Piezometer is an instrument which measures the compressibility of a fluid. If a pressure of p pounds per square inch be applied throughout a liquid whose volume was originally v cubic inches, it will cause this volume to diminish. If the final volume is V - v cubic inches, the compressibility is
A piezometer is provided with arrangements for measuring v and p. The best-known form of the instrument is that devised by Oersted, and it was by means of this that the compressibility of water was first determined with any accuracy. A large bulb provided with a small capillary tube contains the liquid whose compressibility it is required to measure, while a drop of mercury at the top of the liquid in the tube acts as an index. The capillary is graduated, and is open at the top. The whole of this apparatus can be placed inside a large strong glass cylinder, into whose neck a piston tightly fits. This cylinder is filled with water, and a great pressure can be produced throughout its volume by a downward thrust of the piston. Under this increased pressure the experimental liquid is seen to descend in its capillary tube, and the amount of descent is indicated by the bubble of mercury. Previous experiments having determined the value of each graduation on the tube in terms of the total contents of the bulb, the descent of the index measures the decrease of volume (?) which the liquid has suffered. The pressure p may be measured by placing a graduated cylinder of air inside the large vessel. The water will from the first rise some distance in this vessel, and as the pressure is increased the volume of contained air will become visibly less. By suitably graduating this cylinder the increased pressure can be read off. Knowing V, the original volume of our experimental liquid, and having measured p and v, we can determine the required compressibility. An air bubble in the capillary tube is often used as an index instead of the mercury, in which case the liquid must fill the bulb and tube almost to the top. The bubble, whether of air o:' mercury, prevents the water in the outer vessel from coming into contact with the liquid in the tube. Allowance has to be made for the diminution in volume of the bulb itself under the increased pressure.