A barometer is an instrument which is used for measuring atmospheric pressure. There are basically two kinds of barometers which are commonly used; these are mercury barometer and an aneroid barometer. The first has a long narrow glass tube filled with mercury supported in a container of mercury, and the second is equipped with an elastic disk whose size changes as a result of air pressure.
Mercury barometer was discovered by the Italian physicist Evangelista Torricelli in the year 1643. The working principle of these barometers can be easily illustrated as follows: it atmospheric pressure balances a column of mercury, the height of which can be precisely measured. Mercury barometers are often corrected for ambient temperature and the local value of gravity; this is done to increase their accuracy. The most common pressure units include pounds per square inch; dynes per square centimeter; newtons per square meter.
