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Barometers and Hydrometers

Mercury is the only metal that is liquid at room temperature. It expands and contracts evenly with temperature and pressure changes making it useful in devices used for measuring temperature and pressure including the following:

Barometers measure atmospheric pressure. Mercury barometers consist of a glass tube that is closed at one end, with a mercury-filled reservoir at the base. Often, one end of the tube is open to atmosphere so that the elemental mercury is exposed to air. The mercury rises and falls with changes in atmospheric pressure.


Hydrometers measure the specific gravity of liquids. Mercury was often used in hydrometers as a weight, but research indicates that mercury hydrometers are no longer manufactured and sold as new. However, farms, schools and laboratories may still have mercury hydrometers in use. Non-mercury hydrometers contain a lead ballast for weight, and are an alternative to using mercury.