An apparatus is described for measuring linear compressibility of rods and tubes under hydrostatic pressures to 4000 kg/cm2and temperatures to 260°C. The measurements are made by means of a linear‐differential transformer located on the outside of the nonmagnetic high‐pressure container. The differential transformer makes it possible to determine the position of a small iron cylinder affixed to the sample. Compressibility of vitreous silica determined by this method was obtained as −&Dgr;V/V=p(26.43–0.0049t) ×10−7+p2(21.8–0.040t)×10−12; compressibility is expressed in relative volume change,pin kg/cm2andtin °C.