a device for measuring the position of a conductor in an enveloped cable , preferably directly after extruding the plastic envelope , in which at least a supporting means for a sensor is continuously engaging the periphery of the envelope and in which the sensor determines the distance between the conductor and the sensor , and wherein the output signal of the sensor is delivered to a processing and displaying means , wherein the supporting means is defined by a roller driven by the cable which roller is yieldingly supported normal to the rotational axis .