A barometer is a device for measuring the atmospheric pressure. There are different types of barometers. Some of them are described below:
The mercury level in the tube falls until it is about 76 cm (760 mm) vertically above the mercury level. It is the atmospheric pressure acting on the surface of the mercury in the trough that supports the vertical mercury column.The atmospheric pressure is therefore specified by so many millimetres of mercury. For example, the atmospheric pressure is 760 mm Hg at sea level (equivalent to 105 Pa).
Greater the atmospheric pressure, greater is the vertical height of the mercury column. The space above mercury is a vacuum. This empty space is called 'Torricellian vacuum'. This can be shown by tilting the tube till the mercury fills the tube completely.
Generally, a low pressure or a sudden fall in pressure indicates unsettled weather while a high pressure or a rising pressure is associated with fine weather.As the atmospheric pressure changes, the top of the metal box moves up and down. Levers magnify this movement and move a pointer over a circular dial.