What is a Barometer?
A barometer is an instrument that measures atmospheric pressure. Barometers are used by weather forecasters to measure air pressure, because rising or falling atmospheric pressure indicates a coming change in weather.
They are also used to indicate altitude, since atmospheric pressures are lower at high altitudes then at low altitudes. A barometer adapted for measuring altitude is called an altimeter. Because air has weight exerts a pressure. The weight of air at the 'top' of the atmosphere presses down on the lower layers of the air. The barometer is an instrument that measures this air pressure. At sea level, air pressure averages about 103 kPa.
As a general rule, falling air pressure means stormy weather, and rising pressure means more settled weather. There are two main types of barometers, mercury barometers and aneroid barometers.
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How a Mercury Barometer Works
The mercury barometer consists of a glass tube about 3 feet (1 meter) long. The upper end of the tube is closed, and the lower, open end is immersed in a small pan of mercury. The tube itself is filled with mercury except for a few inches at the top, where a vacuum exists. It works on the principle that normal air pressure supports a certain amount of mercury in a tube out of which the air has been drawn. This amount (about 76 cm) varies with changes in pressure. When there is a decrease in air pressure, the column falls. When there is an increase, the column rises.
Two forces balance each other in the mercury barometer. The first is the force exerted by the column of mercury in the tube, which tends to flow down into the pan. The second is the force exerted by the atmosphere on the mercury in the pan, tending to push the mercury up into the tube. The force of the atmosphere, therefore, supports the column of mercury in the tube.
Since the force exerted by the atmosphere and the force of the mercury column must balance each other, any change in the atmospheric pressure causes a change in the height of the mercury column. If atmospheric pressure increases, then more mercury is forced into the tube from the pan, and the column becomes higher. If atmospheric pressure decreases, then it cannot support as high a column of mercury, and some of the mercury runs out into the pan. If there were no atmospheric pressure at all, all of the mercury would run out into the pan.
A scale is placed beside the tube so that the height of the mercury column can be measured, and atmospheric pressure is expressed in terms of this measurement. The average atmospheric pressure at sea level, for example, is equal to 29.92 inches or 760 millimeters of mercury. Some barometers are calibrated in millibars. A millibar is a unit of pressure equal to about one-fifth of an ounce per square inch or one-thirty-second of an inch of mercury.
Although most liquids can be used to make barometers, mercury has several advantages. First, mercury is heavy as compared with water and other substances that are liquid at room temperatures. While a 30-inch column of mercury exerts a pressure equal to that of the atmosphere, a column of water would have to be about 34 feet high to exert such a force. A water barometer, therefore, would have to be about 34 feet high. Also, water freezes at 32° F, while mercury freezes at a much lower temperature. Mercury does not evaporate readily, so that very little mercury vapor enters the vacuum above the mercury in the tube. On the other hand, it is possible for water to evaporate enough to cause errors of more than 6 percent in a water barometer.
How an Aneroid Barometer Works
The aneroid barometer is the type often seen in homes. It works on a different principle from the mercury barometer. It contains a small metal box from which most of the air has been drawn. The box has a flexible lid to which a lever is attached. Changes in air pressure move the lid and its lever. The lever is connected to a pointer, which it moves across a dial to indicate air pressure.
The aneroid barometer consists of a sealed, circular metal box from which the air has been removed, a pointer operated by a lever mechanism, and a spring. The metal box is made of two corrugated, flexible metal disks separated by a spring. When the atmospheric pressure rises, it presses on these disks and compresses the spring, which moves the pointer. When the atmospheric pressure on the disks decreases, the spring pushes the disks farther apart, moving the pointer in the opposite direction. The atmospheric pressure indicated by the pointer is read on a scale similar to the one used with mercury barometers. However, the scale of an aneroid barometer must be calibrated by means of a mercury barometer to make it accurate.
Although the aneroid barometer is less reliable than the mercury barometer, it has several advantages. It is more sensitive, smaller, and portable. It contains no liquids and can be used in ships, planes, and moving vehicles. In addition, a pen can be attached to the pointer to make a permanent record of the barometer readings. Barographs, or aneroid barometers that make a written record of their readings, are widely used in weather stations.
The Italian scientist Evangelista Torrice, an assistant to Galileo, built the first mercury barometer in 1643. In 1647, as part of a series of barometric experiments, the French scientist Blaise Pascal constructed several conventional mercury barometers as well as water and wine barometers 46 feet high. In 1648, Pascal devised a now famous experiment which proved both that atmospheric pressure decreases with altitude and that it is the pressure of the atmosphere that forces a liquid into an evacuated space such as that in a mercury barometer. In this experiment, Pascal's brother-in-law, Florin Perier, carried a mercury barometer up the Puy de Dome in central France. As Perier climbed the 4,800-foot mountain, the column of mercury grew steadily shorter, as Pascal had predicted, while another barometer at the base of the mountain showed no change.
Several different designs for mercury barometers were developed during the 19th century to make the instrument more portable or more sensitive or to make scale adjustment more convenient. Barometers using liquids other than mercury were also invented but are less common than the mercury barometer.
The aneroid barometer was patented by the French inventor Lucien Vidi in 1884.