Читать книгу Common Science - Carleton Washburne - Страница 8

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Experiment 2. Hold a burned-out electric lamp in a basin of water, break its point off, and see what happens.

All the common electric lamps (less than 70 watts) are made with vacuums inside. The reason for this is that the fine wire would burn up if there were any air in the lamps. When you knock the point off the globe, it leaves a space into which the water can be pushed. Since the air is pressing hard on the surface of the water except in the one place where the vacuum in the lamp globe is, the water is forced violently into this empty space.

It really is a good deal like the way mud comes up between your toes when you are barefoot. Your foot is pressing on the mud all around except in the spaces between your toes, and so the mud is forced up into these spaces. The air pressure on the water is like your foot on the mud, and the space in the lamp globe is like the space between your toes. Since wherever there is air it is pressing hard, the only space into which it can force water or anything else is into a place from which all the air has been removed, like the inside of the lamp globe.

The reason that the water does not run out of the globe is this: the hole is too small to let the air squeeze up past the water, and therefore no air can take the place of the water that might otherwise run out. In order to flow out, then, the water would have to leave an empty space or vacuum behind it, and the air pressure would not allow this.

Why water gurgles when it pours out of a bottle. You have often noticed that when you pour water out of a bottle it gurgles and gulps instead of flowing out evenly. The reason for this is that when a little water gets out and leaves an empty space behind, the air pushing against the water starts to force it back up; but since the mouth of the bottle is fairly wide, the air itself squeezes past the water and bubbles up to the top.

Experiment 3. Put a straw or a piece of glass tube down into a glass of water. Hold your finger tightly over the upper end, and lift the tube out of the water. Notice how the water stays in the tube. Now remove your finger from the upper end.

The air holds the water up in the tube because there is no room for it to bubble up into the tube to take the place of the water; and the water, to flow out of the tube, would have to leave a vacuum, which the air outside does not allow. But when you take your finger off the top of the straw or tube, the air from above takes the place of the water as rapidly as it flows out; so there is no tendency to form a vacuum, and the water leaves the tube. Now do you see why you make two holes in the top of a can of evaporated milk when you wish to pour the milk out evenly?