162 



ELECTRICITY 



SECTION VI 

 USES OF ELECTRICAL ENERGY 



186. Electric Motor. The parts of an electric motor 

 are the same as those of the dynamo ( 180) ; in fact, 

 an ordinary dynamo could be made to serve as a motor. 

 The difference is that whereas the armature of a dynamo 

 is turned by some outside means and a current is gen- 

 erated in it, the armature of a motor receives a cur- 

 rent from some outside source and, being turned by 



its action, is able 

 to impart motion to 

 other bodies. 



Fig. 118 may 

 serve to show the 

 action of a motor. 

 The armature ab is 

 to turn about its axle, between the poles (+ and ) 

 of the magnet. A current from a dynamo or battery 

 enters through the brush m, travels around the arma- 

 ture coil ab, and out through n, as shown by arrows. 

 Passing through it in this way, the current makes an 

 electro-magnet of the armature, its negative pole being 

 above and its positive pole below the horizontal position 

 (dotted lines). That is, the part a becomes a pole 

 and the part b a 4- pole. Now since like poles repel and 

 unlike attract each other, a is repelled by the pole 

 of the magnet and attracted by the -f pole ; also b is 

 repelled by + and attracted by . All of these forces 

 tend to make the armature turn about its axle in the 

 direction of the curved arrow. When it has turned so 



FIG. 118 



