The present invention provides a magnet, which comprises a body having a first and a second section, 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 first and a second pole piece that are attached to opposite magnetic pole surfaces of the permanent magnet. The first section comprises a hole that opens into a first cavity of the body, the bottom of the first cavity being delimited at least partly by the second section. The slide is movably arranged in the hole so that the second pole piece is directed towards the bottom of the first cavity, wherein at the first position of the slide, magnetic flux generated by the permanent magnet is short-circuited by the first section, and at the second position of the slide, magnetic flux generated by the permanent magnet is directed through the first and the second section.