a capacitive measuring system including a sensor for measuring the clearance between a movable part and a fixed casing facing the movable part , the sensor is secured in the casing and has an active face facing the movable part to constitute a capacitor whose charge is representative of the clearance . a triaxial cable connects the sensor to an amplifier . the triaxial cable has a central conductive core and two surrounding conductors insulated from one another and from the core . the sensor comprises a central body including a disk with a surface constituting the active face of the sensor made from a metal of the platinum group or an alloy thereof . the central body is connected to the core of the triaxial cable . two conductive shields surround the central body and are connected to respective conductors of the triaxial cable . an electrical shielding or insulation of ceramic or glass is between the central body and the conductive shields . the conductive shields are made of a metal having a substantially lower coefficient of thermal expansion than the central body . the sensor is secured with axial adjustment in the casing , in such manner that accurate active clearance control of a machine , such as a turbo - engine , jet - engine or the like power plant is provided .