An electro-magnet array for use in a sputtering apparatus. The array has a magnetisable core member extending substantially horizontally and having magnetisable outward projections arranged as at least two pairs of symmetrically opposed projections projecting outwardly from the core member. A pole member is associated with each projection and vertically displaced with respect thereto. A magnetisable coupler is arranged to couple each pole piece magnetically to its respective projection. A magnetising coil around each projection is arranged for producing a magnetic field aligned substantially with a horizontal axis of symmetry of its respective projection in dependence upon the direction of flow of electric current through the magnetising coil. The magnetising coils are arranged so that, upon energisation, the magnetic field of one coil of a pair of projections can produce a south pole at an inward side of its respective pole piece while the magnetic field of the coil of the other projection of the pair produces a north pole at an inward side of its respective pole piece. At any moment, at least two inwardly facing south poles are produced on the pole pieces on one side of a vertical plane of symmetry through the array and an equal number of inwardly facing north poles are produced on the pole pieces on the other side of the vertical plane of symmetry.