THE CONTINUOUS CURRENT GENERATOR 123 



e.m.f. is, moreover, in the wrong direction to produce spark- 

 less commutation. The brushes must therefore, be changed 

 in position from the plane A B to the plane A'B' '. 



Effect of Brush Shift. This change in brush position 

 changes somewhat the current distribution in the armature 

 conductors. Some conductors which, before the brush 

 shift, were carrying negative current will now carry posi- 

 tive current, etc. Fig. 77 shows this new distribution of 

 current after the brushes have been moved through the 

 angle a; conductors 38, 39, 40, and 1, which, when the 



FIG. 77. Effect of Shifting the Position of the Brushes. 



brushes were in plane AB, were carrying (+) current, are 

 now carrying ( ) current, and conductors 20, 19, 18, 17, 

 which previously were carrying ( ) current, now carry 

 (+) current. 



Demagnetizing Turns and Cross-magnetizing Turns. The 

 an nature conductors may now be considered in two groups. 

 Those in the angle A'OC, combined with those in the angle 

 DOB' , make up a set of turns whose m.m.f. is in direct oppo- 

 sition to that of the main field; these are called the de- 

 magnetizing turns. Those conductors included in the angle 

 COB', combined with those in the angle A'OD, give a m.m.f 



