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which a revolving field causes, by a kind 

 of electro-magnetic drag, an armature to 

 revolve and give off mechanical work. 

 This action may be represented by the 

 diagram Fig. 12, where A A and B B are 

 two coils placed at right angles, and in 



■<•>' 



the common center, O, of which is placed 

 an armature consisting of a series of coils 

 C, each forming a loop closed on itself. 

 Let there be a difference of phase of 90° 

 between the currents in the two field- 



