The Barometer

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The common barometer simply consists of a bowl of a liquid (usually mercury) and a glass tube. The glass tube (with a vacuum) is placed in the mercury. the mercury rises in the tube due to the gas pressure outside the tube. However, as the mercury rises its weight within the tube increases until it just balances the external gas pressure. The resulting height of the mercury is a direct measure of the external pressure. At sea level the height of the mercury will be 760 mm. The pressure of air at sea level is therefore 760 mmHg.

It is possible to make barometers with other liquids than mercury, however, because of its density mercury is the most convenient.

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C101 Class Notes
Prof. N. De Leon