winding opposes the magnetization and more work is re- 

 quired to magnetize the rod than would be required if the in- 

 duced current did not exist. This additional work is that which 

 produces the induced current. 



64. Electromotive force induced in a straight wire moving side- 

 wise across a uniform magnetic field. Consider a straight wire 

 BB' , Fig. 82, which slides side wise at a velocity of v centime- 



B 



A' 



B' 



Fig. 82. 



ters per second along two straight wires or rails AB and A'B' ', 

 distant / centimeters from each other. The rails AB and 

 A'B' are connected at AA' so that ABB' A' is a closed cir- 

 cuit. The whole arrangement is placed in a uniform magnetic 

 field of intensity H, the direction of the field being perpendic- 

 ular to the plane of the figure and towards the reader. The 

 motion of the wire BB' induces in it an electromotive force E 

 which in its turn produces a current / in the circuit ABB' A' , 

 and because of this current the magnetic field pushes the wire 

 BB' sidewise with a force F as indicated in the figure. The 

 rate at which work is done in moving the wire BB' against the 

 force F at velocity v is Fv ergs per second, and the rate at 

 which work is done by the electromotive force E in maintaining 

 the current / is El ergs per second, E being expressed in 

 abvolts, and / being expressed in abamperes. According to 

 Lenz's law, the work done in moving the wire BB' against the 

 force F goes to maintain the current. Therefore we have 



