40 



Id this way it is found that the mechani- 

 cal energy continues to grow as B is 

 shifted higher up on its circle, until a point 

 in the neighborhood of B 3 is reached, 

 after passing which the energy decreases 

 again. Any position of B between B, 

 and B 3 is stable, an increase or decrease 

 of load resulting in an automatic adjust- 

 ment of the current and of the phases of 

 the electromotive forces ; but if the 

 machine be overloaded beyond the point 

 B 3 , its condition is unstable, as an in- 

 crease of load, tending to further retard 

 the induced electromotive force, must 

 necessarily reduce the capacity of the 

 machine for doing work, and thus bring 

 it out of step with the current. Here is 

 a danger from which direct-current 

 machines are exempt. The power of a 

 direct-current machine, when used as a 

 motor, increases indefinitely with the 

 current, and an overload, if it does not 

 last long enough to burn up the arma- 

 ture, does no harm; but with an alterna- 

 tor, an overload, if it lasts only a few 

 seconds, may bring the armature to a 



