A pair of opposed right-angled electrically conducting spring-strips 12, 13 are secured, one on each side of the vertical central axis of the cover plate 10 with securing means such as rivets or screws 14, 15 which place electrodes of the panels in contact with a source of electric current, typically 120 volts or 220 volts AC. Preferably the ends of the rivets 14, 15 are embedded in the cover plat