Wendy Tetrault, age 8, of South Windham, Maine, for her question:
How does a barometer work?
It's fun to ask a barometer what tomorrow's weather will be like. The useful little gadget looks very complicated. But when you get two things into your head you can understand how it works. One is easy, but the other may be a bit hard to believe. The easy one reminds you of tipping on a seesaw when you go up, your pal on the other end goes down. The other one is about the air. Just because you can't see it, you may think that the filmy stuff does not weigh anything at all. The truth is that it does. It may be hard to believe, but even a shoebox of air has a little feathery weight.
About 300 years ago, some thoughtful people were curious about how much the air weighs. So they invented barometers to measure it. A good way to measure weights is with a pair of balance scales. They work very much like a seesaw. When you put a weight on each side, the heavier one goes down and the lighter one goes up. This tells you which weight is lighter and which is heavier. If both of them weigh exactly the same, neither side goes up or down. The two sides are evenly balanced.
The inventors did not use a pair of scales exactly like this to weigh the air. But they used the same idea. Their barometers balanced the weight of the air with some silvery liquid mercury in a glass tube. The air in a room weighs just a few pounds. However, the whole globe is wrapped in a huge blanket of air that reaches up hundreds of miles. This is the atmosphere. It weighs billions of tons and it presses down on every spot of land and sea. Its weight is called atmospheric pressure.. This is logical because it means the weight of the atmosphere pressing down on the earth.
A barometer weighs just a small sample. For example, the pressure on one square inch is the weight of a tall skinny column of air, an inch square reaching from the ground to the very top of the atmosphere. The first barometers used silvery liquid mercury to weigh a sample of air and so do many modern barometers. The inventor used a small dish and a glass tube with one end open and the other sealed closed. He poured some mercury into the dish and used some more to partly fill the tube. Then he put his thumb over the tube, turned it upside down and set the open end under the mercury in the dish without spilling a drop.
Surprise! All the mercury did not run down the tube, because the air pressed down on top of the mercury in the dish. .The level of mercury in the tube exactly balanced the weight of the air sample. Another surprise.. A ladder of lines beside the tube showed that the level of mercury rises and falls. This proved that the weight of the air sample must change, day by day and even hour by hour. People soon noticed that these changes in atmospheric pressure very often predict changes in the weather. But that, says Andy, is the answer to another question.
Our modern mercury barometers are finer and better, but they still work the same old way. Our aneroid barometer uses no mercury. It is a metal box with the air taken out and a spring sealed inside. Light air pressure lets the top of the box bulge up; heavy.air dents it down. The slightest change moves the delicate spring inside. The spring moves gears and levers that relay the changes to a moving needle on the outside of the box.