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. [Illustration: FIG. 145. An electric motor of commercial size.] _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. CHAPTER NINE MINGLING OF MOLECULES SECTION 41. _Solutions and emulsions._ How does soap make your hands clean? Why will gasoline take a grease spot out of your clothes? If we were to go back to our convenient imaginary switchboard to turn off another law, we should find near the heat switches, and not far from the chemistry ones, a switch labeled SOLUTION. Suppose we turned it off: