Experiment 79. Connect the wires from the laboratory battery to the two binding posts of the toy motor, and make the motor run. Examine the motor and see that it is made of several electromagnets which keep attracting each other around and around.
Motors, and therefore all things that are moved by electricity, including trolley cars and electric railways, submarines while submerged, electric automobiles, electric sewing machines, electric vacuum cleaners, and electric player-pianos, are moved by magnetizing a piece of iron and letting this pull on another piece of iron. And the iron is magnetized by letting a current of electricity flow around and around it.
Fig. 144. A toy electric motor that goes.
Fig. 145. An electric motor of commercial size.
The making of various kinds of electromagnets and putting currents of electricity to work is becoming one of the great industries of mankind. Waterfalls are being hitched up to dynamos everywhere, and the water power that once turned the mill wheels now turns millions of coils of wire between the poles of powerful magnets. The current generated in this way is used for all kinds of work—not only for furnishing light to cities, and cooking meals, heating homes, and ironing clothes, but for running powerful motors in factories, for driving interurban trains swiftly across the country, for carrying people back and forth to work in city street cars, for lifting great pieces of iron and steel in the yards where huge electromagnets are used,—for countless pieces of work in all parts of the globe. Yet the use of electricity is still only in its beginning. Tremendous amounts of water power are still running to waste; there is almost no limit to the amount of electricity we shall be able to generate as we use the world's water power to turn our dynamos.
Application 62. Explain how pressing a telegraph key can make another instrument click hundreds of miles away, and how you can hear over the telephone. Is it vibrations of sound or of electricity that go through the telephone wire, or does your voice travel over it, or does the wire itself vibrate? Explain how electricity can make a car go.
Inference Exercise
Explain the following:
371. When a fuse blows out, you can get no light.
372. If you lay your ear on a desk, you hear the sounds in the room clearly.
373. If you touch a live wire with wet hands, you get a much worse shock than if you touch it with dry hands.
374. A park music stand is backed by a sounding board.
375. The clapper of an electric bell is pulled against the bell when you push the button.
376. A hot iron tire put on a wagon wheel fits very tightly when it cools.
377. Candy will cool more rapidly in a tin plate than in a china plate.
378. When a trolley wire breaks and falls to the ground it melts and burns at the point at which it touches the ground.
379. By allowing the electricity from the trolley wire to flow down through an underground coil of wire, a motorman can open a switch in the track.
380. The bare ends of the two wires leading to your electric lamp should never be allowed to touch each other.