11 



for actual practical work, it is necessary 

 to inquire whether the basis of this 

 theory is in accord with the constructive 

 data to be found in modern machines of 

 this class. Alternators as now con- 

 structed are, with a single exception, 

 multipolar machines. This exception is 

 the high-speed dynamo of the Hon. 

 Charles A. Parsons, Assoc. M. Inst. C. E., 

 which can be converted into an alterna- 

 tor by some slight changes in its arma- 

 ture and commutator ; but all the other 

 machines now in practical use have more 

 than one pair of poles, which are set in a 

 circle, and are so arranged that the coils 

 of the armature sweep past them. As 

 the interpolar space is always very small, 

 the density of lines, emerging from or 

 entering the polar surfaces, may be taken 

 as fairly uniform over the extent of these 

 surfaces. It thus becomes possible to 

 determine for every shape of coil the 

 configuration of polar surface, which will 

 produce that variation in the induction 

 corresponding to the simple sine-func- 

 tion, taken as the basis of the text-book 



