An extrusion resistant ring that is radially expandable and contactable to accommodate the relative movement of two joined bodies relative to each other. The data bus is coupled between the first and second circuit blocks. The detection of one member of the pair activates the switch, which re-routes the other member into the storage loop. The magnetic poles which are adjacent to each other in the circumferential direction respectively extend in the axial directions of the magnetic poles 5 to the inner end surfaces 5b of the magnetic poles so as to virtually interfere with each other in the vicinity of the outer surface of the rotating shaft. Alternatively, in the case where at least one of the outside electrodes is located outside of the above-mentioned areas, no other remaining outside electrodes are arranged at a position that faces that outside electrode across the central electrode.