A magnetic flow controller comprising: a body defining a fluid flow passage having a fluid inlet and a fluid outlet; a stop member mounted across said inlet; and a displaceable sealing member that has an elongate portion of substantially constant diameter and is disposed in said body adjacent said stop member: and wherein said displaceable sealing member is moveable through said flow passage away from said stop member towards said outlet by a force created by a pressure differential between a pressure at the inlet and a pressure at the outlet; and wherein at least one of said stop member and said displaceable sealing member includes a magnet (34, 29), and the other of said stop member and said displaceable sealing member includes a ferromagnetic portion, so as to create a magnetic force, acting on said displaceable sealing member, sufficient to cause the displaceable sealing member to engage said stop member so as to close and seal the flow passage; characterized by the fluid flow passage in said body defining a sealing wall between the fluid inlet and the fluid outlet, said sealing wall and said displaceable sealing member having interacting contours permitting sliding movement of the displaceable sealing member through the flow passage for a pre-selected distance without unsealing the seal between them, said elongate portion of substantially constant diameter of said displaceable sealing member providing a fluid seal while the displaceable sealing member moves through the flow passage so as to build up speed of movement, said elongate portion being of sufficient length to permit said displaceable sealing member to accelerate to a desired speed away from the inlet while maintaining the seal with the flow passage; and said flow passage opening to a relatively large opening such that, only after the displaceable sealing member accelerates through said flow passage towards said fluid outlet and opens said opening and the seal between said displaceable sealing member and the flow passage is unsealed, a volume of fluid at high flux flows through the passage, to permit the measurement of the volume of fluid in a conventional flow meter. a body defining a fluid flow passage having a fluid inlet and a fluid outlet; a stop member mounted across said inlet; and a displaceable sealing member that has an elongate portion of substantially constant diameter and is disposed in said body adjacent said stop member: and wherein said displaceable sealing member is moveable through said flow passage away from said stop member towards said outlet by a force created by a pressure differential between a pressure at the inlet and a pressure at the outlet; and wherein at least one of said stop member and said displaceable sealing member includes a magnet (34, 29), and the other of said stop member and said displaceable sealing member includes a ferromagnetic portion, so as to create a magnetic force, acting on said displaceable sealing member, sufficient to cause the displaceable sealing member to engage said stop member so as to close and seal the flow passage; characterized by the fluid flow passage in said body defining a sealing wall between the fluid inlet and the fluid outlet, said sealing wall and said displaceable sealing member having interacting contours permitting sliding movement of the displaceable sealing member through the flow passage for a pre-selected distance without unsealing the seal between them, said elongate portion of substantially constant diameter of said displaceable sealing member providing a fluid seal while the displaceable sealing member moves through the flow passage so as to build up speed of movement, said elongate portion being of sufficient length to permit said displaceable sealing member to accelerate to a desired speed away from the inlet while maintaining the seal with the flow passage; and said flow passage opening to a relatively large opening such that, only after the displaceable sealing member accelerates through said flow passage towards said fluid outlet and opens said opening and the seal between said displaceable sealing member and the flow passage is unsealed, a volume of fluid at high flux flows through the passage, to permit the measurement of the volume of fluid in a conventional flow meter.