What is an altimeter? State it's principle. How is it's scale calibrated ?

An altimeter is an aneroid barometer, and it is used in aircraft to measure it's altitude.

It's principle is as follows —

As atmospheric pressure decreases with the increase in height above the sea level, therefore a barometer which measures the atmospheric pressure, can be used to determine the altitude of a place above the sea level.

It's scale is calibrated in terms of height of ascent with height increasing towards left because the atmospheric pressure decreases with increase of height above the sea level.