Final answer:
A mercury barometer measures atmospheric pressure using a column of mercury balanced by the air pressure exerted on a pool of mercury, with changes in atmospheric pressure indicated by the rise or fall of the mercury column.
Step-by-step explanation:
A mercury barometer is an instrument that measures atmospheric pressure. It consists of a glass tube closed at one end and filled with mercury. This tube is inverted and submerged in a pool of mercury. The space above the column of mercury is nearly a vacuum, which means there is no air exerting pressure from the inside. The weight of the mercury column, expressed by the formula hpg (height of the mercury column times the density of mercury times the acceleration due to gravity), is exactly balanced by the atmospheric pressure pushing down on the pool of mercury, expressed as Patm. As atmospheric pressure changes, the height of the mercury column adjusts accordingly, which can be observed and measured. This device is not just a weather forecasting tool but can also serve as an altimeter due to the variation of average atmospheric pressure with altitude. The standard measure for atmospheric pressure using mercury barometers is in millimeters of mercury (mm Hg).