71 



ing. The obvious answer is : make the 

 resistance of the circuits as small as 

 possible, and this may be accomplished 

 in three ways : 1. By using wire of 

 large area. This expedient naturally 

 increases the weight, bulk, and cost of 

 the transformer, and must, on account of 

 commercial reasons, not be pushed too far. 

 2. By employing a core approximating to 

 a circular or quadratic section, so as to 

 decrease the perimeter of the coils. 3. 

 By working with a high induction or 

 with a high frequency. 



The mean electromotive force in volts, 

 induced in the coil of a transformer, can 

 be expressed by the following formula — 



e = (q)2 7t nzr 10" 8 (11), 



in which (q) is a coefficient denoting the 

 ratio between the maximum and the 

 mean electromotive force (when the cur- 

 rent-wave may be taken to follow a sine- 

 function, this coefficient is — \ n is 



the frequency, z the total induction in 

 absolute units, and r the number of turns 



