48 



the other machines, notwithstanding the 

 fact that its field is considerably weaker ; 

 but it will readily be understood that the 

 working under these conditions cannot 

 be economical, though it may be per- 

 fectly safe. Now suppose that, instead 

 of giving more steam to the engine, the 

 strength of the field is increased. The 

 point E' will then be shifted further 

 down on the vertical line Oy, and the 

 point A will be similarly shifted further 

 towards O, that is to say, the current 

 will be decreased. "When the field lias 

 been so far strengthened as to make the 

 induced electromotive force equal to the 

 terminal electromotive force, the point 

 E' will occupy the position E", and the 

 terminal electromotive force the position 

 OE"„ whilst no current will be passing. 

 If now more steam is given, the machine 

 will begin to work as a generator, fend 

 the current will reappear and attain its 

 former value, when the power given off 

 by the engine corresponds to the posi- 

 tion E'" of the induced electromotive 

 force. The terminal electromotive force 



