level air pressure is known as normal atmospheric pressure, also called
standard pressure.
The instrument most commonly used for measuring air pressure is the
mercury barometer. The diagram below shows how it operates. Atmospheric
pressure is exerted on the mercury in the dish, and this in turn holds the column of
mercury up in the tube. This column at standard pressure will measure 760
millimeters above the level of the mercury in the dish below.
TIP
Read the top of the meniscus for mercury but the bottom of the meniscus for water.In gas-law problems pressure may be expressed in various units. One
standard atmosphere (1 atm) is equal to 760 millimeters of mercury (760 mm Hg)
or 760 torr, a unit named for Evangelista Torricelli. In the SI system, the unit of
pressure is the pascal (Pa), named in honor of the scientist of the same name, and
standard pressure is 101,325 pascals or 101.325 kilopascals (kPa). One pascal
(Pa) is defined as the pressure exerted by the force of one newton (1 N) acting on
an area of one square meter. In many cases, as in atmospheric pressure, it is more
convenient to express pressure in kilopascals (kPa).
Summary of Units of Pressure