322 POPULAR SCIEXCE MOXTHLY. 



through a coil of only one turn. This last explanation enables 

 us to see how it is that the comparatively small current that can 

 pass through the contact between the trolley wire and the trolley 

 wheel can develop in the motor force sufficient to propel a heavy 

 car up a steep grade. When that small current reaches the car 

 motors it passes through a thousand or more turns of wire, and 

 thus its effect is increased a corresponding munber of times. 



A motor having a single coil of wire upon the armature, as in 

 Fig. 10, would not give very satisfactory results, owing to the fact 

 that the rotative force developed by it would not be uniform. 

 Such motors are made in very small sizes, but never when a ma- 

 chine of any capacity is required. For large machines it is neces- 

 sary to wind the armature with a number of coils, so that the 

 rotating force may be uniform, and also so that the current may 

 be reversed by the commutator without producing sparks so large 

 as to destroy the device. When an armature is wound with a 

 number of coils the direction of the current is reversed, by the 

 commutator, in each coil as it reaches the point where its useful- 

 ness ends, and where, if it continued to flow in the same direction, 

 it would act to hold the armature back. The effect of this rever- 

 sal of the current in one coil after another is to maintain the 

 polarity of the armature practically at the same point, so that the 

 strongest pull is exerted between it and the field magnet poles at 

 all times. To explain clearly the way in which, the commutator 

 reverses the current in one coil at a time it will be necessary to 

 make use of a diagram illustrating what is called a ring armature. 

 Such a diagram is shown in Fig. 11. The ring A is the armature 

 core, and is made of iron; the wire coils are represented as consist- 

 ing of one turn to each coil, and are marked www. The current 

 enters the wire through the spring B, and passes out through C. 

 As can be seen, the current from B can flow through the coils 

 w IV in both directions, thus dividing into two currents, each one 

 of which will traverse one half of the wire wound upon the arma- 

 ture. The two half currents will meet at C. If the armature is 

 rotated the springs B and C (which are called commutator brushes) 

 will pass from one turn of the wire coil to another just back of it 

 as the rotation progresses, and each time that contact is made with 

 a new turn the direction of the current in the turn just ahead will 

 be reversed. The current in the wire as a whole, however, will 

 always be in the same direction — that- is, in all the turns to the 

 right of the two brushes; the current will flow toward the center 

 of the shaft on the front side of the armature, and away from the 

 shaft in all the turns on the left side. As the direction of the cur- 

 rent on opposite sides of the brushes is always the same, the poles 



