Abstract: Apparatus for restricting magnetic oil to a desired location within a FFFB comprising selectably locatable stopper apparatus having a first location near the FFFB for enabling a vacuum to be produced and for restricting the magnetic oil within the FFFB during filling of the FFFB with the magnetic oil and a second location away from the FFFB when filling is not occurring and apparatus for moving the selectably locatable stopper apparatus between the first location and the second location.