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ADVANCED ELECTRICITY AND MAGNETISM. 



The back electromotive force b which is induced in the primary 

 coil of the transformer is very nearly equal and opposite to e' 

 at each instant. For the sake of simplicity let us think of the 

 back electromotive force b as being exactly equal and opposite 

 to e f at each instant. Then b will be represented at each instant 

 by the projection of the rotating line OB which is equal in length 

 to E r and always opposite in direction to E f . 



In the above discussion b is the value at any instant of the 

 electromotive force induced in the primary coil. Let e" be 

 the value at any instant of the electromotive force induced in the 

 secondary coil. Then e" is smaller than b in the same ratio 

 that Z" is smaller than Z' (see Fig. 36), because a definite 



Fig. 37. 



Fig. 38. 



amount of electromotive force is induced in each turn of wire that 

 encircles the core. Therefore the electromotive force e" may be 

 represented at each instant by the projection of the rotating 



Z" 



line E" which is parallel to OB and -=; as long as OB. That 



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is, the line E" whose projection represents e" at each instant 

 is always opposite to the rotating line E' whose projection repre-. 



