Barometers are used to measure atmospheric air pressure in hPa.
Quite often, the former unit Torr (mmHg) can also be found on the dial. Both units are in a defined ratio: 1 Torr = 1.333223684 hPa.
To measure atmospheric air pressure, aneroid barometers use the deformation of a flat, airless metal can. Rising air pressure squeezes the can together, as the air pressure drops, it expands again. This motion, which is only a few hundredths of a millimeter, is transmitted to the pointer with the aid of a fine mechanism, thus displaying the air pressure.