to suppress electromagnetic interference across a wide range of frequencies , in both magnetic and electrical fields , an emissive wire is surrounded by a ferrite core having an outer surface covered with a metal foil . the metal foil on the core or cores is grounded . if multiple cores are used in proximity , the foils of the several cores are electrically coupled together . in one embodiment , an omni - directional strobe light like the one shown in u . s . pat . no . 5 , 155 , 666 is shielded by placing pairs of foil - covered ferrite cores around the wires at each apex of the polygon formed by the interconnected flash tubes , grounding the cores to the nearest lamp reflector , and grounding the reflectors together .