What is Aneroid Barometer its principle and correction?

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Corrections:

Height above sea level
With the increase in the altitude, the air pressure reduces. Hence, the reading is then adjusted to an equivalent sea-level pressure for purposes of reporting. Aneroid barometers have a mechanical adjustment that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude.

Index error
The elasticity of the barometer capsule varies depending on temperature. A bimetallic plate is used for temperature compensation. Once pressure distort an elastic body, it doesn’t completely return to its original shape even after the pressure is relieved. Due to this characteristics called hysteresis,  because of the changes in vacuum chamber’s elasticity over a period of time, index error occurs and  error is accounted as perits prefix given i.e. + or -. It can be found on certificate issued after callibration of the barometer.