Evangelista Torricelli

Barometer (Evangelista Torricelli)

In the early 1600s, Galileo argued that suction pumps were able to draw water from a well because of the "force of vacuum" inside the pump. After Galileo's death, the Italian mathematician and physicist Evangelista Torricelli (1608-1647) proposed another explanation. He suggested that the air in our atmosphere has weight and that the force of the atmosphere pushing down on the surface of the water drives the water into the suction pump when it is evacuated.

In 1646 Torricelli described an experiment in which a glass tube about a meter long was sealed at one end, filled with mercury, and then inverted into a dish filled with mercury, as shown below. Some, but not all, of the mercury drained out of the glass tube into the dish. Torricelli explained this by assuming that mercury drains from the glass tube until the force of the column of mercury pushing down on the inside of the tube exactly balances the force of the atmosphere pushing down on the surface of the liquid outside the tube.

Torricelli predicted that the height of the mercury column would change from day to day as the pressure of the atmosphere changed. Today, his apparatus is known as a barometer, from the Greek baros, meaning "weight," because it literally measures the weight of the atmosphere. Repeated experiments showed that the average pressure of the atmosphere at sea level is equal to the pressure of a column of mercury 760 mm tall. Thus, a standard unit of pressure known as the atmosphere was defined as follows.

1 atm = 760 mmHg

To recognize Torricelli's contributions, some scientists describe pressure in units of "torr," which are defined as follows.

1 torr = 1 mmHg

On a sunny day, at sea level, the weight of a 760-mm column of mercury inside a glas tube balances the weight of the atmosphere pushing down on the pool of mercury that surrounds the tube.  The pressure of the atmosphere is therefore sid to be equivalent to 760 mm Hg.