e=JL: nzr (3) 



It is convenient to compare this value 

 with that representing the electromotive 

 force of a direct-current machine having 

 the same field and external number of 

 wires r (but equally distributed) on the 

 armature. The electromotive force of 

 such a machine is given by the product 



z r — , where N, the number of revolu- 

 60 ' 



tions per minute, may obviously be re- 

 placed by 60n when the machine has 

 two poles, because, in this case, fre- 

 quency and number of revolutions per 

 second are identical. If the direct-cur- 

 rent machine has more than one pair of 

 poles, say, for instance, p pairs, then it 

 is possible to wind the armature in such 

 a way that the electromotive forces due 

 to the different pairs of poles are added, 

 so that the total electromotive force is p 

 times the above value, and the same may 

 obviously be done with the similar ma- 

 chine wound for alternating currents so 



