MANOMETER (from Gk. uav6c, ma nos, thin, rare + pirpoy, rnetron, measure). An instrument for measuring the density or pressure of the air or any gas. A barometer (q.v.) is one form of manometer, as the pressure of the atmosphere is measured by the height of the column of tiler enry which it supports. The manometer in its simplest form would be a glass tube open at both ends and bent into the form of a IT and contain ing a sufficient quantity of some liquid to cover the bend and rise to a small height in each arm. The vessel containing the gas whose pressure is to be ascertained is connected with one arm of the tube, and if the gas is at the same pressure as the atmosphere the liquid will stand at the same level in both tubes. If the gas is at a greater pressure the liquid in the arm of the tube on which it acts will he at a lower level, and the pressure of the gas will be obtained by adding to the pressure of the atmosphere the weight of a column of the liquid whose height is equal to the difference in level in the two tubes. When the pressure of the gas is considerably greater than that of the atmosphere we use mercury on aecount of its high specific gravity, and when the pressures are sufficient a tube with one arm closed can be em ployed and the press ure determined by measuring the extent to which the air is compressed. Now, ac cording to Boyle's or ariotte's law, a pressure exerted on the column of mer cury sufficient to force the air into half the space it occupies at the normal atmos pheric pressure, must become doubled, or 15 pounds to the square inch most be added. Again, to compress the air into half the remaining space, 30 pounds, or double the pressure required for the reduction to the first half, must be added, making in all a pressure of four at mospheres for the re duction to one-fourth the original volume.
It is evident, there fore, that a graduated scale, to exhibit the degrees of pressure, must have its spaces decrease from below upward. If the gas is considerably rarer than the air, as for example in the receiver of an air pump, we employ a shortened barometer consisting of a bent tube with one end closed but filled with mercury, which is supported by the pressure of the atmosphere. In this case the pressure is measured oy the dif ference in level of the two columns, which would be zero were the vacuum perfect.
These manometers are of course constructed in various forms, depending upon the use to which they are to be put, and the tubes and air cham bers are variously constructed. The most common form of manometer is the steam gauge, which may he either a piston actuated by the pressure to move au indicator against the face of a spring, or more commonly a metal tube of elliptical cross section bent into circular shape. One end of this tube is permanently fastened to the ease of the instrument and through it the steam or gas en ters, while the other end is closed lint free to move. It is conneeted with a spring and a series of levers. so that its motion. which depends upon the pressure, is communicated to an indicator moving, over a scale graduated usually in pounds in the United States and England, and atmos pheres in Europe. These steam gauges must of course be adjusted and calibrated by reference to some direct source of pressure. such as would be furnished by a column of liquid in a vertical tube.