96 



the electromotive force. It should also 

 be observed that the strain depends upon 

 the induced electromotive force, whereas 

 the output is computed on the basis of 

 the terminal electromotive force, which is 

 somewhat smaller. As the crest of 

 the current- wave occurs in ordinary 

 work only a very short time after that 

 of induced electromotive force, the 

 mechanical strain to which each coil is 

 subjected twice in each period is that 

 due to the maxima, and not to the 

 mean values of current and electromotive 

 force, and is therefore, roughly speaking, 

 at least twice as great as in a conlinuous- 

 current machine of the same dimensions 

 and output. To provide against these 

 strains, the author inserts between the 

 coils driving-horns of insulated metal or 

 fiber, which pass through the core of the 

 armature, and are secured in their posi- 

 tion by radial bolts. The machine 

 illustrated in Figs. 20 and 21 is one of five 

 120-kilowatt alternators now in course 

 of construction for a central station ; 

 but as it has not yet been tested, it will 



