The first electrode 12 a is formed by a conductive layer approximately 100 ??m thick constructed from a metal such as an alloy of copper and nickel having a resistivity of 10???6 ??cm, a conductive rubber having a resistivity of approximately 10???1 ??cm produced by dispersing a carbon black in an ethylene-propylene rubber (EPM, EPDM), a conductive polymeric material doped with iodine, such as pol