Barometer

The barometer is a device used to measure atmospheric pressure. Its implications are profound, going far beyond its usefulness for predicting the weather. The device has helped scientists understand the nature of the atmosphere and to discover that the atmosphere is finite and does not reach to the stars.

There are two main forms: mercury and aneroid. In the mercury barometer, a glass tube contains mercury and is sealed at the top, while the bottom of the tube opens into a mercury-filled reservoir. The level of mercury in the tube is controlled by the atmosphere that presses down on the mercury in the reservoir. For example, under conditions of high atmospheric pressure, the mercury rises higher in the tube than when atmospheric pressure is low. In the tube, the mercury adjusts until the weight of the mercury in the column balances the atmospheric force.

In aneroid barometers, there is no moving liquid. Instead, a small flexible evacuated metal capsule is used. Inside the capsule is a spring. Small changes in atmospheric pressure cause the capsule to expand or contract. Lever mechanisms within the barometer amplify these small movements, allowing users to read pressure values. If the atmospheric pressure is falling, this change often indicates that stormy weather is likely. Rising air pressure suggests that fair weather with no precipitation is likely.

Italian physicist Evangelista Torricelli is credited with inventing the barometer in 1643. In 1648, Blaise Pascal used a barometer to show that there is less air pushing down at the top of a mountain then there is at the bottom; thus, the atmosphere did not extend forever.