Abstract: Apparatus for measuring the distance between a surface of a workpiece and a predetermined location of the type which measures the capacitance between the predetermined location and the workpiece surface and determines the distance corresponding to the measured capacitance. The apparatus includes a probe disposed at said predetermined location, the probe may be selectively excited to a known electric potential. A support assembly for supporting the workpiece is mounted on the apparatus for rotation relative to the probe. The support assembly is constructed to support the workpiece for rotation and to maintain the workpiece at a substantially constant electric potential. A grounding mechanism for grounding the support assembly includes a fixed grounding member and a liquid ground contact which engages the support assembly for maintaining electrical contact between the support assembly and the fixed grounding member as the support assembly rotates relative to the fixed grounding member.