A barometer measures atmospheric pressure, which changes with weather and altitude. An aneroid barometer uses a flexible metal diaphragm to indicate pressure on a calibrated scale. It was invented in 1840, is portable, and can be used as an altimeter or barograph. However, it is less accurate than a mercury barometer and must be recalibrated regularly. Aneroid barometers are calibrated in millibars or hectopascals and can provide rough weather predictions. They can also be ornamental and antique versions are valuable.
A barometer is a device for measuring atmospheric pressure. Air pressure is affected by weather systems and also changes with altitude, decreasing with increasing altitude as the air presses downward. A barometer can then be used to estimate altitude or, more commonly, to provide useful information in predicting the weather. An aneroid barometer is based on a flexible metal compartment or diaphragm containing air at reduced pressure, connected to a needle which points to a calibrated circular scale. When the air pressure increases it pushes the diaphragm inward and when it decreases the diaphragm moves outward; 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 developed. It was invented by the Italian physicist and mathematician Evangelista Torricelli around 1645 and relied on air pressure pushing down a pool of liquid and pushing it up a vertical pipe. It was soon discovered that mercury, the heaviest liquid available, was the most suitable fluid and this type is known as a mercury barometer. While mercury barometers are highly accurate, they are expensive, bulky, require careful handling, and aren’t very portable.
The aneroid barometer was invented in 1840 by the French scientist Lucien Vidie. This type of barometer can be small and light, it 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 atmospheric pressure, for example. Another advantage of the aneroid barometer is that it can easily be adapted to build a barograph, where the needle is a pen that draws 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 diaphragm sensitivity, so they must be recalibrated regularly, usually every one or two years.
The 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 must be taken into consideration, as a general rule increasing pressure is an indicator of dry, calm weather and reduced pressure is an indicator of rain and wind. For this reason, in addition to the normal calibration, home aneroid barometers often have rough descriptions of expected weather ranging from, for example, “storm”, “rain”, “change”, “good” to “good”. . Home aneroid barometers can be ornamental as well as functional, and antique aneroid barometers are often valuable and sought after.
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