of the embodiment of FIG. 30a; [0056]FIGS. 34a and 34 b are a top view and a side cut-away view, respectively, of the non-magnetic cover of the embodiment of FIG. 30a; [0057]FIGS. 35a and 35 b are a top view and a side cut-away view, respectively, of the tubular stems of the first embodiment; [0058]FIG. 36a is a perspective view showing yet another embodiment of the instant invention, having mult