Application 48. Explain why the stroking of a cat's back will sometimes cause sparks and make the cat's hairs stand apart; why combing sometimes makes your hairs fly apart. Both of these effects are best secured on a dry day, because on a damp day the water particles in the air will let the electrons pass to them as fast as they are rubbed up to the surface of the hair.
Inference Exercise
Explain the following:
291. If you shuffle your feet on a carpet in clear, cold weather and then touch a person's nose or ear, a slight spark passes from your finger and stings him.
292. If you stay out in the cold long, you get chilled through.
293. The air and earth in a greenhouse are warmed by the sun through the glass even when it is cold outside and when the glass itself remains cold.
294. When you hold a blade of grass taut between your thumbs and blow on it, you get a noise.
295. Shadows are usually black.
296. Some women keep magnets with which to find lost needles.
297. You can grasp objects much more firmly with pliers than with your fingers.
298. If the glass in a mirror is uneven, the image of your face is unnatural.
299. A sweater clings close to your body.
300. Kitchens, bathrooms, and hospitals should have painted walls.
CHAPTER EIGHT
ELECTRICITY
Section 33. Making electricity flow.
What causes a battery to produce electricity?
What makes electricity come into our houses?
The kind of electricity you get from rubbing (friction) is not of much practical use, you remember. Men had to find a way to get a steady current of electricity before they could make electricity do any work for them. The difference between static electricity—when it leaps from one thing to another—and flowing electricity is a good deal like the difference between a short shower of rain and a river. Both rain and river are water, and the water of each is moving from one place to another; but you cannot get the raindrops to make any really practical machine go, while the rivers can do real work by turning the wheels in factories and mills.
Within the past century two devices for making electricity flow and do work have been perfected: One of these is the electric battery; the other is the dynamo.