Barometer

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Barometer

A barometer is an instrument for measuring atmospheric pressure. Two kinds of barometers are in common use, a mercury barometer and an aneroid barometer. The first one makes use of a long narrow glass tube filled with mercury supported in a container of mercury, and the second one makes use of a diaphragm whose size changes as a result of air pressure.

Mercury barometers

The principle of the mercury barometer was discovered by Italian physicist Evangelista Torricelli (1608–1647) in about 1643. The principle can be illustrated in the following manner. A long glass tube is sealed at one end and then filled with liquid mercury metal. The filled tube is then inverted and its open end inserted into a bowl of mercury. When this happens, a small amount of mercury metal runs out of the tube, leaving a vacuum at the top of the tube.

Under normal circumstances, the column of mercury in the glass tube stands at a height of about 30 in (76 cm). The column is sustained because air pressure pushes down on the surface of the mercury in the bowl at the bottom of the barometer. At the same time, the vacuum at the top of the glass tube exerts essentially no pressure on the column of mercury. The height of the mercury column in the glass tube, then, reflects the total pressure exerted by the atmosphere at the moment of measurement.

In theory, a barometer could be made of any liquid whatsoever. Mercury is chosen, however, for a number of reasons. In the first place, it is so dense that the column sustained by air pressure is of practicable height. A similar barometer made of water, in comparison, would have to be more than 34 ft (100 m) high. In addition, mercury has a low vapor pressure and does not, therefore, evaporate easily. In a water barometer, the situation would be very different. Water has a much greater vapor pressure, and one would have to take into consideration the pressure exerted by water vapor at the top of the barometer, a factor of almost no consequence with a mercury barometer.

Two important additions needed to increase the accuracy of a barometer are a vernier scale and a thermometer. The vernier allows one to make an even more accurate measurement than is possible by reading the scale itself. The thermometer is needed because the density of mercury and other materials used in the construction of a barometer change with temperature. Most barometers come equipped with thermometers attached to them, therefore, along with conversion charts that permit one to correct barometer readings for a range of actual temperatures.

Modifications to the mercury barometer

The barometer described above is adequate for making rough measurements of atmospheric pressure. When more accurate readings are needed, however, modifications in the basic design of the barometer must be made. The most important factor to be considered in making such modifications is changes that take place in the mercury reservoir at the bottom of the barometer as a result of changes in atmospheric pressure.

When the atmospheric pressure decreases, for example, air pressure is able to sustain a slightly smaller column of mercury, and some mercury flows out of the glass tube into the reservoir. One might hope to find the new pressure by reading the new level of the mercury in the glass tube. However, the level of the mercury in the glass tube must be compared to the level of the mercury in the reservoir, and the latter has changed also as a result of a new atmospheric pressure.

This problem is dealt with in one of two ways. In one instrument, the English Kew barometer, no modification is made in the mercury reservoir itself. Instead, changes that take place in the mercury level within the reservoir, due to changes in atmospheric pressures, are compensated by making small changes in the measuring scale mounted to the glass tube. As one moves upward along the scale, the graduations between markings become slightly smaller to correct for the changing level of the mercury in the reservoir.

A second type of barometer, the Fortin barometer, contains a flexible bag that holds an extra supply of mercury metal. The flow of mercury into and out of that bag and then out of and into the glass tube is controlled by an adjustable screw whose point is moved so as just to touch the surface of the mercury in the reservoir. As atmospheric pressure and mercury levels change, modifications of the adjustable screw keep the mercury level at a constant height.

Aneroid barometer

A major disadvantage of the mercury barometer is its bulkiness and fragility. The long glass tube can break easily, and mercury levels may be difficult to read under unsteady conditions, as on board a ship at sea. To resolve these difficulties, French physicist Lucien Vidie (1805–1866) invented the aneroid (without liquid) barometer in 1843.

An aneroid barometer can be compared to a coffee can whose sides are flexible, like the bellows on an accordion. Attached to one end of the coffee can (aneroid barometer) is a pointer. As atmospheric pressure increases or decreases, the barometer contracts or expands. The movement of the barometer is reflected in the motion of the pointer, which rides up and down with changes in atmospheric pressure.

One way to observe the motion of the pointer is to attach it to the hand on a dial that moves around a circular scale, from low pressure to high pressure. The simple clock-like aneroid barometer hanging on the wall of many homes operates on this basis. Another way to observe the movement of the pointer is to have it rest on the side of a rotating cylinder wrapped with graph paper. As the cylinder rotates on its own axis, the pen makes a tracing on the paper that reflects increases and decreases in pressure. A recording barometer of this design is known as a barograph.

KEY TERMS

Altimeter— An aneroid barometer used to measure altitude.

Barograph— An aneroid barometer modified to give a continuous reading of atmospheric pressures on graph paper.

English Kew barometer— A mercury barometer with a contracting measuring scale and a constant reservoir of mercury.

Fortin barometer— A mercury barometer with a fixed measuring scale and an adjustable reservoir of mercury.

Vapor pressure— The amount of pressure exerted by liquid molecules in the vapor state.

Vernier scale— A movable scale for measuring a fractional part of one division on a fixed scale.

The altimeter

An important application of the aneroid barometer is the altimeter, an instrument used to measure one’s distance above sea level. Atmospheric pressure is a function of altitude. The farther one is above sea level, the less the atmospheric pressure, and the closer one is to sea level, the greater the atmospheric pressure. A simple aneroid barometer can be used to confirm these differences. If the barometer is now mounted in an airplane, a balloon, or some other device that travels up and down in the atmosphere, one’s height above the ground (or above sea level) can be found by noting changes in atmospheric pressure.

The Weather

In order to accurately predict the weather for any particular location the use of barometers are used continuously at several observation points. This barometric system is used to secure data of the motions, sizes, and shapes of air masses as they travel across land masses and bodies of water. Consequently, barometers are essential to meteorology, the study of Earth’s atmosphere and, especially, its weather.