said sheet metal flux guide can be supported under radially resilient elastic stress lying against the inner peripheral surface 22 of the cup 17 the per se annular closed sheet metal flux guide 21 can comprise for this purpose a particularly axis parallel longitudinal slit 23 so that said sheet metal flux guide can be by way of example manually radially compressed for the purpose of being inserted into the cup 17 the said elements 20 are fixed on the inner periphery of the per se annular closed sheet metal flux guide 21 in a positive locking manner or by means of a material connection this causes elastic flexural loadings of the sheet metal flux guide 21 to occur between mutually adjacent elements 20 said loadings being sufficient to expand said sheet metal flux guide radially outwards as it relaxes after being inserted into the cup 17 and as a consequence to relax in any case slightly against the inner peripheral surface 22 of said cup in an expedient manner the sheet metal flux guide 21 becomes adhered to the inner peripheral surface 22 of the cup 17 for which purpose it is useful to exert temporary additional pressure on the elements 20 in the centrifugal direction a central blind hole 24 at the preferably in the cup base 25 is equipped with a bearing 26 for the purpose of receiving the lower free end face 27 of a coaxial shaft.