is shown as a dashed line in fig3 because of the permeability of the round plate material the magnetic field in the region of the round plates is stronger than in the air gap without round plates between the front sides of inner yoke parts 2 a and 2 b fig2 a shows an embodiment of the invention with a spirally wound tune coil 7 a between round plate 3 d and inner yoke part 2 b tune coil 7 b in fig2 b in contrast surrounds the outer circumference of round plate 3 c so that round plate 3 c acts as an iron core of tune coil 7 b tune coil 7 c in fig2 c is wound spirally and arranged in the air gap between round plate 3 a and round plate 3 b tune coils 7 a or 7 c alternatively may have a different type of winding and extend for example in the longitudinal direction the tune coils in fig2 a to 2 c are indicated by three windings the actual design can deviate herefrom the number and arrangement of the tune coils are within the discretion of the person skilled in the art practicing the invention in this case it is possible to use an individual tune coil or any combination of coils and their position in the region of the round plates a modified form of the tune coil is also possible which both surrounds the circumference of a round plate and has an extension into a gap between two round plates or a round plate and an inner yoke part the aforementioned wideroe.