A barometer is an instrument which measures atmospheric pressure. The barometer was invented by the Italian scientist, Evangelista Torricelli.
The barometer in its basic form consists of a bowl of mercury and a tube which is sealed at one end. The tube is filled with mercury. It is then inverted over the bowl. The tube is made to stand vertically over the bowl with the help of a stand.
The level of mercury in the tube now falls in the tube. This is the indication of the atmospheric pressure. The column of mercury in the tube is supported by the atmospheric pressure. The level of mercury in the tube is dependant on the atmospheric pressure.
The standard atmospheric pressure is 76 cms of mercury.
one bar of pressure = 75.0061561303 cms of mercury
Technically, the barometer can also be made using water. However, a very long tube may be required which is not practical.
The standard atmospheric pressure can support a column of water 10 metre high.
The mercury barometer is not a portable instrument. Hence, it has been replaced by the Aneroid Barometer.
See Article on Aneroid Barometer