Torricelli Barometer
The Torricelli barometer, invented by Evangelista Torricelli in 1643, is made by first filling a dish with mercury. Mercury is then poured into a long tube. The tube is filled almost to the top, stoppered and inverted several times to remove air bubbles. The tube is then completely filled with mercury using a dropper. A finger is placed over the top of the tube and the tube is inverted and placed into the dish below the level of the mercury. When the finger is removed, the level of the mercury inside the tube drops until the pressure at the bottom of the column of mercury is equal to the pressure exerted by the surrounding air. Since no air was allowed to enter the tube, the empty space above the mercury column is a vacuum.
We can measure the pressure exerted by the atmosphere by measuring how high the mercury column rises above the level of the mercury in the dish. The pressure (here) is 731 millimeters of mercury.
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