otor that goes.]

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.