The Earth's atmosphere exerts a pressure because of its weight. Atmospheric pressure is determined by altitude and air temperature and other factors that are normally the concern only of meteorologists. The standard sea-level atmospheric pressure for North America is 29.92 inches of mercury (Hg) at 59 degrees Fahrenheit. In metric units, atmospheric pressure is 76 cm of mercury (cm-Hg) or 760 millimeters of mercury (mm-Hg). The mm-Hg and cm-Hg units are frequently used in scientific applications. The mm-Hg unit is equivalent to the "Torr," which is named in honor of Torricelli.
Mercury is the only metal that is a liquid under standard conditions of temperature and atmospheric pressure, or 32 degrees Fahrenheit and 14.5 lb. / inch-squared. Mercury is similar in appearance to lead, and is highly toxic. The density of mercury is 13.5462 grams (g) per cubic centimeter (cm-cubed) at 68 degrees Fahrenheit (20 degrees Celsius), a density higher than that of lead, and this makes it ideal for use in barometers. The reason is that atmospheric pressure is balanced by a height of mercury that is practical for use in an instrument. By comparison, a water barometer would need a tube about 35 feet high, because the density of water (1 g / cm-cubed) is about one-fourteenth that of mercury.
A pressure given in mm-Hg or cm-Hg is converted to any other pressure unit through a process of unit cancellations. For example, an atmospheric pressure of 76 cm-Hg is converted to pounds per square inch (lb / inch-squared), as follows: [13.54 g Hg / cm squared] x [cm-cubed / cm-squared x cm] x [6.4516 cm-squared / inch-squared] x 76 cm Hg x [1 lb Hg / 453.59 g Hg] = 14.636 lb / inch-squared, or 14.636 psi. When the expression is multiplied out, it is seen that units in the numerators cancel with the same units in the denominators, leaving only lb / inch-squared.
The ubiquitous aneroid barometer is practical and popular, although less accurate than the mercury barometer. An aneroid barometer does not use liquid; instead it uses an evacuated, collapsible chamber, or bellows, that responds to changes in atmospheric pressure. The movements of the bellows are transmitted to an indicator needle through a series of delicate linkages. It is common practice to check an aneroid barometer for accuracy against a mercury barometer.