ELECTRICAL MACHINERY 479 



640. Direct-current dynamo. For a great many pur- 

 poses it is desirable to have a direct current, that is, one 

 which always flows in one direction the same as a cur- 

 rent from a cell. To do this some device must be applied 

 to the dynamo just at the point where the current leaves 

 the armature and before it reaches the external circuit. 

 This device as used in a direct-current dynamo is known 

 as a commutator. 



Commutators are practically split rings secured to, but 

 insulated from, the shaft of the armature. They take the 

 place of the accumulating rings of the alternator. Each 

 part of the commutator is insulated from the other parts. 



FIG. 360 



FIG. 361 



Principle of the commutator. Fig. 360 shows a simple 

 commutator connected to a coil which represents an 

 armature. A and B are the segments of the metal ring, 

 each of which is connected to the armature. As the arma- 

 ture rotates in the direction indicated by the arrow the 

 current passes off through the side C, out over the ex- 

 ternal circuit through the segment A, and in through 

 the segment B and side D. When the side D has passed 

 into the position of side C, the current goes out over the 

 circuit in a similar manner. The brushes E and F must 



