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between exciting-power and electromotive 

 force of any given alternator, provided 

 the current flowing through its armature 

 is not so great as to produce a sensible 

 self-inductive effect ; in other words, the 

 formula renders it possible to determine 

 the static, but not the dynamic, charac- 

 teristic. The effect of self-induction is 

 to produce an electromotive force, the 

 phase of which is at right angles to that 

 of the current; and the electromotive 

 force available for doing work in the cir- 

 cuit is the resultant of the induced < 

 tromotive force and that due to self- 

 induction. The mathematical treatment 

 of the problem is of great difficulty, not 

 only because, with the shape of poles and 

 armature coils occurring in modern alter- 

 nators, the electromotive force is a very 

 complicated function of the time, but also 

 because the coefficient of self-induction 

 is not a constant, but varies with the 

 relative position of the coils and poles. 

 Fortunately the coefficient is compara- 

 tively small. In machines containing no 

 iron in the armature, such as the Mordey 



