A mercury barometer is used to measure atmospheric pressure. A barometer can be made from a glass tube that is closed at one end by filling the tube with mercury, stoppering the open end, inverting the tube so that the stoppered end is in a reservoir of mercury, and removing the stopper. Some of the mercury will run out of the tube into the reservoir, leaving a partial vacuum above the mercury in the tube. The height of the mercury reflects the balance between atmospheric pressure on the surface of the mercury in the reservoir and the pressure exerted by the column of mercury in the tube.
An increase in atmospheric pressure will cause the liquid in the tube to rise, while a decrease in atmospheric pressure will permit the liquid in the tube to fall. The atmospheric pressure can be determined by measuring the height of the mercury column, usually in units of millimeters mercury (mmHg).
