Abstract: An electromagnet is mounted on a ferromagnetic support member. A ferromagnetic flapper valve element arranged to control fluid flow from a nozzle orifice is movably mounted on the ferromagnetic support member and controllably moved by the electromagnet. The electromagnet, the support member and the valve element form a substantially closed magnetic circuit with an air gap being provided between the electromagnet and the valve element. Ferromagnetic and paramagnetic spacers are disposed between the electromagnet and the support member. The air gap between the electromagnet and the valve element is determined by the combined thickness of the spacers and another gap in the magnetic circuit is determined by the thickness of the paramagnetic spacer.