A round copper rod 1 which was 50 mm. thick and 2.5 m. long was provided at one place on its circumference with a groove 2 which has 10 mm. deep and 4 mm. wide, so that it ran parallel to the axis. Sand-blasted titanium sheets 3 which were 240 mm. long and 150 mm. wide were pressed into groove 2 at intervals of about mm. The thickness of the titanium sheets 3 was a few mm. greater than the internal width of groove 2 in the round copper rod 1. An interval of 20 mm. was left between the individual titanium sheets 3. Thereafter, the round copper rod was tin-plated at moderately elevated temperature and the entire assembly was then covered with a layer of ebonite 4 so that the entire copper surface and the adjacent zones of the titanium sheets 3 were covered, the outer edge 5 of the ebonite cover 4 being no closer to the copper than 10 mm. at any point. Apertures 6 were provided in titanium sheets 3 at those places which were not covered with the ebonite covering 4 for attaching anodes, not shown, which may be made either of graphite, platinized titanium, magnetite, or tantalum.