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142 loose hank into a more compact mass of coils. On opening the switch, the elasticity of the single turns causes them to spread out again from each other. In such a coil all wires carry the same current, they are more or less parallel, and the direction of flow is the same. The force which this simple experiment reveals is but feeble, but in kind it is the same force which comes into play when we use electricity for driving a 1000 horse-power rolling-mill, or a tram-car or a railway train. The difference is merely one of degree as regards the magnitude of the force, and of suitable arrangement of the parts of the machine, so that instead of one spasmodic jerk of the outermost loose coils of our hank we shall get a sustained rotary movement of all the coils.

The increase in the magnitude of the force is brought about by the use of iron. That the increase is very considerable may be shown by a simple experiment: Take two paper tubes about an inch in diameter and four inches long. Wind on each about ten layers of fine cotton-covered copper wire, so as to get a long coil containing about 500 or 1000 turns in all. Leave the two ends of the wire long enough to serve as suspending wires of the