A device for assisting a relative movement of two parts of a mechanical system, for example a ball bearing or a gear device, configured to form a device with variable capacity, thereby enabling mechanical energy to be recovered and converted into electrical energy. At least one among driving and driven elements of the device includes a conductive zone separated by a variable distance during the relative movements of the elements with respect to each other of the conductive parts of the device, the conductive zone and the conductive parts being coupled to different equipotentials.