Monitoring the weather

The pressure of the atmosphere at the surface of the earth will balance the weight of a column of liquid. Mercury is favoured because it is so heavy compared with other liquids, and the barometer tube can be relatively short - less than a metre.

Mercury-in-glass barometers, being expensive, cumbersome and difficult to transport, have largely been replaced by 'aneroid' barometers. These consist of a flexible metal cell or bellows from which some air has been evacuated. When the air pressure rises it presses the sides of the cell together, and when the pressure falls the cell opens up again. The cell is connected to a pointer on a dial. It can also operate a pen that moves across a graduated paper chart wound around a drum for either 24-hour or 7-day rotation: this is a barograph.

(below) Aneroid barometer