Arc lamp

Arc lamps are lighting devices that generate illumination through an electric arc formed between two electrodes. Originally developed in the early nineteenth century, arc lamps represent one of the earliest forms of practical electric lighting. Although largely superseded by more efficient technologies in everyday applications, arc lamps remain significant in specialised fields requiring intense, high-quality light sources.

Principles and Operation

An electric arc forms when a gas becomes ionised and conducts electricity. In an arc lamp, a high-voltage pulse initiates the discharge between the electrodes—an action known as striking the arc. After ionisation, the arc can be sustained at a lower voltage. The ignition process requires a ballast and an igniter: the ballast regulates electrical current, while the igniter provides the initial high-voltage pulse. These components must be matched to the lamp’s specifications to ensure correct functioning.
As current passes through the ionised gas, it produces temperatures of several thousand degrees Celsius. The glass envelope surrounding the arc can reach considerable temperature levels, often exceeding 500°C. Some lamp types require a cooling period before restriking the arc, referred to as cold restrike, whereas others permit immediate hot restrike. Atmospheric electrical discharges, such as lightning, follow similar physical principles of ionisation driven by potential differences in the atmosphere.
Modern arc lamps frequently use gases such as xenon or mercury vapour. Xenon arc lamps emit intense white light and are common in projection systems, searchlights, and other contexts demanding high-brightness illumination. Low-pressure mercury lamps form the basis of fluorescent lighting, where ultraviolet emission from the arc is converted into visible light by phosphor coatings inside the tube.

Types of Arc Lamps

Arc lamps vary according to the materials used for their electrodes and the gases employed within their envelopes. The major types include:

• Carbon arc lamps: Early devices using two carbon rods in free air.
• Mercury vapour lamps: Gas discharge lamps producing ultraviolet radiation.
• Xenon arc lamps: High-intensity lamps generating continuous bright white light.

Specialised designs also exist for laser pumping, microscopy, and industrial processes. The Vortek waterwall plasma arc lamp, developed in the 1970s, is notable as one of the most powerful continuous light sources, capable of output exceeding 300 kW.

Carbon Arc Lamps

The carbon arc lamp was the first widely used electric light. Its operation relies on two carbon electrodes placed in open air. To start the lamp, the rods are touched together, allowing current to flow and establish an arc. When the rods are separated, the arc persists across the gap, and the intense heat causes the carbon to vaporise. Because the electrodes are consumed during use, their spacing requires continuous adjustment. Early mechanisms employed solenoids to regulate the gap by linking current flow with electrode movement.
The Yablochkov candle represented a simplified carbon arc design without mechanical regulators. However, it could not be restarted once extinguished and had a limited operating life.
The spectrum of carbon arc lamps approximates sunlight more closely than many artificial light sources. However, unenclosed arcs also emit substantial infrared and ultraviolet radiation. Enclosing the arc in glass reduces these harmful wavelengths. Carbon arc lamps were eventually replaced in mainstream use by incandescent lamps and later by gas-discharge technologies, though they remain valuable in applications requiring intense ultraviolet light or simulations of solar radiation for materials testing.
Within the carbon arc, the arc itself consists largely of carbon vapour in a plasma state, emitting light mainly in the ultraviolet and violet regions. Most visible and infrared radiation is produced by incandescence from the positive carbon electrode. The high resistance of carbon forces electrons to enter the anode at extremely hot points, creating a bright glowing crater with temperatures between approximately 3300°C and 3600°C. This property allows the emitted radiation to approximate the blackbody spectrum of the Sun when filtered to remove ultraviolet and infrared components.

Historical Development

Humphry Davy first demonstrated the carbon arc light in the early nineteenth century using charcoal electrodes powered by a large battery. The precise year of this demonstration varies among historical accounts, with dates ranging from 1802 to 1809. Observing the arched shape of the flame due to convection currents, Davy referred to the apparatus as an arch lamp, a term later transformed into arc lamp.
By the late nineteenth century, arc lighting was widely adopted for public illumination of streets, squares, and large buildings. However, early systems faced challenges such as flickering and hissing arcs. In the 1890s, Hertha Ayrton analysed these instabilities, demonstrating that they arose from interactions between oxygen and carbon electrodes. Her research contributed significantly to understanding the physics of electric arcs and improving lamp performance.
In the United States, commercial development of arc lamps was initially hindered by the limited availability of consistent electrical power. As stable power supplies became more common, engineers refined arc lighting systems, and they saw widespread use before eventually being displaced by incandescent lights in the early twentieth century. Nevertheless, carbon arc systems continued in specialised applications such as cinematic projection and searchlights until after the Second World War.