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| image title | Mercury barometer |
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| caption | This barometer uses a mercury-filled tube to indicate the amount of pressure that the atmosphere exerts on a surface. The tube is sealed at its upper end and is initially set up completely filled with mercury when horizontal. It is raised to the vertical while keeping the open end submerged in mercury. The mercury sags, creating a vacuum in the sealed end. The pressure of the air is responsible for supporting the column of mercury. About 760 mm of mercury can be supported in this way, several cm or so more when the pressure is higher and several cm or so less when it is lower. Barometers of the age of the one shown here are often marked in inches rather than millimetres. This one also includes markers and labels for the mid-latitude weather associated with changing pressure. A thermometer is included for good measure. See also Aneroid barometer, MI010. |
| code | MI009 |
| Section | Meteorological instruments |
| Subsection | Surface-based instruments |
| image credit | © The Open University |
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