as explained in detail hereinbelow the magnetically balanced design of fingers 34 that is shown in fig4 and 5 may be particularly advantageous in this context since it may permit the motor to start rotation of the camshaft with low power input furthermore because of the magnetic balancing the pump mechanism may develop only minimal inertia while it is in motion and may therefore stop immediately due to the small amount of friction that exists in the mechanism and hold its position stably when the motor stops cams 52 may have an oblong shape or a special crescent shaped design the special crescent shaped cams may enhance the magnetic balancing by extending the range of rotation of the camshaft over which the corresponding finger holds conduit 50 closed this design may provide additional mechanical tolerance and may facilitate the low power start and stable stop of the mechanism that are described above exemplary cam designs are described further in the above mentioned u s and pct publications with the sort of mechanical and magnetic design that is shown in fig3 5 for example the inventors have found it possible to run pump 22 on an internal battery at an intermediate flow rate 125 ml h for.