Carbon Arc Lamp
Lamps consisting of two rods of carbon in open air or in a glass enclosure. The ends of the rods touch each other and are connected to a current source. By subsequently separating the rods, a discharge arc is produced that brings the ends of the rods to bright incandescence.
Carbon Arc Electric Lamps
In contrast to what many people think, it is not the incandescent lamp, but the carbon arc lamp that was the first electric light source used. Already in 1810 Humphry Davy demonstrated in the Royal Institution in London a bright arc between two pieces of charcoal connected to 2,000 voltaic cells [1, 2, 3]. Electric carbon arc lighting really took off after the introduction of steam-driven generators around 1850, some 30 years before the introduction of the incandescent lamp. The earliest practical application of electric light was an arc lamp used to simulate the sun in the opera of Paris in 1849 . Arc lamps with their concentrated light of high intensity were, in their infancy, especially used for beacon and search lights. From 1870 onwards arc lamps became popular for the lighting of streets, factory halls, railway stations, and big department stores. Huge structures, sometimes called moonlight towers, were used in cities to illuminate large areas instead of using many small masts with gas lanterns. The arc lamp had such a high intensity that it was seldom used in domestic lighting.
From the beginning of the twentieth century, incandescent electric lighting quickly replaced carbon arc lighting installations. Up to the 1950s, extremely high-intensity arc lamps were still used in search lights, in film studios, and in cinema projectors, until the short-arc gas discharge xenon lamp took over. Today, electrical arcs are used not for lighting but for industrial purposes, as, for example, in plasma torches and welding apparatus where an arc is created between the one welding rod and the metal material to be welded.
The gap between the rods is just a few millimeters, and the light-emitting area therefore is so small that concentrated light of high intensity is created. The carbon rods burn away with time; in a DC supply, the positive rod burns more quickly than the negative rod because it becomes hotter. The distance between the rods has to be adapted regularly as the arc will extinguish if the distance becomes too large. Many different mechanisms have been invented to perform this automatically. After some time the rods become so short that they have to be replaced.
An arc lamp has a negative-resistance characteristic (like all gas discharge lamps) and needs therefore a resistor, usually an inductive coil, in its electric circuit to limit the current.
Materials and Construction
Common Carbon Rods
Charcoal was originally used for the electrodes, but charcoal burns away rapidly. It was soon discovered that rods made out of carbon have a much longer life. Hard molded carbon rods were therefore used which later got a core of soft carbon. DC-operated lamps used for the anode a thicker carbon rod than for the cathode to make them burning away with the same rate. In AC-operated lamps the burning rate of some 20 mm per hour of anode and cathode is, of course, the same [1, 3]. The rods have a diameter of 10 to slightly more than 15 mm and were made as long as possible, up to some 500 mm, giving a lifetime of up to 24 h.
Jablochkoff’s Parallel Electrodes
Carbon Rods with Additives (Flame Arc Lamps)
Just before 1900, fluorides of certain metals (including rare earth metals) were added to the carbon rods. When the electrodes become hot, the metallic salts evaporate and take part in the arc discharge, enveloping the arc as a flame, hence the name of flame arc. Both the lumen output and the luminous efficacy increase considerably with a factor between 2 and 4. These types are therefore also referred to as “high-intensity arc lamps.” Rare earth additives emit a line spectrum resulting in bright white light. Other types of additives emit different colors of light as, for example, calcium, emitting an explicitly yellow light, and strontium, red light. In this way light sources emitting specific spectra suitable for chemical and photographic processes were produced .
As has been mentioned, the distance between the rods has to be adapted regularly as the arc will extinguish if the distance becomes too large in the process of burning off material from the rods. Simple hand-regulated devices were designed where, by turning one screw, both electrodes were adjusted so that the light-emitting area remained at the same location. These systems have long been used in arc lamps for cinema projection.
Open carbon arc lamps have a light output of up to some 4,000 lm (500 W versions) at a luminous efficacy of some 4–8 lm/W. Enclosed lamps have a 10–15 % lower output and efficacy. Flame arc lamps, using carbon rods with additives, have a light output up to 15,000 lm (500 W versions) at efficacies between 15 and 30 lm/W . Some arc lamps designed for use in search lights have wattages of more than 20 kW. Beam intensities of up to 5,000 million candela have been reported with mirror diameters of more than 2 m.
Arc lamps are often not rated by power but by the current they draw. Lamps with currents from 5 to 1,000 amp have been produced.
The correlated color temperature of some 3800 K of arc lamps  is much higher than what one was accustomed to with oil, candle, and gas lighting. The high-intensity flame arc lamps have relatively high color temperatures, depending on the material, up to 5,000 K. The spectrum of flame arc lamps extends well into the ultraviolet part (UV-A, B, and C), so that care is required with open arc lamps. Special arc lamp devices for tanning purposes have in fact been produced.
Arc lamps produce a buzzing sound which in interiors was experienced as annoying.
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