A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier is a type of electrical rectifier used for converting high-voltage or high-current alternating current (AC) into direct current (DC). It is a type of cold cathode gas-filled tube, but is unusual in that the cathode, instead of being solid, is made from a pool of liquid mercury and is therefore self-restoring. As a result mercury-arc valves, when used as intended, are far more robust and durable and can carry much higher currents than most other types of gas discharge tube. Some examples have been in continuous service, rectifying 50-ampere currents, for decades. Invented in 1902 by Peter Cooper Hewitt, mercury-arc rectifiers were used to provide power for industrial motors, electric railways, streetcars, and electric locomotives, as well as for radio transmitters and for high-voltage direct current (HVDC) power transmission. They were the primary method of high power rectification before the advent of semiconductor rectifiers, such as diodes, thyristors and gate turn-off thyristors (GTOs). These solid state rectifiers have almost completely replaced mercury-arc rectifiers thanks to their lower cost, maintenance, and environmental risk, and higher reliability.
History
In 1882 Jules Jamin and G. Maneuvrier observed the rectifying properties of a mercury arc. The mercury arc rectifier was invented by Peter Cooper Hewitt in 1902 and further developed throughout the 1920s and 1930s by researchers in both Europe and North America. Before its invention, the only way to convert AC current provided by utilities to DC was by using expensive, inefficient, and high-maintenance rotary converters or motor–generator sets. Mercury-arc rectifiers or "converters" were used for charging storage batteries, arc lighting systems, the DC traction motors for trolleybuses, trams, and subways, and electroplating equipment. The mercury rectifier was used well into the 1970s, when it was finally replaced by semiconductor rectifiers.
Operating principles
Operation of the rectifier relies on an electrical arc discharge between electrodes in a sealed envelope containing mercury vapor at very low pressure. A pool of liquid mercury acts as a self-renewing cathode that does not deteriorate with time. The mercury emits electrons freely, whereas the carbon anodes emit very few electrons even when heated, so the current of electrons can only pass through the tube in one direction, from cathode to anode, which allows the tube to rectify alternating current. When an arc is formed, electrons are emitted from the surface of the pool, causing ionization of mercury vapor along the path towards the anodes. The mercury ions are attracted towards the cathode, and the resulting ionic bombardment of the pool maintains the temperature of the emission spot, so long as a current of a few amperes continues. While the current is carried by electrons, the positive ions returning to the cathode allow the conduction path to be largely unaffected by the space charge effects which limit the performance of vacuum tubes. Consequently, the valve can carry high currents at low arc voltages (typically 20–30 V) and so is an efficient rectifier. Hot-cathode, gas discharge tubes such as the thyratron may also achieve similar levels of efficiency but heated cathode filaments are delicate and have a short operating life when used at high current. The temperature of the envelope must be carefully controlled, since the behaviour of the arc is determined largely by the vapor pressure of the mercury, which in turn is set by the coolest spot on the enclosure wall. A typical design maintains temperature at 40 °C (104 °F) and a mercury vapor pressure of 7 millipascals. The mercury ions emit light at characteristic wavelengths, the relative intensities of which are determined by the pressure of the vapor. At the low pressure within a rectifier, the light appears pale blue-violet and contains much ultraviolet light.
Construction The construction of a mercury arc valve takes one of two basic forms — the glass-bulb type and the steel-tank type. Steel-tank valves were used for higher current ratings above approximately 500 A.
Glass-bulb valves
The earliest type of mercury vapor electric rectifier consists of an evacuated glass bulb with a pool of liquid mercury sitting in the bottom as the cathode. Over it curves the glass bulb, which condenses the mercury that is evaporated as the device operates. The glass envelope has one or more arms with graphite rods as anodes. Their number depends on the application, with one anode usually provided per phase. The shape of the anode arms ensures that any mercury that condenses on the glass walls drains back into the main pool quickly to avoid providing a conductive path between the cathode and respective anode. Glass envelope rectifiers can handle hundreds of kilowatts of direct-current power in a single unit. A six-phase rectifier rated 150 amperes has a glass envelope approximately 600 mm (24 inches) high by 300 mm (12 inches) outside diameter. These rectifiers will contain several kilograms of liquid mercury. The large size of the envelope is required due to the low thermal conductivity of glass. Mercury vapor in the upper part of the envelope must dissipate heat through the glass envelope in order to condense and return to the cathode pool. Some glass tubes were immersed in an oil bath to better control the temperature. The current-carrying capacity of a glass-bulb rectifier is limited partly by the fragility of the glass envelope (the size of which increases with rated power) and partly by the size of the wires fused into the glass envelope for connection of the anodes and cathode. Development of high-current rectifiers required leadwire materials and glass with very similar coefficients of thermal expansion in order to prevent leakage of air into the envelope. Current ratings of up to 500 A had been achieved by the mid-1930s, but most rectifiers for current ratings above this were realised using the more robust steel-tank design.
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