Inference Exercise
Explain the following:
301. Mexican water jars are made of porous clay; the water that seeps through keeps the water inside cool.
302. When you crank an automobile, electricity is generated.
303. Potatoes will not cook any more quickly in water that is boiling violently than in water that is boiling gently.
304. When you brush your hair on a winter morning, it sometimes stands up and flies apart more and more as you continue to brush it.
305. You cannot see a person clearly through a ground-glass window, although it lets most of the light through.
306. There is a layer of coarse, light-colored gravel over the tar on roofs, to keep the tar from melting.
307. It is very easy to slip on a well-waxed hardwood floor.
308. If you have a silver filling in one of your teeth and you touch the filling with a fork or spoon, you get a slight shock.
309. You can shake a thing down into a bottle when it will not slip down by itself.
310. If you rub a needle across one pole of a magnet three or four times in the same direction, then float it on a cork in water one end of the needle will point north.
Section 34. Conduction of electricity.
How does electricity travel?
Why do you get a shock if your hands are wet when you touch a live wire?
If you were to use a piece of string instead of a copper wire to go from one pole of a battery to another or to spin between the poles of the magnet of the dynamo, you could get no flow of electricity to speak of. Electrons do not flow through string easily, but they flow through a copper wire very easily. Anything that carries, or conducts, electricity well is called a good conductor. Anything that carries it poorly is called a poor conductor. Anything that allows practically no electricity to pass through it is called an insulator.
Experiment 65.[5] Turn on an electric lamp. Turn it off by opening the knife switch. Cover the blade of the knife switch with a fold of paper and close it. Will the lamp glow? Try a fold of dry cloth; a fold of the same cloth wet. Connect the blade to the slot with a piece of iron; with a piece of glass; with porcelain; with rubber; with dry wood; with wood that is soaking wet; with a coin. Which of these are good conductors of electricity? Which could be used as insulators?
[Footnote 5]: Read footnote, page [226], before doing this experiment.
