Barometers

This subcategory includes barometers and barometer & clock sets.

The aneroid barometer was invented in 1843 by French scientist Lucien Vidie. It uses a small, flexible metal box called an aneroid cell (capsule), which is made from an alloy of beryllium and copper. The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen. In addition, the mechanism is made deliberately "stiff" so that tapping the barometer reveals whether the pressure is rising or falling as the pointer moves.

The actual reading of the instrument depends on its location, because air pressure decreases at altitudes above sea level (and increases below sea level).

Aneroid barometers have a mechanical adjustment for altitude that allows the equivalent sea level pressure to be read directly.

[Source: Wikipedia]