which windings w are located on drum 4 the annular gap provided between the annular body r and the circumferential surface of drum 4 has a width of about 5 mm the conical part of hollow body k has a cone angle of about 120 u00b0 the height of the projections v above the inner wall is between 2 and 5 mm the diameter of the wart shaped projections v being approximately 10 to 20 mm in the case of a cone angle of about 120 u00b0 fig3 illustrates a conical hollow body k whose shape corresponds to that of the hollow body k in fig1 and 4 i illustrates a straight generatrix of the hollow body k alternatively the generatrix could also be a bow line ii which is convex relative to drum axis x or a concave bow line iii as outlined in fig5 and 6 the hollow body k grips over the face end of drum 4 in such a case the outer diameter of drum 4 is smaller than the inner diameter at the large diameter end of hollow body k however it is also possible to position the hollow body k according to fig5 at some distance in front.