The mercury-arc valve
Interest in DC transmission came back again when the mercury-arc valve came onto the scene. The mercury-arc valve is a sealed bulb filled with mercury vapor that uses a steel anode (later made up of carbon) and a mercury cathode (Figure 1)
Once an arc is initiated between the anode and the cathode, the current flowing in the arc ionizes the mercury vapor by its heat. The bombardment of ionsat the interface of the arc and the mercury causes ions to be released. The steel anode does not emit electrons at the operating temperature. There is a flow of electrons from the mercury cathode but not in the reverse direction. The mercury-arc valve acts as a diode and can be applied in a rectifier circuit: as soon as the voltage across it becomes positive, the valve conducts the current, and it isolates when the c 13urrent passes through zero. Mercury-arc valves can also be applied for DC to AC conversion: an auxiliary electrode inside the bulb receives a voltage pulse that initiates an arc between the anode and the cathode at an arbitrary point in the cycle. So mercury valves can perform both AC to DC and DC to AC conversion and made it possible to combine the transformation advantages of AC with the transmission advantages of DC.The mercury-arc valve was first demonstrated by its inventor Peter Cooper Hewitt in 1902.