MANOMETER (Gr. avos, thin or loose; erpov, a measure)

This article appears in Volume V17, Page 594 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: MAL-MAR

MANOMETER (Gr. avos, thin or loose; erpov, a measure) , an

instrument

INSTRUMENT (Lat. instrumentum, from insincere, to build up, furnish, arrange, prepare)

for measuring the pressures exerted by gases or vapours. An alternative name is pressure gauge, but this

term

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 U 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
2 Manoeuvres incidentally afford an excellent opportunity of testing new patterns of equipment, transport or other materiel under conditions approximating to those of active service.
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: (I) 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

PART

of a closed limb of the tube. In the " open tube " form (fig. I) the pressure on
the surface a is equal to the pressure on the surface at b (one atmos1,here) 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

ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

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. I.et 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

INSTRUMENTS

are equally serviceable for deter-

mining

MINING

pre. sores less than one atmosphere. In laboratory practice, e.g. when it is required to determine the degree of exhaust of a water

pump

PUMP

, a common form consists of a

vertical

VERTICAL (from Lat. vertex, highest point)

glass tube having its lower end immersed in a basin of mercury, and its upper end connected by means of an intermediate

vessel

VESSEL (O. Fr. vaissel, from a rare Lat. vascellum, dim. of vas, vase, urn)

to the exhaust. The mercury rises it the tube, and the difference between the barometric height and the length of the mercury column gives the pressure attained.

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