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A barometer is a device for measuring atmospheric pressure. The pressure of the air is affected by weather systems and also changes with altitude, decreasing with increasing height as there is progressively less air pressing down. A barometer can therefore be used to estimate altitude or, more commonly, to provide information that helps in forecasting the weather. An aneroid barometer is based on a flexible metal compartment or diaphragm containing air at reduced pressure, connected to a needle that points to a calibrated circular scale. When air pressure increases it pushes the diaphragm inwards and when it decreases the diaphragm moves outwards; these movements determine the position of the needle on the scale, giving an indication of the pressure.
The term “aneroid” means “without fluid” and contrasts with the first type of barometer to be developed. This was invented by the Italian physicist and mathematician Evangelista Torricelli around 1645 and relied on the pressure of the air pushing down on a pool of liquid and pushing it up a vertical tube. It was soon found that mercury — the heaviest liquid available — was the most suitable fluid, and this type is known as a mercury barometer. Although mercury barometers are very accurate, they are expensive, bulky, require careful handling and are not very portable.
The aneroid barometer was invented in the 1840s by the French scientist Lucien Vidie. This type of barometer can be small and lightweight, is suitable for use in the home and some types can even be carried around — to be used as an altimeter to estimate altitude based on air pressure, for example. Another advantage of the aneroid barometer is that it can easily be adapted to construct a barograph, where the needle is a pen that traces a line on a roll of paper, providing a continuous record of pressure changes over time. Among the disadvantages of aneroid barometers are that they are less accurate than mercury barometers and subject to gradual changes in the sensitivity of the diaphragm, so that they need to be recalibrated regularly, typically every one to two years.
Calibration of an aneroid barometer is usually in millibars, a commonly accepted unit of air pressure, but can be in hectopascals, which are equivalent. Although other factors need to be taken into account, as a general rule increased pressure is an indicator of dry, calm weather and reduced pressure an indicator of rain and wind. For this reason, in addition to the normal calibration, aneroid barometers in the home often have rough descriptions of the expected weather ranging from, for example, “stormy," “rain,” “change,” “fair” through to “fine”. Aneroid barometers for home use can be ornamental as well as functional and antique aneroid barometers are often valuable and sought after.