the insulation layer 3 alternatively to the recesses 10 it is also conceivable to form longitudinal elevations which necessarily result on the underside in corresponding longitudinal groove like depressions at the opposing edge region 11 a number of elevations 6 a corresponding to the number of recesses 10 or longitudinal depressions is formed on each insulation layer again nub like in the example shown as the example further shows elevations 6 and depressions 7 are arranged over the rest of the surface in the manner already known from fig2 i e the number of elevations 6 depressions 7 over the surface is significantly larger than the number of the few elevations 6 a in the edge region 11 if now two insulation layers 3 are arranged next to one another forming an overlap region 8 be they plates strips or foils the elevations 6 a formed in the region of the edge section 11 engage in the corresponding longitudinal recesses 10 as shown in fig6 since the recesses 10 are longitudinal i e represent elongated holes it is not a matter of the exact positioning of the elevations 6 a relative to the recesses 10 i e these insulation layers can be integrated in different radial planes.