Page:Popular Science Monthly Volume 56.djvu/337

Rh of the armature will remain under B and C, therefore the relation between the position of the poles 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 commutator. The segments s s are connected with the ends of the armature coils c c c, but are separated from each other by some kind of material that will not conduct electricity—that is, they are electrically insulated. As will be noticed from this, the armature

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 shown in Fig. 1 is of the type called drum armature, 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 the of armature may be used, the winding is always such that the direction of the current through the wire coils is reversed progressively, 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