Dynamo-electric Machines. 283 



ance. And this must ultimately depend on the governing 

 function of the prime mover. 



The following example leads to an unstable solution. 



Same machine with constant field = 53. It is required to 

 keep the difference of potential between the armature- brushes 

 constant. This includes the case of constant potential between 

 the ends of leads. 



R is the resistance of armature (with leads), 



r external resistance; 



T 



e= ■==» — E is to be kept constant. 

 sx-\-r 



(2),=const.= ^E = ^^ 

 Re 



R + r = R 





%vl- 



p -^ l (m-e). 



(Vi dp a _dp 9 __ 2rvWP m 

 K } dv dv ' R(R + r) + R 



%l( 2rE \ 



+ el 



R\ B, + r 

 (since v= |) 



__We 

 ~ TT 



and the governing function which satisfies the required condi - 

 tion gives rise to unstable motion. 



I will now consider the simplest assumption that can be 

 made in connexion with the ordinary main-circuit machine. 

 It is that, according to the law of electromagnets generally 

 accepted, the excitation of the magnets is proportional to the 

 number of coils and to the current circulating in them, or 



%> = knC; 



nC may be called current-turns. 



