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Arc lamp
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An arc lamp or arc light is the general term for a class of lamps that produce light by an electric arc (also called a voltaic arc). The lamp consists of two electrodes typically made of tungsten which are separated by a gas. The type of lamp is often named by the gas contained in the bulb; including neon, argon, xenon, krypton, sodium, metal halide, and mercury.
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Encyclopedia
An arc lamp or arc light is the general term for a class of lamps that produce light by an electric arc (also called a voltaic arc). The lamp consists of two electrodes typically made of tungsten which are separated by a gas. The type of lamp is often named by the gas contained in the bulb; including neon, argon, xenon, krypton, sodium, metal halide, and mercury. The common fluorescent lamp is actually a low-pressure mercury arc lamp.
The electric arc in an arc lamp consists of gas which is initially ionized by a high voltage and therefore becomes electrically conductive. To start an arc lamp, a very high voltage is pulsed across the lamp to "ignite" or "strike" the arc across the gas. This requires an electrical circuit with an igniter and a ballast.
The ballast is wired in series with the lamp and performs two functions, when the power is first switched on, the igniter/starter (which is wired in parallel across the lamp) sets up a small current through the ballast and starter. This creates a small magnetic field within the ballast windings. A moment later the starter interrupts the current flow from the ballast, the ballast has a high inductance therefore it tries to maintain the current flow (The ballast opposes any change in current through it) it cannot as there is no longer a 'circuit'.
As a result, a high voltage appears across the ballast momentarily - to which the lamp is connected, therefore the lamp receives this high voltage across it which 'strikes' the arc within the tube/lamp. The circuit will repeat this action until the lamp is ionized enough to sustain the arc.
When the lamp sustains the arc the ballast performs its second function to limit the current to that needed to operate the lamp. The lamp, ballast and igniter are rated 'matched' to each other so you must replace these parts with the same rating as the failed component or the lamp will not work.
The lamp (the colour of light) as its electrical characteristics change with temperature and time. Lightning is a similar principle where the atmosphere is ionized by the high potential difference (voltage) between earth and storm clouds.
The temperature of the arc in an arc lamp can reach several thousand degrees Celsius. The outer glass envelope can reach 500 degrees Celsius, therefore before servicing one must ensure the bulb has cooled sufficiently to handle. Often if these type of lamps are turned off or lose power supply they will not strike again for several minutes.
Carbon arc lamp
In popular use, the term arc lamp means carbon arc lamp only.
In a carbon arc lamp, the electrodes are carbon rods in free air. To ignite the lamp, the rods are touched together, thus allowing a relatively low voltage to strike the arc. The rods are then slowly drawn apart, and electric current heats and maintains an arc across the gap. The tips of the carbon rods are heated to incandescence, creating light. The rods are slowly burnt away in use, and need to be regularly adjusted to maintain the arc. Many ingenious mechanisms were invented to effect this automatically, mostly based on solenoids. In the simplest form (which was soon superseded by more smoothly acting devices) the electrodes are mounted vertically. The current supplying the arc is passed in series through a solenoid attached to the top electrode. If the points of the electrodes are touching (as in start up) the resistance falls, the current increases and the increased pull from the solenoid draws the points apart. If the arc starts to fail the current drops and the points close up again.
History
The concept was first demonstrated by Sir Humphry Davy in the early 19th century (1802, 1805, 1807 and 1809 are all mentioned), using charcoal sticks and a 2000-cell battery to create an arc across a 4-inch gap. He mounted his electrodes horizontally and noted that, because of the strong convection flow of air, the arc formed the shape of an arch. He coined the term "arch lamp", which was contracted to "arc lamp" when the devices came into common usage.
There were attempts to produce the lamps commercially after 1850 but the lack of a constant electricity supply thwarted efforts. Thus electrical engineers began focusing on the problem of improving Faraday's dynamo. The concept was improved upon by a number of people including William Staite and Charles F. Brush. It was not until the 1870s that lamps such as the Yablochkov candle were more commonly seen. In 1877, the Franklin Institute conducted a comparative test of dynamo systems. The one developed by Brush performed best, and Brush immediately applied his improved dynamo to arc-lighting. In 1880, he established the Brush Electric Company.
The harsh and brilliant light was found most suitable for public areas, being around 200 times more powerful than contemporary filament lamps. There were three major advances in the 1880s:
- The arcs were enclosed in a small tube to slow the carbon consumption (increasing the life span to around 100 hours).
- Flame arc lamps were introduced where the carbon rods had metal salts (usually magnesium, strontium, barium, or calcium fluorides) added to increase light output and produce different colours.
- František Križík invented a mechanism to allow the automatic adjustment of the electrodes.
In the US, patent protection of arc-lighting systems and improved dynamos proved difficult and as a result the arc-lighting industry became highly competitive. Brush's principal competition was from the team of Elihu Thomson and Edwin J. Houston. These two had formed the American Electric Company in 1880, but it was soon bought up by Charles A. Coffin, moved to Lynn, Massachusetts, and renamed the Thomson-Houston Electric Company. Thomson remained, though, the principal inventive genius behind the company patenting improvements to the lighting system. Under the leadership of Thomson-Houston's patent attorney, Frederick P. Fish, the company protected its new patent rights. Coffin's management also led the company towards an aggressive policy of buy-outs and mergers with competing manufacturers. Both strategies reduced competition in the electrical lighting manufacturing industry. By 1890, the Thomson-Houston company was the dominant electrical manufacturing company in the US (Noble, 6-10). Nikola Tesla received U.S. Patent 447920, "Method of Operating Arc-Lamps" (March 10, 1891), that describes a 10,000 cycles per second alternator to suppress the disagreeable sound of power-frequency harmonics produced by arc lamps operating on frequencies within the range of human hearing.
Around the turn of the century arc-lighting systems were in decline but nonetheless, Thomson-Houston controlled key patents to urban lighting systems. This control slowed the expansion of incandescent lighting systems being developed by Thomas Edison's Edison General Electric Company. Conversely, Edison's control of direct current distribution and generating machinery patents blocked further expansion of Thomson-Houston. The roadblock to expansion was removed when the two companies merged in 1892 to form the General Electric Company (Noble, 6-10).
Arc lamps were also used in some early motion-picture studios to illuminate interior shooting; one problem was that such lights output such a high level of ultra-violet light that many early film actors and actresses needed to wear sunglasses when not in front of the camera to relieve sore eyes resulting from the ultra-violet light. The problem was eventually solved by simply adding a sheet of ordinary window glass in front of the lamp (which is opaque to the ultra-violet). By the dawn of the "talkies", arc lamps had been replaced in film studios with other forms of bright lights. In 1915, Elmer Ambrose Sperry began manufacturing his invention of a high-intensity carbon arc searchlight. These were used aboard warships of all navies during the 20th century for signals at sea and for illuminating an enemy. In the 1920s carbon arc lamps were sold as family health products, a substitute for natural sunlight.
The arc lamps were soon superseded by the more efficient and longer-lasting filament lamps in most roles, remaining in only certain niche markets such as cinema projection and searchlights but even in these applications, conventional carbon arc lamps are finally being pushed into obsolescence by xenon arc lamps.
See also
External links
- National High Magnetic Field Laboratory
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