A valve having a body inside which slides a plugging member integral with a keeper forming part of a magnetic circuit, which circuit comprises the walls of the aforementioned body, and a core located inside the same. The body presents a bottom wall having a hole through which slides the aforementioned plugging member, and the keeper presents an annular projection located on one side in relation to the aforementioned wall and having an outside diameter greater than that of the aforementioned hole, so as to provide for substantially axial magnetic flow lines between the aforementioned bottom wall and the keeper.