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In 1643, Evangelista Torricelli, a student of Galileo, invented the mercury barometer. His original design is similar to barometers used today, which is made up of a long glass tube (39 inches high) with one open and one closed end. After the air was removed from the tube, the open end was lowered into a dish of mercury. Mercury would ascend in the tube to balance the weight of the air above the dish. As the atmospheric pressure would change, the amount of mercury in the tube would change to maintain the balance. Mercury was used because it is denser than water, meaning that the changes in pressure would not lift the liquid as high. Mercury was also convenient at the time. Since Torricelli’s time, the dangers of handling and exposure to mercury were discovered and the use of mercury in barometers has been discontinued.

The most common barometer used in homes is called an aneroid barometer. This type of barometer does not contain any liquid, instead there is a flexible metal box inside of the instrument. Most of the air is removed from the sealed metal box and an internal spring keeps the box from collapsing. As the air pressure changes, the box flexes:

• If the box compresses, it means the pressure is rising.
• If the box expands, it means the pressure is falling.

As the box flexes, it moves gears and levers, which translate to movements of a pointer on a dial to indicate the current pressure readings. This mechanical aneroid barometer is less precise than a mercury barometer.