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.
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