Atmospheric Pressure and the Barometer

The SI unit of pressure is the pascal (Pa), named for Blaise Pascal (1623—1662), a French scientist who studied pressure 1 Pa = 1 N/m. A related pressure unit is the bar 1 bar = 10 Pa = 10 N/m. Thus, the atmospheric pressure at sea level we just calculated, 100 kPa, can be reported as 1 bar. (The actual atmospheric pressure at any location depends on weather conditions and altitude.) Another pressure unit is pounds per square inch (psi, Ibs/in. ). At sea level, atmospheric pressure is 14.7 psi. 

Assume the top of your head has a surface area of 10 in. x 10 in. How many 

In the seventeenth century many scientists and philosophers believed that the atmosphere had no weight. Evangelista Torricelli (1608-1647), a student of Galileo s, 

What happens to h, the height of the mercury column, if the atmospheric pressure increases  

Although Torricelli s explanation met with fierce opposition, it also had supporters. Blaise Pascal, for example, had one of Torricelli s barometers carried to the top of a mountain and compared its reading there with the reading on a duplicate barometer at the base of the mountain. As the barometer was carried up, the height of the mercury column diminished, as expected, because the amount of atmosphere pressing down on the mercury in the dish decreased as the instrument was carried higher. These and other experiments eventually prevailed, and the idea that the atmosphere has weight became accepted. 

Standard atmospheric pressure, which corresponds to the typical pressure at sea level, is the pressure sufficient to support a column of mercury 760 mm high. In SI units, this pressure is 1.01325 X 10 Pa. Standard atmospheric pressure defines some common non-SI units used to express gas pressure, such as the atmosphere (atm) and the millimeter of mercury (mm Hg). The latter unit is also called the torr, after Torricelli 1 torr = 1 mm Hg. Thus, we have 

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