Barometers are instruments to measure atmospheric pressure. There are two main types of barometers - mercury and aneroid. The mercury barometers measure the atmospheric pressure by means of height of mercury column pushed up a glass tube by the atmospheric. The aneroid barometer works on the basis of expansion and contraction of a metal chamber under the influence of atmospheric.
The mercury barometer is more accurate but less sensitive. That is though the mercury barometer indicates more accurately the absolute atmospheric pressure, it is not suitable for detecting very small changes in the pressure. Not only that the mercury barometer measures atmospheric pressure using using the primary effect of of atmospheric pressure that defines it. In comparison, the aneroid barometer measures atmospheric pressure based on its effect in compressing a metal chamber. Thus while the atmospheric pressure can be directly measured by a mercury barometer, in case of aneroid barometer it needs to be graduated by comparing the pressure shown by it against pressure known by other methods. Thus the aneroid barometer itself depends on mercury barometer for its correct graduation.
Mercury barometer uses the height of the mercury column in a vacuum tube inverted on bowl filled with mercury,to show the atmospheric pressure .The height of the mecury column rises up or comes down in accordance with the atmospheric pressure over the bowl.It has an advantage over other liquid barometers, as the density of mercury is higher than other liquids particularly water, for which we need the height of a column of more than 34 feet. But yet the mercury barometer is much bulky diffcult to carry , as compared to the aneroid barometer, which uses the principle of pressure on a partially vaccum cell subject to the external atmospheric pressure. Compared to mercury bameters, aneroid barometrs are more handy and easy to read.