The present invention relates to a magnet, which comprises a body and a slide arranged to be movable relative to the body between a first and a second position. The slide comprises a permanent magnet, and a cylindrical first pole piece and a second pole piece, which are attached to opposite magnetic pole surfaces of the permanent magnet. The body comprises a first and a second section made of magnetic material and being separated from each other by a third section that is made of non-magnetic material. The first section comprises a cylindrical recess into which the slide is movably arranged so that, at the first position of the slide, the cylindrical first pole piece, the permanent magnet and at least part of the second pole piece are located inside the cylindrical recess, and at the second position of the slide, at least part of the cylindrical first pole piece is located inside the cylindrical recess and the second pole piece is located outside the cylindrical