FIG. 1 shows a pneumatic cylinder 34 with a piston 4 that carries an activation magnet 2. Piston 4 can move in the movement direction 7 over the entire distance 6 to be monitored. The activation magnet 2 has a magnetic orientation that is parallel to the movement direction 7 of piston 4. The activation magnet 2 comprises a permanent magnet 3. The magnetic lines (not shown) of a magnetic field extend outside permanent magnet 3 between a north pole N and a south pole S. As shown, piston 4 is at position C.