Arc lamp
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An arc lamp is the general term for a class of lamps that produce light by an electric arc (or 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, sodium, metal halide, and mercury. The common fluorescent lamp is actually a low-pressure mercury arc lamp where the inside of the bulb is coated with a light-emitting phosphor. Lightning could be thought of as a type of natural arc lamp, or at least a flash lamp.
The electric arc in an arc lamp consists of gas which is initially ionized by a voltage and is therefore electrically conductive. To start an arc lamp, usually a very high voltage is needed to "ignite" or "strike" the arc. This requires an electrical circuit sometimes called an "igniter", which is part of a larger circuit called the "ballast". The ballast supplies a suitable voltage and current to the lamp as its electrical characteristics change with temperature and time. The ballast is typically designed to maintain safe operating conditions and constant light output over the life of the lamp. The temperature of the arc can reach several thousand degrees Fahrenheit.
In a carbon arc lamp, the electrodes are carbon rods. 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 vaporized during the process and need to be regularly adjusted to maintain the arc. 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 Křižík invented a mechanism to allow the automatic adjustment of the electrodes.
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 two companies merged in 1892 to form the General Electric Company (Noble, 6-10).
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
- Electric light
- Léon Foucault
- Graphite
- František Křižík
- Light bulb
- List of light sources
- Manual metal arc welding
- Movie projector
- Walther Nernst
- Photolithography
- Praseodymium
- Stage lighting
- Timeline of lighting technology
- Tesla patents
- Yablochkov candle
- Pavel Yablochkov
References
| L i g h t i n g and L a m p s | ||
|---|---|---|
| Incandescent: | Incandescent light bulb>Conventional - Halogen - Parabolic Aluminized Reflector (PAR) |
|
| Fluorescent: | Compact fluorescent lamp>Compact Fluorescent (CFL) - Linear fluorescent - Induction lamp | |
| Gas discharge: | High-intensity discharge lamp>High-Intensity Discharge (HID) - Mercury-vapor - Metal-halide - Neon - Sodium vapor | |
| Electric arc: | Arc lamp - Hydrargyrum medium-arc iodide>HMI - Xenon arc - Yablochkov candle | |
| Combustion: | Carbide lamp>Acetylene/Carbide - Candle - Gas lighting - Kerosene lamp - Limelight - Oil lamp - Safety lamp | |
| Other types: | Sulfur lamp - Light-emitting diode>Light-Emitting Diode (LED) - Optical fiber - Plasma | |
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