Arc lamp

An arc lamp is an electric light that produces light by an arc of electrical current passing through ionized gas between two electrodes, oftentimes made of tungsten. The gas within the bulb often consists of argon, metal halide, mercury, neon, sodium, or xenon. The fluorescent light is a commonly used arc lamp that is a type of mercury arc lamp whose bulb has been coated with phosphor.

Long before the incandescent electric light bulb was invented, arc lamps had given birth to the science of electric lighting. In the early 1800s, when the first large batteries were being built, researchers noticed that electric current would leap across a gap in a circuit, from one electrode to the other, creating a brilliant light. English chemist Sir Humphry Davy (1778–1829) is credited with discovering this electric arc and inventing the first arc lamp, which used carbon

electrodes. Yet the electric arc lamp remained a curiosity for decades. Many scientists gave public demonstrations of arc lighting, and the invention of automatic controls in the 1840s made it possible for arc lamps to be used in special applications such as lighthouses, theaters, and microscopes. But arc lamps still relied on expensive batteries or generators as their source of power.

Then a flurry of inventions brought arc lighting into widespread use. First came the development in 1871 of a relatively cheap source of electricity, the dynamo, a type of generator that produces direct current power. Public interest quickly reawakened, and people began installing arc lighting in factories, mills, and railway stations; actually, any place light was required over a large, open space. France pioneered this field, though Great Britain and the United States soon followed. The next step forward was the electric candle, a type of arc lamp invented in 1876 by Pavel Jablochkoff (1847–1894), a Russian engineer who later moved to Paris, France. This device, which could burn for two hours without adjustment, eliminated the need for expensive automatic controls. Although defects soon led to its downfall, this arc lamp greatly stimulated the development of electric lighting and increased the demand for better generating equipment.

By this time, American scientists were active in improving and installing arc lighting systems. In 1877, a dynamo invented earlier by William Wallace (1825–1904) and American inventor and electrician Moses Farmer (1820–1893) was adapted for arc lighting by Wallace. This was probably the first commercial arc lamp made in the United States. Around the same time, American Charles Brush’s (1849–1929) arc lamp, which used magnets to move the electrodes, could be lit by remote control. He also invented a way to operate multiple arc lamps from a single dynamo, which greatly improved upon the European method. In 1879, Brush demonstrated his first streetlight system in Cleveland, Ohio, a success that led many other American and European cities to install Brush arc lighting. Finally, a team of two American electrical engineers, Edwin Houston (1847–1937) and Elihu Thomson (1853–1937), introduced an arc lighting system that wasted less electricity by maintaining a constant current. Two years later, in 1881, they patented automatic controls for the system.

At the beginning of the twentieth century, after other improvements to arc lamps, spin-off technologies began to spring from the original concept. Scientists knew that electricity, when passed through certain gases at very low pressures, would discharge light, producing a glow instead of an arc. Although high voltage was needed to start the process, a much lower voltage would sustain it. American engineer Peter Hewitt (1861–1921) invented a starting device and developed the first discharge light, which used mercury vapor in a glass tube. Soon, higher pressure lamps began to be developed, using either mercury or sodium vapor.

In contrast to arc lamps and incandescent light bulbs, discharge lamps deliver nearly all their energy in the form of visible light or ultraviolet rays, rather than producing large amounts of useless heat. The color of the light varies depending on the gas. Mercury gives a bluish light, which can be corrected to look more natural by coating the tube with phosphors, while sodium vapor light is distinctly yellow. Both types provide excellent illumination for large areas such as roadways, shopping malls, parking lots, and exhibit halls. Mercury lamps are used where the quality of light is an aesthetic concern in a city’s downtown area, for example, while sodium lights work well where visibility is more important than appearance. Metal halide lamps, a recent development, produce a spectrum that is ideal for use when color television reception is needed, so they are often used in sport stadiums and athletic fields. Fluorescent lamps and neon lights are also variations of discharge lamps.

Meanwhile, the original arc lamp has come full circle. Ironically, today’s extremely powerful versions make use of the lamp’s heat, rather than its light. These high-tech arc lamps, which can simulate the heat of the sun, have proved useful in testing aerospace materials and hardening metal surfaces.