1911 Encyclopædia Britannica/Manometer
MANOMETER (Gr. μανός, thin or loose; μέτρον, a measure), an instrument for measuring the pressures exerted by gases or vapours. An alternative name is pressure gauge, but this term may conveniently be restricted to manometers used in connexion with steam-boilers, &c. The principle of hydro-statics suggest the most common forms. Suppose we have a ∪ tube (fig. 1), containing a liquid: if the pressures on the surfaces of the liquid be equal, then the surfaces will be at the same height. If, on the other hand, the pressure in one limb be greater than the pressure in the other, the surfaces will be at different heights, the difference being directly proportional to the difference of pressures and inversely as the specific gravity of the liquid used.
limb. Let the length of the air column be h′, then its pressure is h⁄h′ atmospheres. The difference in height of the mercury columns in the two limbs is 2(h–h′) and the pressure in the open limb is obviously equal to that of a column of mercury of length 2(h–h′), plus h⁄h′atmospheres. These instruments are equally serviceable or determining pressures less than one atmosphere. In laboratory practice, e.g. when it is required to determine the degree of exhaust of a water pump, a common form consists of a vertical glass tube having its lower end immersed in a basin of mercury, and its upper end connected by means of an intermediate vessel to the exhaust. The mercury rises in the tube, and the difference between the barometric height and the length of the mercury column gives the pressure attained.