Although three-terminal capacitors have been used to measure accurately small displacements in air and vacuum, it is usually impractical to use them in dielectric liquids, due to the strong dependence of the permittivity on purity and temperature. A differential-capacitance transducer in the form of a guarded disk between parallel plates was only slightly affected by variation in temperature over the range 18 to 90°C, and was unaffected by variation in the relative permittivity of air or liquid dielectric from 1 to 49. Displacements up to 2·5 mm could be measured with a precision better than 0·01 μm.