In the embodiments depicted in FIGS. 6 and 7, a low cost insulating layer with high insulating properties and a resistivity greater than 1012 ??cm may be obtained if insulating layers 72 and 74 are formed of a resin including polystyrene, acrylic, phenol, polyester, silicon elastomer, polyurethane, epoxy resin, polyimide, cellulose or natural rubber.