A coil structure has a coil yoke (6) and a coil unit (1) that has a cylindrical hollow chamber (12) inside and is wound round in an outer area with a wire structure (2). Between the coil unit and the coil yoke there is a tube (10) moving on bearings with a smaller diameter for its opening than the cylindrical hollow chamber. The tube is a perforated disk with magnetic properties. The coil unit has a recess (11) in which the magnetic disk is located.
