In order to prevent the occurrence of glow discharge at sharp corners and edges of core legs for transformers, reactors and the like which are formed of a plurality of sheets, tape is wound around the bundle of sheets, the tape being formed of a backing of glass fiber or the like having high mechanical strength and impregnated with an electrically semi-conducting and thermosetting material, the successive turns of tape overlapping, the winding being under high tension, the screen after heat treatment and curing being quite stiff and having good mechanical and electrical contact with the core leg along its longitudinal edges.