A barometer measures atmospheric pressure. It consists of:
- a tray, which contains mercury on which atmospheric pressure is acting;
- a tube, with the mercury column, whose weight balances the atmospheric pressure. The tube is made of glass, usually about 80 cm long, with an internal diameter of approximately 7 mm. Above the mercury in the tube there's an absolute vacuum. Next to the column, a thermometer measures the temperature of the mercury;
- a scale used to measure the barometric height, the difference in level between the two free surfaces of mercury in the tray and in the pipe.