The device used to measure atmospheric pressure is called a barometer. Barometers can be classified mainly into two types: mercury barometers and aneroid barometers. A mercury barometer uses mercury in a glass tube to measure air pressure based on the height of the mercury column, while an aneroid barometer employs a small, flexible metal box that expands and contracts with pressure changes. Barometers are essential tools in meteorology, helping to predict weather changes and assess altitude.