DYNAMOS. 721 



With Frohlich's formula, we should put 



K' B-s/J 



A + - -I 

 K' 



and we should have 



E Cn *!~ab i 



K a + b 



A + -- I 

 K' 



The maximum of the second member does not exactly corre- 



spond to the maximum of the factor -- -, in consequence of Jb 

 occurring in the denominator of the following factor; we should 

 thus diminish &, and therefore make a>b. 



Suppose that the machine excited is shunt-wound ; a part only I' 

 of the principal current is used for the inductors, and the other part 

 I - 1' is taken at the terminals by a useful resistance x. We have 

 then 



and the expression for the efficiency is 



,- * 

 a+ * 



bx atf- 



b + x x 



This efficiency is a maximum, when the resistance x satisfies the 

 condition 



aft ab 



a + b a 



VOL. II. AAA 



