The Torricelli Barometer

A barometer is an instrument that measures atmospheric pressure. Barometers are used by weather forecasters to measure air pressure, because rising or falling atmospheric pressure indicates a coming change in weather
Evangelista Torricelli was an Italian physicist. He was born in 1608 and died in 1647. He studied mathematics in Rome, and there became fascinated by the work of Galileo, whom he aided in the preparation of his Discorsi. Toriccelli succeeded Galileo, on the latter’s death, in the chair of philosophy and mathematics at Florence. Toriccelli is famous for balancing the weight of a column of mercury against the pressure of the atmosphere, the principle of the barometer. The space above the mercury in a brometer is still called the Torricellian vacuum.

The Torricelli barometer is made by first filling a dish with mercury which is afterwards poured into a long tube. The tube is filled almost to the top, stopped 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.
A safer way to fill the Torricelli barometer is to use a vacuum pump to create a vacuum at the top of the barometer. This draws the mercury up into the tube from the dish at the bottom. Continuing to draw a vacuum does not change the level of the mercury inside the tube.  Closing the valve maintains the vacuum when the hose is removed.
Modern barometers measure air pressure in inches or millimeters of mercury or in units called bars and millibars. The bar is a unit of pressure in the metric system, and a millibar equals 1/1,000 bar. Scientists record most pressure measurements in millibars. The atmospheric pressure at sea level averages 1,013 millibars, which equals 29.92 inches (760 millimeters) of mercury. To compare measurements taken at various altitudes and temperatures, scientists first adjust them to the values that would be measured at sea level at 32°F (0°C). Torricelli also developed the concepts of momentum and impetus, and solved Fermat’s triangle problem for the Torricelli poin. All in all the Toriccelli barometer is an amazing tool used to measure atmospheric pressure. Toriccelli barometer also stands as the base of many modern barometers being a point of reference in our history.

toriccelli barometer