12$ ELEMENTS OF ELECTRICITY AND MAGNETISM. 



ab which is built up of sheet iron stampings, is wound uniformly 

 with insulated wire as indicated in Fig. 86, the ends of the wire 

 being spliced together and soldered so that the winding is endless. 

 This iron ring with its winding of wire is called the armature of 

 the machine, and it rotates, as indicated by the curved arrow, be- 

 tween the poles 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, because exactly equal and opposite 

 electromotive forces are induced on the opposite sides c and d 

 of the ring, as shown diagrammatically in Fig. 87 in which the 



Fig. 87. 



Fig. 88. 



circle adbc represents the endless wire on the ring. A steady, 

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

 winding on the ring by keeping the terminals of an external cir- 

 cuit /, Fig. 88, in metallic contact with the windings on the ring 

 at a and b. For this purpose the insulation may be removed 

 from the outer portions of the wire windings on the ring and two 

 stationary metal or carbon brushes SS, Fig. 88, may be 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 

 connected to insulated copper bars near the axis of rotation as 

 shown in Fig. 89. Sliding contact is then made with these 

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



