A mercury barometer is a device that uses the various physical properties of mercury contained in a tube to measure barometric pressure, also known as air pressure or atmospheric pressure. Changes in pressure are used to predict imminent weather changes, so barometers are useful tools for weather forecasting. When the barometric pressure drops, for example, it is possible that storms, rain, or wind are on the way. An increase in barometric pressure, on the other hand, could indicate that dry, fair weather could appear soon. Interpreting barometric readings can be quite accurate, but it is an imprecise science — there is no guarantee that the weather will act as expected.
There are two primary components to a mercury barometer: a mercury-filled tube that is closed on one end and a mercury reservoir. The tube of mercury is inverted in the reservoir so that the closed end faces up and the open end is partially submerged in the reservoir. Some of the mercury flows from the tube into the reservoir, creating a vacuum at the top of the closed end of the tube. The tube is generally about 3 feet (0.9 meters) tall and includes markings at regular intervals, usually in inches or millimeters, that are used to quantify changes in pressure.
The height of the mercury in the tube changes with changing atmospheric pressure because the mercury in the reservoir is sensitive to these changes. When the barometric pressure increases, force is applied to the surface of the mercury reservoir, forcing the level in the tube to increase. Conversely, when pressure drops, less force is applied and the level of mercury in the tube decreases. The weight of the mercury in the tube, then, is balanced against the weight of the air above the reservoir. The markings on the side of the tube are used to measure the pressure levels as precisely as possible.
A mercury barometer is a commonly used tool in science, particularly in chemistry. There are many units used to measure pressure; for years, millimeters of mercury, or mm Hg, was the unit of choice. One millimeter of mercury is commonly referred to as one torr, after Evangelista Torricelli who invented this barometer. One atmosphere is equal to 29.9 inches (760 mm) of mercury, or 760 mm H; atmospheres are the units of pressure used in the international standard, or SI, system of measurement. Standard pressure is one atmosphere, or 760 millimeters of mercury, and is the base pressure used in many scientific calculations.