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80-457: 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) in

160-411: A galvanic cell the anode is the wire or plate having excess negative charge as a result of the oxidation reaction. In an electrolytic cell , the anode is the wire or plate upon which excess positive charge is imposed. As a result of this, anions will tend to move towards the anode where they will undergo oxidation. Historically, the anode of a galvanic cell was also known as the zincode because it

240-578: A Mercury arc valve to provide power to the tram which carries visitors between its two sites. Special types of single-phase mercury-arc rectifiers are the Ignitron and the Excitron . The Excitron is similar to other types of valve described above but depends critically on the existence of an excitation anode to maintain an arc discharge during the half-cycle when the valve is not conducting current. The Ignitron dispenses with excitation anodes by igniting

320-421: A circuit is opposite to the direction of electron flow, so (negatively charged) electrons flow from the anode of a galvanic cell , into an outside or external circuit connected to the cell. For example, the end of a household battery marked with a "+" is the cathode (while discharging). In both a galvanic cell and an electrolytic cell , the anode is the electrode at which the oxidation reaction occurs. In

400-497: A direction "from East to West, or, which will strengthen this help to the memory, that in which the sun appears to move", the anode is where the current enters the electrolyte, on the East side: " ano upwards, odos a way; the way which the sun rises". The use of 'East' to mean the 'in' direction (actually 'in' → 'East' → 'sunrise' → 'up') may appear contrived. Previously, as related in the first reference cited above, Faraday had used

480-413: 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. 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

560-419: A more electrically reactive (less noble) metal attached to the vessel hull and electrically connected to form a cathodic protection circuit. A less obvious example of this type of protection is the process of galvanising iron. This process coats iron structures (such as fencing) with a coating of zinc metal. As long as the zinc remains intact, the iron is protected from the effects of corrosion. Inevitably,

640-506: A negative charge, the direction of electron flow is opposite to the direction of conventional current. Consequently, electrons leave the device through the anode and enter the device through the cathode. The definition of anode and cathode is different for electrical devices such as diodes and vacuum tubes where the electrode naming is fixed and does not depend on the actual charge flow (current). These devices usually allow substantial current flow in one direction but negligible current in

720-442: A public asylum) forced a public debate of eugenics. Anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode , an electrode of the device through which conventional current leaves the device. A common mnemonic is ACID, for "anode current into device". The direction of conventional current (the flow of positive charges) in

800-599: A single tank. As solid-state metal rectifiers became available for low-voltage rectification in the 1920s, mercury arc tubes became limited to higher voltage and especially high-power applications. Mercury-arc valves were widely used until the 1960s for the conversion of alternating current into direct current for large industrial uses. Applications included power supply for streetcars, electric railways, and variable-voltage power supplies for large radio transmitters. Mercury-arc stations were used to provide DC power to legacy Edison -style DC power grids in urban centers until

880-407: 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

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960-439: A smoother direct current. Three phase operation can improve the efficiency of the transformer as well as providing smoother DC current by enabling two anodes to conduct simultaneously. During operation, the arc transfers to the anodes at the highest positive potential (with respect to the cathode). In HVDC applications, a full-wave three-phase bridge rectifier or Graetz-bridge circuit was usually used, each valve accommodated in

1040-423: A starting electrode. The starting electrode is brought into contact with the pool and allowed to pass current through an inductive circuit. The contact with the pool is then broken, resulting in a high emf and an arc discharge. The momentary contact between the starting electrode and the pool may be achieved by a number of methods, including: Since momentary interruptions or reductions of output current may cause

1120-401: A steel tank with ceramic insulators for the electrodes is used, with a vacuum pump system to counteract slight leakage of air into the tank around imperfect seals. Steel-tank valves, with water cooling for the tank, were developed with current ratings of several thousand amps. Like glass-bulb valves, steel-tank mercury arc valves were built with only a single anode per tank (a type also known as

1200-401: 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

1280-413: Is electrically linked to the protected system. As a result, the metal anode partially corrodes or dissolves instead of the metal system. As an example, an iron or steel ship's hull may be protected by a zinc sacrificial anode , which will dissolve into the seawater and prevent the hull from being corroded. Sacrificial anodes are particularly needed for systems where a static charge is generated by

1360-489: 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

1440-442: 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. The construction of

1520-460: Is the positively charged electron collector. In a tube, the anode is a charged positive plate that collects the electrons emitted by the cathode through electric attraction. It also accelerates the flow of these electrons. [REDACTED] In a semiconductor diode , the anode is the P-doped layer which initially supplies holes to the junction. In the junction region, the holes supplied by

1600-411: Is used to coat steel, when a breach of the coating occurs it actually accelerates oxidation of the iron. Another cathodic protection is used on the impressed current anode. It is made from titanium and covered with mixed metal oxide . Unlike the sacrificial anode rod, the impressed current anode does not sacrifice its structure. This technology uses an external current provided by a DC source to create

1680-619: The Nelson River DC Transmission System high-voltage DC-power-transmission project. The valves for the Inter-Island and Kingsnorth projects used four anode columns in parallel, while those of the Nelson River project used six anode columns in parallel in order to obtain the necessary current rating. The Inter-Island link was the last HVDC transmission scheme in operation using mercury arc valves. It

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1760-464: 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. A conventional mercury-arc rectifier is started by a brief high-voltage arc within the rectifier, between the cathode pool and

1840-484: The 1930s and 1940s, leading to the first truly practical mercury-arc valve for HVDC transmission, which was put into service on the 20 MW, 100 kV HVDC link from mainland Sweden to the island of Gotland in 1954. Uno Lamm's work on high voltage mercury-arc valves led him to be known as the "Father of HVDC" power transmission and inspired the IEEE to dedicate an award named after him, for outstanding contributions in

1920-595: The 1950s. In the 1960s, solid-state silicon devices, first diodes and then thyristors , replaced all lower-power and lower voltage rectifier applications of mercury arc tubes. Several electric locomotives, including the New Haven EP5 and the Virginian EL-C , carried ignitrons on board to rectify incoming AC to traction motor DC. One of the last major uses of mercury arc valves was in HVDC power transmission, where they were used in many projects until

2000-435: The 1970s, when it was finally replaced by semiconductor rectifiers . 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

2080-479: The 1970s. These solid state rectifiers have almost completely replaced mercury-arc rectifiers thanks to their higher reliability, lower cost and maintenance and lower environmental risk. 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,

2160-561: The AC waveform are utilised. The cathode is connected to the + side of the DC load, the other side being connected to the center tap of the transformer secondary winding, which always remains at zero potential with respect to ground or earth. For each AC phase, a wire from each end of that phase winding is connected to a separate anode "arm" on the mercury-arc rectifier. When the voltage at each anode becomes positive, it will begin to conduct through

2240-433: The action of flowing liquids, such as pipelines and watercraft. Sacrificial anodes are also generally used in tank-type water heaters. In 1824 to reduce the impact of this destructive electrolytic action on ships hulls, their fastenings and underwater equipment, the scientist-engineer Humphry Davy developed the first and still most widely used marine electrolysis protection system. Davy installed sacrificial anodes made from

2320-444: The anode (even though it is negative and therefore would be expected to attract them, this is due to electrode potential relative to the electrolyte solution being different for the anode and cathode metal/electrolyte systems); but, external to the cell in the circuit, electrons are being pushed out through the negative contact and thus through the circuit by the voltage potential as would be expected. Battery manufacturers may regard

2400-422: The anode and the other the cathode according to the roles the electrodes play when the battery is discharged. This is despite the fact that the roles are reversed when the battery is charged. When this is done, "anode" simply designates the negative terminal of the battery and "cathode" designates the positive terminal. In a diode , the anode is the terminal represented by the tail of the arrow symbol (flat side of

2480-474: The anode combine with electrons supplied from the N-doped region, creating a depleted zone. As the P-doped layer supplies holes to the depleted region, negative dopant ions are left behind in the P-doped layer ('P' for positive charge-carrier ions). This creates a base negative charge on the anode. When a positive voltage is applied to anode of the diode from the circuit, more holes are able to be transferred to

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2560-409: The anode is the positive terminal imposed by an external source of potential difference. The current through a recharging battery is opposite to the direction of current during discharge; in other words, the electrode which was the cathode during battery discharge becomes the anode while the battery is recharging. In battery engineering, it is common to designate one electrode of a rechargeable battery

2640-523: The anode, anions (negative ions) are forced by the electrical potential to react chemically and give off electrons (oxidation) which then flow up and into the driving circuit. Mnemonics : LEO Red Cat (Loss of Electrons is Oxidation, Reduction occurs at the Cathode), or AnOx Red Cat (Anode Oxidation, Reduction Cathode), or OIL RIG (Oxidation is Loss, Reduction is Gain of electrons), or Roman Catholic and Orthodox (Reduction – Cathode, anode – Oxidation), or LEO

2720-406: 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. For larger valves,

2800-401: The arc each time conduction is required to start. In this way, ignitrons also avoid the need for control grids. In 1919, the book "Cyclopedia of Telephony & Telegraphy Vol. 1" described an amplifier for telephone signals that used a magnetic field to modulate an arc in a mercury rectifier tube. This was never commercially important. Mercury compounds are toxic, highly persistent in

2880-525: The arc has been established, it cannot be stopped by grid action, because the positive mercury ions produced by ionisation are attracted to the negatively charged grid and effectively neutralise it. The only way of stopping conduction is to make the external circuit force the current to drop below a (low) critical current. Although grid-controlled mercury-arc valves bear a superficial resemblance to triode valves, mercury-arc valves cannot be used as amplifiers except at extremely low values of current, well below

2960-1051: The birth of Ann Cooper Hewitt (July 28, 1914-1956). Hewitt later married Andrews in 1918, right after his divorce to Work, and formally adopted Ann. Prior to Hewitt, Andrews was married in 1902 to Dr. Peder Sather Bruguiere (brother of American photographer Francis Bruguière , brother-in-law of heiress Margaret Post Van Alen and grandson of banker Peder Sather ) and in 1907 to wealthy New York broker Alexander Turner Stewart Denning. After Hewitt, Andrews married in 1922 to Baron Robert Frederic Emile Regis D'Erlanger and in 1926 to George William Childs McCarter (grandson of American author Hannah Mary Bouvier Peterson , great-grandson of Judge John Bouvier and nephew-in-law of American publisher George William Childs ). Peter Cooper Hewitt died in 1921. His will left two-thirds of his estate to Ann and one-third to her mother Marion; but Ann's portion would revert to her mother if Ann (Gay Bradstreet) died childless. In 1935, just before Ann's 21st birthday when she would have attained legal majority, she

3040-425: The cathode spot to extinguish, many rectifiers incorporate an additional electrode to maintain an arc whenever the plant is in use. Typically, a two or three phase supply of a few amperes passes through small excitation anodes . A magnetically shunted transformer of a few hundred VA rating is commonly used to provide this supply. This excitation or keep-alive circuit was necessary for single-phase rectifiers such as

3120-411: The cathode tanks either water-cooled or air-cooled. Single-phase mercury-arc rectifiers were rarely used because the current dropped and the arc could be extinguished when the AC voltage changed polarity. The direct current produced by a single-phase rectifier thus contained a varying component (ripple) at twice the power supply frequency , which was undesirable in many applications for DC. The solution

3200-431: The cathodic protection. Impressed current anodes are used in larger structures like pipelines, boats, city water tower, water heaters and more. The opposite of an anode is a cathode . When the current through the device is reversed, the electrodes switch functions, so the anode becomes the cathode and the cathode becomes anode, as long as the reversed current is applied. The exception is diodes where electrode naming

3280-407: The critical current needed to maintain the arc. Mercury-arc valves are prone to an effect called arc-back (or backfire ), whereby the valve conducts in the reverse direction when the voltage across it is negative. Arc-backs can be damaging or destructive to the valve, as well as creating high short-circuit currents in the external circuit, and are more prevalent at higher voltages. One example of

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3360-412: 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

3440-431: The current through the electrodes reverses direction, as occurs for example in a rechargeable battery when it is being charged, the roles of the electrodes as anode and cathode are reversed. Conventional current depends not only on the direction the charge carriers move, but also the carriers' electric charge . The currents outside the device are usually carried by electrons in a metal conductor. Since electrons have

3520-426: The depleted region, and this causes the diode to become conductive, allowing current to flow through the circuit. The terms anode and cathode should not be applied to a Zener diode , since it allows flow in either direction, depending on the polarity of the applied potential (i.e. voltage). In cathodic protection , a metal anode that is more reactive to the corrosive environment than the metal system to be protected

3600-523: The disused deep-level air-raid shelter at Belsize Park . After they were no longer needed as shelters, Belsize Park and several other deep shelters were used as secure storage, particularly for music and television archives. This led to the mercury-arc rectifier at the Goodge Street shelter featuring in an early episode of Doctor Who as an alien brain, cast for its "eerie glow". Auckland's Museum Of Transport And Technology (MOTAT) still employs

3680-630: The early 1970s, including the HVDC Inter-Island link between the North and South Islands of New Zealand and the HVDC Kingsnorth link from Kingsnorth power station to London . However, starting about 1975, silicon devices have made mercury-arc rectifiers largely obsolete, even in HVDC applications. The largest ever mercury-arc rectifiers, built by English Electric , were rated at 150  kV , 1800 A and were used until 2004 at

3760-586: The environment, and present a danger to humans and the environment. The use of large quantities of mercury in fragile glass envelopes presents a hazard of potential release of mercury to the environment should the glass bulb be broken. Some HVDC converter stations have required extensive clean-up to eliminate traces of mercury emitted from the station over its service life. Steel tank rectifiers frequently required vacuum pumps, which continually emitted small amounts of mercury vapor. Peter Cooper Hewitt Peter Cooper Hewitt (May 5, 1861 – August 25, 1921)

3840-434: The evacuated tube due to being heated by a filament, so electrons can only enter the device from the external circuit through the heated electrode. Therefore, this electrode is permanently named the cathode, and the electrode through which the electrons exit the tube is named the anode. The polarity of voltage on an anode with respect to an associated cathode varies depending on the device type and on its operating mode. In

3920-409: The event of a later convention change it would have become West to East, so that the East electrode would not have been the 'way in' any more. Therefore, "eisode" would have become inappropriate, whereas "anode" meaning 'East electrode' would have remained correct with respect to the unchanged direction of the actual phenomenon underlying the current, then unknown but, he thought, unambiguously defined by

4000-458: The excitron and for mercury-arc rectifiers used in the high-voltage supply of radiotelegraphy transmitters, as current flow was regularly interrupted every time the Morse key was released. Both glass and metal envelope rectifiers may have control grids inserted between the anode and cathode. Installation of a control grid between the anode and the pool cathode allows control of the conduction of

4080-509: The field of HVDC. Mercury arc valves with grading electrodes of this type were developed up to voltage ratings of 150 kV. However, the tall porcelain column required to house the grading electrodes was more difficult to cool than the steel tank at cathode potential, so the usable current rating was limited to about 200–300 A per anode. Therefore, Mercury arc valves for HVDC were often constructed with four or six anode columns in parallel. The anode columns were always air-cooled, with

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4160-680: The first successful precursors of the cruise missile . Hewitt's first wife was Lucy Bond Work. Work was the daughter of Franklin H. Work (1819–1911), a well-known stockbroker and protégé of Cornelius Vanderbilt , and his wife, Ellen Wood (1831–1877), who was the sister of Frances Ellen Work . Thus he was an uncle of Maurice Roche, 4th Baron Fermoy , the maternal grandfather of Diana, Princess of Wales . Cooper Hewitt and his first wife had no children and divorced in December 1918. While married to Work, Hewitt had an extramarital relationship with Marion (aka Maryon) Jeanne Andrews that resulted in

4240-408: The following examples, the anode is negative in a device that provides power, and positive in a device that consumes power: In a discharging battery or galvanic cell (diagram on left), the anode is the negative terminal: it is where conventional current flows into the cell. This inward current is carried externally by electrons moving outwards. In a recharging battery, or an electrolytic cell ,

4320-469: The future. Since the later discovery of the electron , an easier to remember and more durably correct technically although historically false, etymology has been suggested: anode, from the Greek anodos , 'way up', 'the way (up) out of the cell (or other device) for electrons'. In electrochemistry , the anode is where oxidation occurs and is the positive polarity contact in an electrolytic cell . At

4400-399: 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

4480-478: The lamp was frequently augmented by a standard incandescent lamp. The two together provided a more acceptable color while retaining some efficiency advantages. In 1902, Hewitt developed the mercury arc rectifier , the first rectifier that could convert alternating current power to direct current without mechanical means. It was widely used in electric railways , industry, electroplating, and high-voltage direct current (HVDC) power transmission. Although it

4560-433: The least reactive materials for anodes. Platinum erodes very slowly compared to other materials, and graphite crumbles and can produce carbon dioxide in aqueous solutions but otherwise does not participate in the reaction. In a battery or galvanic cell , the anode is the negative electrode from which electrons flow out towards the external part of the circuit. Internally the positively charged cations are flowing away from

4640-650: The lion says GER (Losing electrons is Oxidation, Gaining electrons is Reduction). This process is widely used in metals refining. For example, in copper refining, copper anodes, an intermediate product from the furnaces, are electrolysed in an appropriate solution (such as sulfuric acid ) to yield high purity (99.99%) cathodes. Copper cathodes produced using this method are also described as electrolytic copper . Historically, when non-reactive anodes were desired for electrolysis, graphite (called plumbago in Faraday's time) or platinum were chosen. They were found to be some of

4720-456: The magnetic reference. In retrospect the name change was unfortunate, not only because the Greek roots alone do not reveal the anode's function any more, but more importantly because as we now know, the Earth's magnetic field direction on which the "anode" term is based is subject to reversals whereas the current direction convention on which the "eisode" term was based has no reason to change in

4800-543: The mercury vapor from the cathode. As the anodes of each AC phase are fed from opposite ends of the centre tapped transformer winding, one will always be positive with respect to the center tap and both halves of the AC Waveform will cause current to flow in one direction only through the load. This rectification of the whole AC waveform is thus called full-wave rectification . With three-phase alternating current and full-wave rectification, six anodes were used to provide

4880-451: The more straightforward term "eisode" (the doorway where the current enters). His motivation for changing it to something meaning 'the East electrode' (other candidates had been "eastode", "oriode" and "anatolode") was to make it immune to a possible later change in the direction convention for current , whose exact nature was not known at the time. The reference he used to this effect was the Earth's magnetic field direction, which at that time

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4960-402: The negative electrode as the anode, particularly in their technical literature. Though from an electrochemical viewpoint incorrect, it does resolve the problem of which electrode is the anode in a secondary (or rechargeable) cell. Using the traditional definition, the anode switches ends between charge and discharge cycles. In electronic vacuum devices such as a cathode-ray tube , the anode

5040-406: 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

5120-445: The other direction. Therefore, the electrodes are named based on the direction of this "forward" current. In a diode the anode is the terminal through which current enters and the cathode is the terminal through which current leaves, when the diode is forward biased . The names of the electrodes do not change in cases where reverse current flows through the device. Similarly, in a vacuum tube only one electrode can emit electrons into

5200-687: The problems caused by backfire occurred in 1960 subsequent to the electrification of the Glasgow North Suburban Railway where steam services had to be re-introduced after several mishaps. For many years this effect limited the practical operating voltage of mercury-arc valves to a few kilovolts. The solution was found to be to include grading electrodes between the anode and control grid, connected to an external resistor - capacitor divider circuit. Dr. Uno Lamm conducted pioneering work at ASEA in Sweden on this problem throughout

5280-406: 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

5360-418: The triangle), where conventional current flows into the device. Note the electrode naming for diodes is always based on the direction of the forward current (that of the arrow, in which the current flows "most easily"), even for types such as Zener diodes or solar cells where the current of interest is the reverse current. In vacuum tubes or gas-filled tubes , the anode is the terminal where current enters

5440-455: The tube. The word was coined in 1834 from the Greek ἄνοδος ( anodos ), 'ascent', by William Whewell , who had been consulted by Michael Faraday over some new names needed to complete a paper on the recently discovered process of electrolysis . In that paper Faraday explained that when an electrolytic cell is oriented so that electric current traverses the "decomposing body" (electrolyte) in

5520-429: The two electrodes and the liquid mercury; later he developed the inductive electrical ballast to start the tube. The efficiency was much higher than that of incandescent lamps , but the emitted light was of a bluish-green unpleasant color, which limited its practical use to specific professional areas, like photography, where the color was not an issue at a time where films were black and white. For space lighting use,

5600-444: The valve in the non-conducting state, a negative bias of a few volts or tens of volts is applied to the grid. As a result, electrons emitted from the cathode are repelled away from the grid, back towards the cathode, and so are prevented from reaching the anode. With a small positive bias applied to the grid, electrons pass through the grid, towards the anode, and the process of establishing an arc discharge can commence. However, once

5680-452: The valve, thereby giving control of the mean output voltage produced by the rectifier. Start of the current flow can be delayed past the point at which the arc would form in an uncontrolled valve. This allows the output voltage of a valve group to be adjusted by delaying the firing point, and allows controlled mercury-arc valves to form the active switching elements in an inverter converting direct current into alternating current. To maintain

5760-443: The zinc coating becomes breached, either by cracking or physical damage. Once this occurs, corrosive elements act as an electrolyte and the zinc/iron combination as electrodes. The resultant current ensures that the zinc coating is sacrificed but that the base iron does not corrode. Such a coating can protect an iron structure for a few decades, but once the protecting coating is consumed, the iron rapidly corrodes. If, conversely, tin

5840-505: Was an American electrical engineer and inventor, who invented the first mercury-vapor lamp in 1901. Hewitt was issued U.S. patent 682,692 on September 17, 1901. In 1903, Hewitt created an improved version that possessed higher color qualities which eventually found widespread industrial use. Hewitt was born in New York City , the son of New York City Mayor Abram Hewitt and the grandson of industrialist Peter Cooper . He

5920-412: Was believed to be invariant. He fundamentally defined his arbitrary orientation for the cell as being that in which the internal current would run parallel to and in the same direction as a hypothetical magnetizing current loop around the local line of latitude which would induce a magnetic dipole field oriented like the Earth's. This made the internal current East to West as previously mentioned, but in

6000-559: Was educated at the Stevens Institute of Technology and the Columbia University School of Mines . In 1901, Hewitt invented and patented a mercury-vapor lamp that was the forerunner of the fluorescent lamp . A gas-discharge lamp , Hewitt's invention used mercury vapor produced by passing current through liquid mercury. His first lamps had to be started by tilting the tube to make contact between

6080-694: Was formally decommissioned on 1 August 2012. The mercury arc valve converter stations of the New Zealand scheme were replaced by new thyristor converter stations. A similar mercury arc valve scheme, the HVDC Vancouver Island link was replaced by a three-phase AC link. Mercury arc valves remain in use in some South African mines and Kenya (at Mombasa Polytechnic - Electrical & Electronic department). Mercury arc valves were used extensively in DC power systems on London Underground , and two were still observed to be in operation in 2000 at

6160-494: Was hospitalized for appendicitis. Ann's mother told the surgeons that Ann was "feebleminded" and paid them to sterilize her while performing her appendectomy. Ann retaliated by suing her mother in San Francisco court and telling the press about Maryon's gambling and alcohol addictions. The mother-daughter dispute riveted the public; and the unconventional use of sterilization (it occurred in private practice, not

6240-489: Was largely replaced by power semiconductor devices in the 1970s and 1980s, it is still used in some high power applications. In 1903, Columbia University awarded Hewitt the degree of Honorary Doctorate of Science in recognition of his work. In 1907, he developed and tested an early hydrofoil . In 1916 , Hewitt joined Elmer Sperry to develop the Hewitt-Sperry Automatic Airplane , one of

6320-448: Was to use two-, three-, or even six-phase AC power supplies so that the rectified current would maintain a more constant voltage level. Polyphase rectifiers also balanced the load on the supply system, which is desirable for reasons of system performance and economy. Most applications of mercury-arc valves for rectifiers used full-wave rectification with separate pairs of anodes for each phase. In full-wave rectification both halves of

6400-407: Was usually composed of zinc. The terms anode and cathode are not defined by the voltage polarity of electrodes but the direction of current through the electrode. An anode is an electrode of a device through which conventional current (positive charge) flows into the device from an external circuit, while a cathode is an electrode through which conventional current flows out of the device. If

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