78 



means to obtain it. As a matter of fact, 

 most makers of alternators have started 

 with a very much higher frequency than 

 they have finally adopted. It should 

 also be remembered that the higher the 

 frequency, the greater is the loss of con- 

 ductivity in the mains, owing to the 

 unequal distribution of current through- 

 out their cross-section. 



Motors. 



Machines, for the conversion of the 

 electrical energy of alternating currents 

 into mechanical energy, have not yet 

 been brought to such a state of perfec- 

 tion that the problem may be considered 

 as solved — and, in fact, very little is at 

 present known as to the details of such 

 machines and the results achieved. Mo- 

 tive power may be obtained from an 

 alternating current in either of three dis- 

 tinct methods : 



1. By the employment of an alternator 

 with separately excited field, and some 

 means to first run it up to speed before 

 switching the current on. Various de- 



