Barometers are used to measure atmospheric pressure in hPa.
Quite often the former unit Torr (mmHg) can be found on the dial. Both units have a defined ratio: 1 Torr = 1.333223684 hPa.
Aneroid barometers use the deformation of a flat empty metal can measure atmospheric pressure. Increasing air pressure compresses the can while decreasing air pressure expands it again. This movement, which is only a few hundredths of a millimetre, is transmitted to the pointer by means of a fine mechanism and the air pressure is thus displayed.