Like glass-bulb valves, steel-tank mercury arc valves were built with only a single anode per tank (a type also known as the excitron) or with multiple anodes per tank. Multiple-anode valves were usually used for multi-phase rectifier circuits (with 2, 3, 6 or 12 anodes per tank) but in HVDC applications, multiple anodes were often simply connected in parallel in order to increase the current rating.