18 



THE DIRECT-CURRENT MOTOR 



CH. I 



condition of the dynamo for different magnet currents, and 

 in no way involves the idea of motion. 



When a current is made to pass through the armature 

 of a dynamo, the magnets being excited, the field due to 

 the magnets is shifted and the distribution of magnetisa- 



A M P E R t S 



I N 



FIG. 9 



678 

 ARMATURE 



tion changed, the result often being an alteration in the 

 area of the magnetisation curve and a diminution in the 

 value of M . The amount of this diminution depends very 

 largely upon the form, of the pole pieces. 



In some machines where the value of the induction factor 



