Barometer is an instrument used to measure the atmospheric pressure (air pressure) around the earth. It helps in forecasting the weather and shows how the atmospheric changes occur when the air pressure varies. Evangelista Torricelli of Italy devised the first barometer in 1643 by using a glass tube and mercury. The same principle is followed even today for accurate reading of atmospheric pressure.
The atmospheric pressure is the force exerted on a unit surface area by the weight of the atmospheric air. The atmospheric pressure can be measured in terms of the height of a liquid column, which exerts a downward pressure equal to the atmospheric pressure. There are two types of barometers-mercury barometer and aneroid barometer.
Mercury Barometer: - A mercury barometer uses mercury as the liquid. Mercury is used because it does not stick to the walls of the container and it has a high density. A mercury column of 76 cm long, which is able to balance the normal atmospheric pressure, is used in it. A mercury barometer consists of a glass tube about 35 cm long, which is closed at one end. The tube is filled with mercury and is inverted over an open vessel containing mercury. The mercury in the glass tube exerts a downward pressure on the mercury in the open vessel. So, the mercury in the tube falls to a height of 76 cm above the level of mercury in the open vessel. When the atmospheric pressure increases, the height of mercury column increases. Similarly, when the atmospheric pressure decreases, the height of mercury column also decreases. Mercury barometer is more accurate. But it is bulky and inconvenient to carry.
Aneroid Barometer: - The aneroid barometer is more common. It is a dry barometer in which no liquid is used. It functions by using an airtight metal box that responds to changes in air pressure. It consists of an evacuated metal drum having a spring with a pointer attached to it. The pointer moves over a circular scale when the atmospheric pressure changes.