29 



or less perfect way in which the alterna- 

 tor transforms mechanical into electrical 

 energy, but merely an indication of the 

 commercial value of any given type for 

 producing a maximum amount of exter- 

 nal work with a minimum weight and 

 cost of the materials employed. The 

 electrical efficiency of the machine can 

 also be found from the diagram. Accord- 

 ing to a well-known law, the internal 

 energy of the machine is equal to half 

 the product of maximum current and 

 maximum induced electromotive force 

 multiplied with the cosine of the angle 

 of lag & between the two. 



Internal energy = ^ E f I cos $, 

 when Ej is the maximum value of the 

 induced electromotive force and I the 

 maximum value of the current. The ex- 

 ternal energy available for doing work is 

 the product of the mean current and 

 mean terminal electromotive force, which 

 is equal to half the product of their 

 maximum values. 



External energy = 1 EI, 

 when E is the maximum value of the 



