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 hydrostatics 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.
Two forms are in use: (1) the “open-tube,” in which the pressure in one limb is equal to the atmospheric pressure, and (2) the “closed-tube,” in which the experimental pressure is balanced against the liquid column and the air compressed into the upper part of a closed limb of the tube. In the “open tube” form (fig. 1) the pressure on the surface a is equal to the pressure on the surface at b (one atmosphere) plus the hydrostatic pressure exerted by the liquid column of height a b. The liquid commonly used is mercury. If a scale be placed behind the limbs of the tube, so that the difference a b can be directly determined, then the pressure in a is at once expressible as P + a b in millimetres or inches of mercury, where P is the atmospheric pressure, known from an ordinary barometric observation. In the “closed tube” form (fig. 2) the calculation is not so simple, for the variation of pressure on the mercury surface in the closed limb has to be taken into account. Suppose the length of the air column in the closed limb be h when the mercury is at the same height in both tubes. Applying the experimental pressure to the open end, if this be greater than atmospheric pressure the mercury column will rise and the air column diminish in the closed 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 for 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.