Barometers
A barometer is an instrument that measures atmospheric pressure. It uses water, mercury or a small metal cell to measure pressure. Short-term weather changes can be forecast from changes in atmospheric pressure.
Mercury Barometers
Mercury barometers usually consist of a glass tube approximately 30 in (76 cm) in height. The top end is closed and the open-ended base sits in a mercury reservoir. Pressure from the atmosphere pushes mercury up the glass column until the weight of mercury in the column equals the force of pressure exerted on the reservoir.
Aneroid Barometers
An aneroid barometer use a small metal box made from an alloy of beryllium and copper called an aneroid cell. The cell is evacuated and contains a spring to prevent collapse. Changes in outside air pressure cause the cell to expand and contract. The cell is attached to mechanical levers which amplify the movements and display them using a needle on a face similar to a clock.
