across the element. As the absolute values of the potentials V1, V2 can be high, the field gradients at the surface of small wires or small individual resistor units would be too high to withstand the insulation strength of the atmospheric air used for the construction. Therefore, the larger diameter of the shields lowers the field gradients to an acceptable magnitude. A further reduction of these gradients is achieved by the helical arrangement, as now the helix might be assumed to form a cylinder of much larger diameter, across which the potential continuously decreases from the top to the bottom. These statements could be confirmed by a computation of the very complex field distribution of the three-dimensional structure. The h.v. end of the resistor is fitted with a large ‘stress ring’ which again prevents concentration of electrical field and thus corona or partial discharge formation. A corona-free design is absolutely necessary to avoid leakage currents, which would decrease the overall resistance value