This paper describes an instrument capable of measuring the roundness of an object with a resolution of 5 m&mgr;. The instrument consists of a precision spindle with hydrostatic fluid bearings and a capacitance displacement transducer. In this scheme, the displacement transducer is part of a capacitance bridge, which is made self balancing by closing the servo loop to the nulling capacitor through the recorder pen motor. Also, an instrument using a multiple beam interferometer for the displacement transducer calibration is briefly described.