Mercury arc Valve

A mercury arc valve converts high-voltage or high-current alternating current into direct current. It is an electrical rectifier often used to provide power for industrial motors, electric railways, streetcars, and electric locomotives.

Peter Cooper Hewitt, in 1902, invented the mercury arc rectifier. It was further developed by researchers throughout the 1920s and 1930s. Prior to solid-state devices, mercury arc rectifiers were one of the more efficient rectifiers. Silicon diode and thyristor devices have largely made mercury arc rectifiers largely obsolete since 1975. They do remain in use in some South African mines, on the Manx Electric Railway on the Isle of Man, and the HVDC Vancouver Island link between Vancouver Island and the Canadian Mainland.

The design of one mercury vapor electric rectifier consists of an evacuated glass bulb with a pool of liquid mercury at the bottom acting as a cathode. Over the mercury curves the glass bulb which condenses mercury evaporated in the operation of the device. In order to prevent “backfire” the design of the arms and envelope is intended to stoop an arc from forming between the anodes. Glass envelope rectifiers can produce hundreds of kilowatts of direct-current power in a single unit. Due to the low thermal conductivity of glass the envelope must be large. The mercury vapor in the upper part of the envelope must give up heat through the glass envelope to condense and return to the cathode pool.

How much current the rectifier can carry is limited in part to the size of the wires fused into the glass envelope. They also require leadwire materials and glass with a very similar coefficient of thermal expansion in order to stop leakage of air into the envelope.
A metal tank with ceramic insulators for the electrodes is used for larger valves along with a vacuum pump system to counteract slight leakage of air into the tank around the imperfect seals.

The control grids for both glass and metal envelope rectifiers are inserted between the anode and cathode allowing for the conduction of the rectifier to be controlled. This allows for a delay in the instant at which the arc transfers to the anode on the alternating current waveform. The temperature of the envelope must be controlled since the working pressure within the envelope is set by the coolest spot on the enclosure wall.

Once an arc is formed the electrons are emitted from the surface of the pool causing ionization of mercury vapor along the path towards the anodes. Conventional mercury arc rectifiers are started by a brief high-voltage arc between the cathode pool and a starting electrode. This starting electrode is brought into contact with the pool and allowed to pass current through an inductive circuit.
The brief contact between the starting electrode and the pool can be achieved by allowing an external electromagnet to pull the electrode into contact with the pool. The electromagnet can also be arranged to tip the bulb of a small rectifier allowing mercury from the pool to reach the starting electrode. The largest ever rectifiers were used until 2004 at the Nelson River Bipole high-voltage DC power transmission project.

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