A barometer is a meteorological instrument that measures air pressure. Used primarily in weather forecasting, a barometer uses different methods to measure the weight of the air above the instrument. There are two major types of barometers that use different methods to ascertain air pressure measurements, known as mercury and aneroid barometers.
Mercury barometers are the older form of instrument, and are still used today for weather forecasts. Invented in the 17th century by an Italian physicist, these instruments are very accurate, but can be dangerous due to the high levels of mercury needed to operate the barometer. Mercury barometers are also very delicate and fairly large, and generally need to stay in one place permanently. The instrument consists of a large glass tube with a closed top and open bottom, which sits inside a larger container. Both the container and the tube are filled with mercury, and the tube may have measurement markings on the glass or along the sides.
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A mercury barometer works by equalizing pressure between the mercury in the large container and that within the tube. As atmospheric pressure rises, the weight of the air on the mercury of the container drives the mercury higher up the tube. When pressure falls, the air becomes lighter, and the mercury in the container exerts less force on that in the tube, allowing the level to fall.
The aneroid barometer works quite differently than the mercury instrument, and was invented almost exactly two centuries later. A French inventor, Lucien Vidie, was able to develop the new version in the mid-19th century, thanks to improvements in metalworking and technology. Aneroid barometers typically consist of a very small, flexible capsule, which is attached to levers that can move a needle on a dial, allowing the measurement to be read.
The capsule in an aneroid barometer works by using a flexible metal that is welded shut to create an interior vacuum. When pressured, the capsule walls squeeze, exerting pressure on an attached lever that moves a measurement dial. The greater the pressure, the greater the squeeze, and the higher the air pressure dial rises. While aneroid barometers work very well for general readings, they tend not to be as accurate as mercury versions. On the other hand, aneroid barometers are sturdier, less dangerous, and easy to transport.
Either type of modern barometer works extremely well, thanks to centuries of fine-tuning on both types of device. They are frequently used to predict the coming of storms, rain, or other inclement weather, which usually result in low-pressure readings. Fair weather can also be accurately predicted if a barometer shows a high pressure reading, which is linked with dry air and clear skies.