Locking devices are provided that can be opened by alternate users with different keys. A control unit serves to provide at least one reference voltage and at least one reference voltage with opposite phase. The amplifier comprises a buffer amplifier that receives the differential input voltage and produces a differential input current. The retaining member is displaceable relative to the main part, at least in the longitudinal direction of the thread, to provide a self-locking threaded element that is inexpensive to manufacture and enables both simple tightening and loosening of the threaded element.