Editing Arc lamp (section)

== Carbon arc lamp ==
[[Image:Arc lamp-ignition PNr°0035.jpg|thumb|A [[carbon]] arc lamp, cover removed, on the point of ignition. This model requires manual adjustment of the electrodes]]
[[Image:Lichtbogen 3000 Volt.jpg|thumb|An electric arc, demonstrating the “arch” effect.]]
[[Image:Arc light and battery.jpg|thumb|Early experimental carbon arc light powered by liquid batteries, similar to Davy's]]
[[Image:Medical arc lamp.jpg|thumb|Medical carbon arc lamp [[Light therapy|used to treat skin conditions]], 1909]] 
[[Image:Staite-Petrie Lamp 1847.png|thumb|Self-regulating arc lamp proposed by William Edwards Staite and [[William Petrie (electrical engineer)|William Petrie]] in 1847]]

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.<ref name="Whelan" />  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 and the carbon vaporizes.<ref name="Whelan" />   The rods are slowly burnt away in use, and the distance between them needs to be regularly adjusted in order to maintain the arc.<ref name="Whelan" />

Many ingenious mechanisms were invented to control the distance automatically, mostly based on [[solenoid]]s. In one of the simplest mechanically-regulated forms (which was soon superseded by more smoothly acting devices) the electrodes are mounted vertically. The current supplying the arc is passed in [[Series circuit|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 again.

The [[Yablochkov candle]] is a simple arc lamp without a regulator, but it has the drawback that the arc cannot be restarted (single use) and a limited lifetime of only a few hours.

===Spectrum===
The spectrum emitted by a carbon-arc lamp is the closest to that of sunlight of any lamp. One of the first electric lights, their harsh, intense output usually limited their use to  lighting large areas. Although invisible wavelengths were unknown at the time of their invention, unenclosed lamps were soon discovered to produce large amounts of infrared and harmful ultraviolet-radiation not found in sunlight. If the arc was encased in a glass globe, it was found that many of these invisible rays could be blocked. However, carbon-arcs were soon displaced by safer, more efficient, versatile, and easier to maintain incandescent and [[gas-discharge lamp]]s. Carbon-arc lamps are still used where a close approximation of sunlight is needed, for testing materials, paints, and coatings for wear, fading, or deterioration, or, for example, spacecraft materials that are to be exposed to sunlight at orbits closer than Earth's.<ref>''Publications of the Goddard Space Center, 1959-1962, Volume 2'' by M. Schach and JH. Boeckel - National Aeronautics and Space Administration, 1967, Page 571</ref>

The arc consists of pure carbon-vapor heated to a plasma state. However, the arc contributes very little of the light output, and is considered non-luminous, as most of its emission occurs in spectral lines in the violet and UV portions of the spectrum. Most of the carbon spectra occurs in a very broad line centered at 389 nm (UV-A, just outside the visual spectrum), and a very narrow line at 250 nm (UV-B), plus some other less-powerful lines in the UV-C. 

Most of the visible and IR radiation is produced from [[incandescence]] created at the positive electrode, or anode. Unlike the tungsten anodes found in other arc lamps, which remain relatively cool, carbon produces much higher resistance and the electrons are forced to enter the anode at the hottest point, generating tremendous amounts of heat that vaporizes the carbon and creates a pit in the anode's surface. This pit is heated from 6000 to 6500 degrees Fahrenheit (3300 to 3600 degrees Celsius, just below its melting point), causing it to glow very brightly with incandescence. Due to this, the electrodes were often placed at right angles from each other with the anode facing outward to keep from blocking its light output. Since carbon has the highest melting point of any element, it is the only lamp whose blackbody radiation is capable of nearly matching the Sun's temperature of 10,000 degrees Fahrenheit (5500 degrees Celsius), especially when filters are used to remove most of the IR and UV light.<ref>''Clinical Medicine and Surgery Volume 35'' by Herman Goodman - American Journal of Clinical Medicine, 1928, Page 159-161</ref>