Dictionary.net

An instrument for measuring the force of the blood in the vessels. It consists of a bent glass tube, the lower bent part of which is filled with mercury. A brass head is fitted into the artery, and a solution of carbonate of soda is interposed between the mercury and the blood, which is allowed to enter the tube for the purpose of preventing its coagulation. The pressure of the blood on the mercury in the descending portion of the bent tube causes the metal to rise in the ascending portion; and the degree to which it rises indicates the pressure under which the blood moves.