page_title: Arc lamp
section_title: Carbon arc lamp
hierarchical_section_title: Arc lamp / Carbon arc lamp
caption_attribution_description: English: An arc lamp during ignition Deutsch: Eine Kohlebogenlampe während der Zündung
context_page_description: An arc lamp or arc light is a lamp that produces light by an electric arc. The carbon arc light, which consists of an arc between carbon electrodes in air, invented by Humphry Davy in the first decade of the 1800s, was the first practical electric light. It was widely used starting in the 1870s for street and large building lighting until it was superseded by the incandescent light in the early 20th century. It continued in use in more specialized applications where a high intensity point light source was needed, such as searchlights and movie projectors until after World War II. The carbon arc lamp is now obsolete for most of these purposes, but it is still used as a source of high intensity ultraviolet light.
  The term is now used for gas discharge lamps, which produce light by an arc between metal electrodes through an inert gas in a glass bulb. The common fluorescent lamp is a low-pressure mercury arc lamp. The xenon arc lamp, which produces a high intensity white light, is now used in many of the applications which formerly used the carbon arc, such as movie projectors and searchlights.
context_section_description: 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 and the carbon vaporizes. The carbon vapor in the arc is highly luminous, which is what produces the bright light.   The rods are slowly burnt away in use, and the distance between them needs to be regularly adjusted in order to maintain the arc.
  Many ingenious mechanisms were invented to affect the distance automatically, mostly based on solenoids. In one of the simplest mechanically-regulated forms (which was soon superseded by more smoothly acting devices) the electrodes are mounted vertically.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 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.
  The Yablochkov candle is a simple arc lamp without a regulator, but it has the drawbacks that the arc cannot be restarted (single use) and a limited lifetime of only a few hours.
caption: A carbon arc lamp, cover removed, on the point of ignition. This model requires manual adjustment of the electrodes
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