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Mercury-arc valve
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<p>A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier is a type of electrical <a href="/facts/Rectifier/me8XDxND">rectifier</a> used for converting high-<a href="/facts/Voltage/EN2po6N2">voltage</a> or high-<a href="/facts/Electric_current/Dc1xdnUX">current</a> <a href="/facts/Alternating_current/qxICS3xa">alternating current</a> (AC) into <a href="/facts/Direct_current/r9MNUx84">direct current</a> (DC). It is a type of <a href="/facts/Cold_cathode/da9mlNne">cold cathode</a> <a href="/facts/Gas-filled_tube/uPNhhy7I">gas-filled tube</a>, but is unusual in that the cathode, instead of being solid, is made from a pool of liquid <a href="/facts/Mercury_(element)/WlxfoHva">mercury</a> 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-<a href="/facts/Ampere/MScqkCgz">ampere</a> currents, for decades. </p><p>Invented in 1902 by <a href="/facts/Peter_Cooper_Hewitt/aDtP2N8C">Peter Cooper Hewitt</a>, mercury-arc rectifiers were used to provide power for industrial motors, <a href="/facts/Railway_electrification_system/WksMTQ4u">electric railways</a>, <a href="/facts/Tram/gSaKExyR">streetcars</a>, and <a href="/facts/Electric_locomotive/580UInEr">electric locomotives</a>, as well as for radio <a href="/facts/Transmitter/nFYghtKv">transmitters</a> and for <a href="/facts/High-voltage_direct_current/FDIob86A">high-voltage direct current</a> (HVDC) power transmission. They were the primary method of high power rectification before the advent of <a href="/facts/Semiconductor/zFf3mGTE">semiconductor</a> rectifiers, such as <a href="/facts/Diode/JyaoyGnk">diodes</a>, <a href="/facts/Thyristor/DYPV2ftl">thyristors</a> and <a href="/facts/Gate_turn-off_thyristor/rdBy6wmf">gate turn-off thyristors</a> (GTOs). These solid state rectifiers have almost completely replaced mercury-arc rectifiers thanks to their lower cost, maintenance, and environmental risk, and higher reliability. </p>