Capacitance determination of the area thermal expansion of dielectric crystals

Both the area thermal expansion and the dielectric constant of a solid insulator are determined by a capacitance technique in which the change in area of a capacitor is first caused by the variation with temperature of the dimensions of the dielectric, and then by the variation over the same temperature range of the dimensions of copper electrodes, the same sample being used in both cases. This method is particularly convenient for small, flat crystals under hydrostatic pressure and for anisotropic crystals. It is believed that a similar method could be used to measure the compressibility. The apparatus and calculations are described, and the results of a test run on the thermal expansion of NaCl are compared with previous data. Between 77 and 250 K, the measured linear thermal expansion L (T)/L (300) of NaCl agrees with the results of a Fabry-Perot etalon experiment to within 0.03%, giving differences in the coefficient of linear expansion of 10% in the worst case.