#### What is atmospheric pressure? What are the uses of atmosferic pressure? Devices work by using of atmospheric pressure?

At one time or another you have used what is called suction. You suck up lemonade through a straw, and pumps suck up water through pipes. Medicine droppers and fountain pens are filled by suction. Many people think that suction is caused by a pull from the inside of an empty space. This is what everyone thought long ago. In those days, even scientists did not know what you have already learned: Air is pulled downward by gravity just as solids and liquids are. The downward pull of gravity causes air to have pressure. What really makes things go into an empty space is not a pull from the inside but a push from the outside. This push is produced by atmospheric pressure.

Many devices that we use every day work by atmospheric pressure. The first step in using atmospheric pressure in any device is to make the air pressure less inside a closed space. When this has been done, the pressure of the atmosphere tends to force air back into the closed space. Then the device works as we want it to work.

As you already know, scientists are quite sure that air is made up of tiny, invisible molecules, which are bouncing against each other and against other things at great speed. inside a pumped-up tire, air molecules are striking the walls. Each molecule that strikes a wall gives it a little push. Since billions of molecules are striking the walls each second, the total outward push is very great. So the pressure inside a tire is caused by air molecules striking the walls. To make the air pressure less inside a closed space, we must reduce the number of molecules that strike its sides every second.

There are three ways to make fewer molecules strike the sides each second: (1) Take out some of the air. (2) Enlarge the space. (3) Cool the air. Each of these three ways will lower the air pressure inside a closed space. Lowering the air pressure inside a closed space is called creating a vacuum. A perfect vacuum is an empty space from which everything, even air, has been removed. To create a perfect vacuum, every molecule would have to be taken out of the closed space. No one has ever been able to do this. A partial vacuum is what is actually created. The space above the mercury in the glass tube of a barometer is a partial vacuum. So is the space inside the flat, round box of an aneroid barometer.

When you press a rubber suction cup against a smooth surface, some of the air inside is forced out. Then when the cup comes back to its original shape, there is less air in it. This creates a partial vacuum inside, while on the outside the full pressure of the atmosphere is still acting. Since the air pressure on the outside is greater than on the in-side, the cup is held firmly against the smooth surface. Squeezing the rubber bulb of a medicine dropper forces out much of the air. The tip of the glass tube is then dipped into a liquid. When the bulb springs back to its original shape, a partial vacuum is created inside. Then atmospheric pressure on the surface of the liquid forces some of the liquid up into the dropper.

Many pumps use atmospheric pressure. When you work the handle of a lift pump, air is removed from the cylinder as the piston moves upward. The air in the pipe below expands into the cylinder through the lower valve. The valve is like a little trap door that lets air or water go up but not down. When the piston starts downward, the lower valve closes and keeps the air in the cylinder from going back into the pipe. As this air is compressed by the piston, it pushes the upper valve open and escapes. More and more air is removed from the pipe as the piston goes up and down.

The pressure inside the pipe becomes less and less. As this happens, atmospheric pressure on the water in the well pushes the water higher and higher in the pipe. At last, the water reaches the cylinder of the pump. Then the piston lifts the water out, just as it lifted the air out at first. So this kind of pump works by removing air from the inside of the pipe so that atmospheric pressure will push water into it. Then the piston lifts the water out through the spout. Sometimes the piston and valves of a pump become so worn that the pump is no longer airtight. Then water must be poured in to fiil the cracks so that the piston will remove air from the pipe. This is called priming the pump.

About 300 years ago, an Italian nobleman had a pump put 40 feet above the water in a well. Al-though the pump seemed to be in good working order, it would not deliver any water. You know that atmospheric pressure will push mercury up only about 30 inches. Mercury is 13.6 time as dense as water. So atmospheric pressure can push water up 13.6 times as high as it can push mercury (13.6 X 30 inches = 408 inches, or 34 feet). In other words, atmospheric pressure would force water up only about 34 feet into a perfect vacuum. This means that even a perfect pump would not work if its cylinder were more than 34 feet above the water in a well. Usually, pump cylinders are set not more than 25 feet above the water.

Centrifugal pump uses centrifugal force together with atmospheric pressure to pump water. inside the circular case is a paddle wheel, called the impeller, which spins very rapidly. To make a better vacuum, this kind of pump is usually primed before the impeller starts spinning. The rapid spinning of the impeller throws the water toward the outside of the case, creating a partial vacuum in the center. One pipe leads water away from the outside of the case, and the other lets water into the center. Atmospheric pressure pushes the water up through the intake pipe into the center of the pump. Centrifugal pumps can move water faster for their size than any other kind of pump. They are used in such places as automobile cooling systems, water-supply systems, and in the basements of skyscrapers to fiil the water tanks up above.

Vacuum cleaners also use centrifugal force and atmospheric pressure inside a vacuum cleaner is a centrifugal pump driven by an electric motor. The spinning paddle wheel, or rotor, of the pump throws air out of the case into a closely woven cloth bag or something like it. When this happens, a partial vacuum is created at the center of the case. Then atmospheric pressure forces more air into the nozzle of the cleaner and into the center of the pump. When the nozzle is close against a rug, the pressure of the atmosphere forces air through the rug and into the cleaner. The rapidly moving air carries dirt with it into the cloth bag. The bag catches the dirt and lets the air pass through. Some vacuum cleaners have a revolving brush to loosen dirt, hair, and lint and help pick them up.

All devices that work by using atmospheric pressure are alike. In each device, there is some way of creating a partial vacuum. This lowers the air pressure in or around the device. Then the greater pressure of the atmosphere pushes against something and does what we want done.