an instrument for measuring atmospheric pressure

The atmosphere exerts a pressure because air has weight and is being pulled to the earth by the force of gravity. For this reason atmospheric pressure depends on the height of air above the point at which it is being measured and is lower on top of a mountain than it is at sea level.

In atmospheric pressure measurement, a column of a standard liquid (one with known density) in a vertical tube is used and as gravity is approximately constant at all points on the earth's surface, it is only necessary to record the column height. Mercury is the most common liquid used because it is extremely dense and so only needs a short column. One atmosphere is the pressure at the bottom of a column of mercury only 30 inches high. Using water, whose density is about 1/13 that of mercury, one atmosphere is about 34 feet of water.

Mercury Barometer

The fact that the atmosphere has weight and exerts pressure was first demonstrated by Galileo Galilei in the early 17th century, but it was his pupil Evangelista Torricelli who worked out the principle of the mercury barometer. The two scientists had already determined that the limit to which air pressure on water in a well could lift water up a pump pipe was 32 feet, but Torricelli found that one some days pumps could lift water a little higher and on other days a little less than 32 feet. Torricelli determined that the pressure cannot always be the same, that it must change a little from day to day and, perhaps, even from hour to hour. In 1643, Torricelli decided that air pressure could be measured by a column of mercury, and the mercury barometer was subsequently invented.

In principle, the weight of a column of mercury is balanced against the weight of the air. To do this, a glass tube more than 30 inches long, sealed at one end, is completely filled with mercury. This is then placed upright with its sealed end uppermost and its open end dipped into an open bowl of mercury. The pressure on the open surface of the mercury is the atmospheric pressure, and this must balance the pressure created by the column of mercury. The mercury level in the tube therefore falls until the weight of the column exactly balances the atmospheric pressure; by falling it creates a vacuum at the top of the glass tube. That fact that the bottom of the tube is submerged prevents air from entering the tube, which would let all the mercury run out.

All modern mercury barometers are based on Torricelli's basic design, and only differ by a few modifications which enable more accurate and consistent readings to be made.

The diagrams below show the principle of the mercury barometer. At far left, both ends of the tube are open and the atmospheric pressure is equal on both sides. In the middle, one tube end is sealed and evacuated. The pressure now affects one side only. Finally, the open end is replaced by a shallow dish and the closed end by a single inverted glass tube. The height of the column of mercury in the tube is used to measure the atmospheric pressure acting on the open dish of mercury.

Fortin Barometer

In the simple mercury barometer, any change in the column height will mean a slight change in the mercury level in the bowl. Yet, as it is from this level that the column height is measured, this would mean using a moving scale to find the height at the top of the column. The Fortin barometer, which was designed by Jean Fortin and first brought into use in the early 19th century, overcomes this problem by employing an adjustable container so that the open mercury level can be raised or lowered to a fixed point. To obtain a pressure reading the adjusting screw is turned, compressing the flexible leather bag until the mercury level in the cistern reaches the tip of the ivory pointer. The difference between the two mercury levels can then be read from the fixed scale and movable vernier -- which increases the accuracy of the reading -- by the column.

Kew Barometer

The Kew barometer has a fixed cistern and allowance is made for changes in the open mercury level by altering the spacing of the column scale. If both the column and the cistern are perfectly cylindrical then the change in the column scale compared to the true inch or millimeter will always be in a fixed ratio. This ratio is made very close to equality by making the barometer column very narrow compared with the diameter of the cistern.

Kew barometers are often used in ships, where motion can affect the accuracy of the readings by causing oscillations in the mercury column. This is overcome by introducing a restriction in the column which dampens such oscillations without unduly affecting the sensitivity of the device.

A further modification is an air trap to prevent any air from entering the column from the cistern and reaching the vacuum at the top of the tube, which might happen if the barometer was tilted or shaken. Kew barometers are designed to be portable, usually being carried upside down with the mercury completely filling the glass tube.

Aneroid Barometer

The term aneroid means without liquid. Although not offering quite the same degree of sensitivity or accuracy as the mercury type, it has the advantage of being a robust instrument useful in such applications as altimeters, and generally where mobility is required (an aneroid altimeter is exactly the same machine as a barometer, and is used to measure height by air pressure). The aneroid barometer was invented by Italian scientist Lucien Vidie in 1843.

The principle of the aneroid barometer is that a sealed metal chamber, sometimes called the bellows, expands and contracts with the changes in atmospheric pressure. This expansion and contraction can be suitably amplified with a rack-and-pinion arrangement or levers to move a pointer on a scale. The chamber is usually made of thin sheet nickel-silver alloy or hardened and tempered steel. High grade aneroid barometers have a series of steel diaphragms formed into a complete unit and corrugated to provide greater flexibility. Temperature compensation is necessary, since the diaphragms expand and contract with temperature changes, and their elasticity alters. Any air within the chamber also leads to unwanted temperature effects from expansion and contraction, and so a high vacuum is generally created in the chamber. Collapse of the chamber under atmospheric pressure is resisted by the springy nature of the diaphragms.

Aneroid barometers are calibrated against a standard mercury barometer, and require frequent readjustment.

Measuring Altitude

Soon after Torricelli invented the mercury tube method for measuring atmospheric pressure, scientists began wondering if the tube could be used to show the difference in pressure at sea level and at the tops of mountains. It had been shown that the weight of the air supported the fluid in the tube, so it was supposed that the mercury should fall farther on a mountaintop, since there is less air to hold the mercury up. Changes in the level of the mercury could, then, be used to measure altitudes. Mathematician Blaise Pascale proved the theories to be true, and scientists subsequently named the mercury tube barometer, which means pressure-measure. Since then, barometers of all types have been helpful in measuring heights, as well as to tell how high an airplane is flying above sea level. Barometers used in airplanes are called altimeters.

Forecasting Weather

The barometer has made possible the science of forecasting changes in the weather. When air gets thinner, it weighs less and the pressure lessens, or lowers. The air cannot hold up the weight of the mercury in the barometer tube, so the mercury in the tube falls. When the air pressure is greater than the weight of the mercury in the tube, more mercury is forced up into the tube, so the mercury rises. Because damp air weighs less than dry air, falling mercury usually foretells an approaching storm, while rising mercury usually means fair weather. For these reasons, barometers are important tools for the weatherman.

The barometer below is similar to ones found in many homes. To determine changes in the weather, the user first turns a hand-controlled needle so that it points to the current reading on the dial. Changes in air pressure affect the metal diaphragm in the center of the dial, moving a needle attached to the diaphragm. Later, when the barometer is read again, the space between the hand-controlled needle and the needle attached to the diaphragm will show if the barometric pressure is rising or falling.

PRINT SOURCES
The Illustrated Science and Invention Encyclopedia New York: H.S. Stuttman Co., 1977
Our Wonderful World New York: Grolier Incorporated, 1965

SEE ALSO
Galileo Galilei
Evangelista Torricelli

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This page was last updated on 12/15/2017.