25a and 25 b are a top view and a side cut-away view, respectively, of the magnetic rings of the embodiment of FIG. 24a; [0045]FIGS. 26a and 26 b are a top view and a side cut-away view, respectively, of the base washers of the embodiment of FIG. 24a; [0046]FIGS. 26c and 26 d are a top view and a side cut-away view, respectively, of the base plates of the embodiment of FIG. 24a; [0047]FIGS. 27a an