PIEZOMETER (from Gk. pirzein, to press ,uirpov, metros, nlea,ure). An instrument for measuring the compressibility of fluids. The name was given by Oersted (q.v.) to a piece of apparatus which he devised in 1822, and with which he was able to determine the compres sibility of water, mercury, alcohol, and other liquids. It consisted of a cylinder of thick glass closed at either end by a brass cap. A tube of glass or brass containing a piston or a screw plug was fitted to the upper cap, by means of which considerable pressure could be exerted on the liquid in the cylinder below. This cylinder was suflieiently large to contain a glass flask whose neck was drawn out into a fine tube. like that of a thermometer. graduated into equal divisions, each of which represented a certain fraction of the volume of the flask. The flask was filled with the liquid whose compressibility was to be determined, and a small thread of mercury introduced into the fine tube. It was then placed inverted in the cylinder, which was filled with water, being so supported that the end of the fine tube dipped into a small vessel of mercury at the bottom. In this was placed another tube closed at its upper end. which
acted as a manometer, enabling the observer to measure the pressure sustained by the liquid by the height of the mercury column. When the piston was pushed or screwed down. the liquid in the cylinder was put under pressure. This pressure was communicated to the liquid in the flask, which was compressed by a small amount, as was shown by the use of the mercury in the fine tube. Knowing the volnme of the flask and the value of the divisions of the tube, the amount of compressibility was readily ascer tained by the pressure shown by the manometer, and in this way Oersted found that the com pressibility of water for one atmosphere of pressure amounted to 46.65 millionths. Colladon and Sturm, who performed a series of similar experiments, made this value 49.65 millionths, while Grassi somewhat later fixed the com pressibility of water at 0° centigrade at 50 millionths. and at 53° centigrade at 44 mil lionths. With the piezometer it was also demon strated that liquids are perfectly elastic, since when the pressure is removed the mercury sinks to its former position. showing that the liquid returns to its original volume.