One interrupter structure comprises a plurality of longitudinally spaced interrupter elements mounted on a core rod, each element comprising a plurality of blade like members each of at least approximately spherical segment profile in side elevation, the members extending mutually radially outward relative to one another to touch or nearly touch the said surface or {HEAD ~ surfaces} adjacent {TAIL ~ the elements} 
One interrupter structure comprises a plurality of longitudinally spaced interrupter elements mounted on a core rod , each element comprising a plurality of blade like members each of at least approximately spherical segment profile in side elevation , the members extending mutually radially outward relative to one another to touch or nearly touch the said surface or { HEAD ~ surfaces } adjacent { TAIL ~ the elements }
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