A working surface of the electrode head, designed to be essentially hemispherical in shape, is made of a suitable high-temperature-resistant metal. The electrode head is supplied with power via an electrical connecting line. The rear surface of the electrode head forming the sectional surface of the hemisphere is planar and is covered with an insulating cover made of a ceramic material. The transitional region from the working surface to the rear surface does not have an edge but instead is rounded with a minimum radius of curvature, which is substantially greater than one-thirtieth of the width, which corresponds to the hemisphere diameter (=twice the radius of the hemisphere), which is the dimension of the electrode head in the direction of its maximum extent. The relatively large radius of curvature prevents the main activity of the electrode from occurring at its boundary due to excessively high local current densities.
