The linear movement of a flexible steel cable, which can have a length of up to 2 m, is converted into an angular movement with the aid of a measuring drum. The measuring drum is coupled via a gear to a conductive plastic potentiometer. The restoring force of the spring drive holds the measuring cable tight at all times and prevents any sagging which would otherwise induce an error. The measuring drum moves axially on a spindle ensuring that the cable is wound up precisely and reproducibly wrap for wrap in the helical groove of the drum. The cable entry contains a brush to remove dust.