WHAT MAKES THE TROLLEY CAR GO. 



323 



of the armature will remain under B and C, therefore the relation 

 between the position of the jDoles of the armature and the field 

 magnet will be the same substantially as that illustrated in Fig. 10, 

 and, as a result, the force tending to produce rotation will at all 

 times be the greatest possible for the strength of the current used 

 and the size of the magnets. 



Armatures are wound with a number of turns of wire in each 

 coil, unless the machine is very large, and present an appearance 

 more like Fig. 12. In this figure the brushes are arranged to make 

 contact with the outer surface of the ring C, which is the commu- 

 tator. The segments s s are connected with the ends of the arma- 

 ture coils c c c, but are separated from each other by some kind 

 of material that will not conduct electricity — that is, they are elec- 

 trically insulated. As will be noticed from this, the armature 



Fig. 1-2. 



Figs. 11, 12. — Diagrams illustrating the Method of winding Armatures of Electric 

 Motors and Generators. 



in Fig. 11 acts as a commutator as well as an armature, its 

 outer surface performing the former office. In the winding the 

 difference between Figs. 11 and 12 is simply in the number of 

 turns in each coil, there being one turn in Fig. 11 and several 

 in Fig. 12. 



The armature shoAvn in Fig. 1 is of the type called drum arma- 

 ture, but it can be wound so as to produce the same result as the 

 ring, although it is not so easy to explain this style of winding. It 

 will be sufficient for the present explanation to say that whatever 

 t^^ie of armature may be used, the winding is always such that the 

 direction of the current through the wire coils is reversed pro- 

 gressively, so that the magnetic polarity is maintained practically 

 at the same point; therefore there is a continuous pull between 

 this point of the armature core -and the poles of the field magnet. 

 The commutator is secured to the armature shaft, and the brushes 



