130 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



The vndtipolar direct-current dynamo. Figure 90 shows a 

 ring armature rotating inside of a crown of six inwardly project- 

 ing field magnet poles. The electromotive force induced in the 

 windings as they sweep across the pole faces cannot produce cur- 

 rent in the endless wire that is wound on the ring, because the 

 electromotive forces induced under the north poles are just 

 balanced by the electromotive forces induced under the south 

 poles, as shown diagrammatically in Fig. 91. To utilize the in- 



Flg. 90. 



duced electromotive forces eeeeee, Fig. 91, for the production of 

 direct current, six brushes aaa and bbb, Fig. 90, should be used. 

 Three of these brushes maintain contact with the windings at 

 aaa, and through all three of these brushes current flows out of 

 the armature to one terminal of a receiving circuit. The other 

 three brushes maintain contact with the windings at bbb, Fig. 91, 

 all three of these brushes are connected to the other terminal of 

 the receiving circuit and current flows into the armature through 

 all three. The three brushes aaa together constitute the positive 

 terminal of the armature, and the three brushes bbb together 

 constitute the negative terminal of the armature. 



Number of current paths in the armature between positive and 



negative brushes. In the bipolar direct-current dynamo two 



brushes are used as shown in Fig. 88, and the current which 



enters the armature at the negative brush divides into two parts 



10 



