THE DYNAMO. 



39 



An iron ring ab, Fig. 17, which is built up of sheet-iron 

 stampings, is wound uniformly with insulated wire as indicated 

 in the figure, the ends of the wire being spliced together and sol- 

 dered so that the winding is endless. 

 This iron ring with its winding of wire 

 is called the armature; it rotates, as 

 indicated by the curved arrow, between 

 the poles N S of a strong field magnet. 

 The wires on the outside of the iron 

 ring have electromotive forces induced 

 in them as they move across the pole 

 faces of the field magnet and cut the 

 lines of force. These electromotive 

 forces cannot, however, produce current in the endless wire that 

 is wound on the armature, inasmuch as exactly equal and oppo- 

 site electromotive forces are induced on the opposite sides, c and 

 d y of the ring, as shown diagrammatically in Fig. 18, in which 

 figure the circle adbc represents the endless wire winding on the 

 ring. 



A steady, or very nearly steady, current can be taken from the 

 winding on the ring through an outside receiving circuit /, Fig. 

 19, by keeping the terminals of this circuit in metallic contact 

 with the windings on the ring at a and b as shown in Fig. 19. 

 For this purpose the insulation may be re- 

 moved from the outer portions of the wire 

 windings on the ring, and the two stationary 

 metal or carbon brushes 55, Fig. 19, ar- 

 ranged to rub at a and b as the ring rotates. 

 In practice, wire leads are soldered to the 

 various turns of wire on the ring and con- 

 nected to insulated copper bars near the axis 

 of rotation, as shown in Fig. 20. Sliding contact is then made 

 with these copper bars instead of with the turns of wire at a and 

 b directly. This set of copper bars constitutes what is called the 

 commutator. 



Fig. 19. 



