The mercury-gravity manometer consists of a uniform diameter straight glass tube with a reservoir containing mercury. The pressure chamber of the reservoir communicates with the compression cuff through a rubber tube. When pressure is exerted on the mercury in the reservoir it falls, and the mercury in the glass tube rises. Since the weight of the mercury is dependent on gravity, which is constant, a given amount of pressure will always support a column of mercury of the same height. The mercury-gravity manometer is the most accurate, does not require recalibration, and is the standard for measuring BP.