one half its frequency f r i e at a frequency f r 2 during which a bubble is generated by nucleator 38 at every other or alternate 360 u00b0 rotation of the in plane field h r as denoted by vector 12 the first generated bubble from nucleator 38 and input channel 18 has passed around feed ring 16 and first achieved stable position 17 as represented by the illustrated configuration of fig2 the nucleator 38 continues generating bubbles at this same one half in plane field h r frequency f r such that after this first generated bubble has completed two complete cycles of the feed ring to achieve stable position 17 the second time the feed ring 16 is filled with particular reference to fig3 there is presented a schematic diagram of the embodiment of fig1 illustrating the positions of the bubbles after the first generated bubble has completed two complete cycles around the feed ring 16 of fig1 at this time then with the bubbles occupying every next adjacent stable position of feed ring 16 as illustrated in fig3 nucleator field source 36 is turned off whereby nucleator 38 is no longer required to perform its nucleation and generation of the bubbles in input channel 18 once the feed ring 16 is filled with.