09 



obtained. Existing knowledge of the 

 subject is not sufficient for the determi- 

 nation of this frequency by a mere 

 theoretical investigation, but the experi- 

 ence obtained by several makers of these 

 machines, during the last few years, tends 

 to show that the best frequency for 

 nearly all types is below 100. Something 

 more will be said on this point later on. 



Transformers. 



The theory of transformers is so sim- 

 plo, and so generally understood, that it 

 would be waste of time to enter into it at 

 length in this place. Modern transform- 

 ers are provided with a core of laminated 

 iron, which is symmetrically placed in 

 respect to the two windings, so that the 

 same induction passes through both the 

 primary and the secondary coils. Under 

 these conditions, the electromotive force 

 induced in the two circuits is simply pro- 

 portional to the number of convolutions 

 they respectively contain, and the ter- 

 minal electromotive forces are found 

 by adding in the case of the primary, 



