Mercury Barometers
Mercury barometers are constructed first by filling with mercury a glass tube that is closed at one end and inverting it into an open dish of mercury, and then by using a vacuum pump attached to the top of a glass tube. Finally three differently shaped mercury barometers are constructed to show that shape has no effect on the mercury level.
Keywords
atmospheric pressure, barometric pressure, barometer
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Torricelli barometer
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The Torricelli barometer, invented by Evangelista Torricelli in 1643, is made by first filling a dish with mercury. Mercury is then poured into a long tube. The tube is filled almost to the top, stoppered and inverted several times to remove air bubbles. The tube is then completely filled with mercury using a dropper. A finger is placed over the top of the tube and the tube is inverted and placed into the dish below the level of the mercury. When the finger is removed, the level of the mercury inside the tube drops until the pressure at the bottom of the column of mercury is equal to the pressure exerted by the surrounding air. Since no air was allowed to enter the tube, the empty space above the mercury column is a vacuum.
We can measure the pressure exerted by the atmosphere by measuring how high the mercury column rises above the level of the mercury in the dish. The pressure (here) is 731 millimeters of mercury.
Vacuum pump
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A safer way to fill a barometer is to use a vacuum pump to create a vacuum at the top of the barometer. This draws the mercury up into the tube from the dish at the bottom. Continuing to draw a vacuum does not change the level of the mercury inside the tube. Again, the pressure is 731 millimeters of mercury. Closing the valve maintains the vacuum when the hose is removed.
Standard atmospheric pressure is 760 millimeters of mercury but barometer readings vary with weather conditions and are lower at higher elevation.
Barometer shape
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Because the height of the mercury is dependent on the atmospheric pressure, changing the shape or volume of the barometer does not change the level of the mercury. Three differently shaped barometers are all filled with mercury by creating a vacuum at the top of the tube. Even though they have very different shapes, the mercury level is the same in all three.
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