Hewitt Invents the Mercury-Vapor Lamp

Date 1901

Peter Cooper Hewitt developed a lamp based on gaseous discharge, leading to significant developments such as the fluorescent light.

Locale Ringwood Manor, New Jersey

Key Figures

• Peter Cooper Hewitt (1861-1921), American electrical engineer
• Heinrich Geissler (1814-1879), German instrument maker
• Julius Plücker (1801-1868), German physicist

Summary of Event

For all of recorded history, humankind has possessed the desire to bring light into enclosed spaces and to banish darkness with sources of artificial light. Artificial light was first provided in the form of controlled flame, beginning with simple torches and wood fires. Devices such as oil lamps and candles were more effective, and indeed these are often still used today. In the nineteenth century, dramatic advances were made in the field of artificial lighting. The first of these was the development of gas lighting. Flammable gases for use as fuel could be derived from a multitude of sources. By the 1850’s, the streets of several urban areas were illuminated by gaslight.

In modern times, light generated in some fashion by electricity has all but supplanted other sources for everyday use. People have long been familiar with electricity in the form of lightning and static electricity, and such notables as Benjamin Franklin performed experiments to discover the nature of electricity. Luigi Galvani, an Italian physicist, made a critical observation in 1780 when he noticed that an electrical spark caused a frog leg to twitch. Alessandro Volta studied this phenomenon, and his work resulted in a man-made battery, or voltaic cell, for the production of an electrical current in 1794. The battery simplified the construction of a reliable source of electrical current and prompted an outpouring of research involving electricity.

The first commercially successful electric light was the electric arc lamp, in which an electrical discharge between the slightly separated tips of two electrodes produces a very strong light. Arc lights were not practical for indoor use, but they were very effective for outdoor applications, particularly street lighting. The light produced was of good quality; however, maintaining the network required a fairly elaborate and labor-intensive support system. In particular, arc lights gave a very favorable appearance to the exteriors of buildings, which led the governing bodies of some cities to dictate their continued use even after suitable alternative systems were available. Of course, construction of these lighting systems was possible only after the development of machinery to provide reliable electric current.

Thomas Alva Edison envisioned a complete system of electrical supply—municipal and residential lighting—and electrical power for other applications. Although Edison is recognized, along with Joseph Wilson Swan, as the developer of the incandescent lightbulb, it is important to realize that he saw this invention as part of a larger picture. It was his determination that fostered public awareness of and desire for electric lighting, leading to the mammoth electrical generating facilities in existence today. Edison’s electric light relied on the passage of a current through a filament, which became heated to incandescence. For most domestic uses, the incandescent bulb is still the lighting of choice today, even though it is relatively inefficient. Much of the energy provided to a lightbulb is lost as heat.

While Edison struggled with the electric light and development of power transmission, other researchers were exploring areas of physics and chemistry that would lead to the development of another type of lighting device. As early as 1675, the French astronomer Jean Picard had observed a discharge of light from a mercury barometer. Subsequently, it was understood that this kind of discharge was stimulated by static electricity, although the cause of the flash of greenish-blue light was unknown. The mercury barometer, invented by Evangelista Torricelli, was simply a long glass tube sealed at one end, filled with mercury, and then placed open end down in a vat of mercury. When such an instrument is prepared properly, the mercury in the tube drops down to a height that is supported by air pressure. Above the mercury in the sealed end of the tube, a good vacuum is created that contains only a slight amount of mercury vapor. This type of vacuum is isolated and thus not easily studied.

In 1855, the German inventor Heinrich Geissler developed a simple pump that used a moving column of mercury to generate a good vacuum in a tube. This development came at an opportune time for Geissler, as many physicists were investigating the possibility of forcing electricity through a vacuum. Julius Plücker, a German physicist and contemporary of Geissler, performed a key experiment in this field. Plücker succeeded in passing current through one of these tubes, and in 1858 reported what came to be known as cathode rays. Other physicists studied these rays, including Sir William Crookes, who proposed that the charge flowed through the tube because of movement of some sort of charged particle. Joseph John Thomson conclusively proved this postulate in the late 1890’s and thus is credited with proving the existence of the electron.

It was therefore known by the beginning of the twentieth century that in a near vacuum, current in the form of electrons can flow between two electrodes and that electricity can cause a discharge of light in a near vacuum, as Picard had observed. Armed with this knowledge, Peter Cooper Hewitt tried to devise a way to establish a discharge in a glass tube containing mercury vapor at very low pressure. Plücker had shown that when an element is stimulated, it emits only certain characteristic wavelengths of light, or what is commonly known as a line spectrum. This is in contrast to the light of an incandescent bulb, which is a continuous spectrum. Hewitt hoped to use the electrical discharge through the mercury vapor to generate light.

In principle, the electrons traveling through the tube would collide with mercury atoms. This collision would contribute energy to the mercury atoms, causing an electron in each atom to become energized and move to a high-energy state. When the electron returned to its original energy level, light would be emitted with an energy characteristic of the transition. A benefit of this type of light would be that only visible light would be emitted, so the light source would generate very little heat. The tube that Hewitt marketed commercially was 2.5 centimeters (almost 1 inch) in diameter and about 127 centimeters (49.5 inches) in length and was very similar in appearance to modern fluorescent lights. In Hewitt’s original lamp, the glass tube was hung at a slight angle and contained a small amount of liquid mercury. To activate the light, the user tilted the tube so that a stream of mercury stretched from one electrode to the other. The current flow vaporized the mercury, and the mercury vapor was then stimulated by the current to produce light. Hewitt’s first lamp was a direct-current lamp. One of the major obstacles to a discharge lamp was that a large potential was needed to start the discharge, although a much lower voltage sufficed to maintain the discharge once established. This was why Hewitt developed the starting system. Changes that Hewitt incorporated into subsequent versions of his lamp included the addition of an electromagnet to tilt the lamp and an induction coil that provided high voltage to start the lamp.

One of the major drawbacks of the light generated was that it produced distortions in the appearance of objects because of the lack of red light in the mercury spectrum. The quality of light was quite useful in photography, however. Further developments would make the light compatible with alternating current and in subsequent years other inventors developed commercially successful lights based on the principle of electrical discharge through a vapor.

Significance

Hewitt’s invention of the mercury-vapor lamp represented an attempt to use a novel means of obtaining light. Initially, practical applications of the lamp were limited, primarily because of the quality of the light produced. Hewitt developed the mercury rectifier to convert alternating current to direct current and refined the lamp to improve its color characteristics. Many factories used this improved model as a source of illumination. One property of the mercury lamp is that in addition to certain wavelengths of visible light, it emits large quantities of ultraviolet light. Ultraviolet light does not pass through ordinary glass, but quartz is transparent to it. By producing a light with a quartz tube, Hewitt marketed a source of ultraviolet light that was found to be of great utility in biological applications. A related modern use of similar quartz tube lamps is in tanning beds.

Placing the electrodes closer together and increasing the pressure of vapor result in the generation of more intense light. This type of lamp commonly utilizes either mercury or sodium. The sodium vapor lamp, developed in the early 1930’s, produces a distinctly yellow light. This type of lamp was widely used as an external source of illumination, especially in streetlights, all but supplanting mercury lamps until refined models became available. Modern city lighting often includes mercury lamps in the vicinity of downtown areas, where the light produced seems to enhance the appearance of buildings, and sodium lights in rural areas and at intersections, where the main concern is visibility of the road and motor vehicles.

A related and equally familiar development in the field of gas discharge lighting was introduced in 1910 by Georges Claude, a French chemist who placed various types of gases in tubes. He found that certain elements produced distinct colors. In particular, neon produced a very attractive discharge. By altering the mixture of gases, one could produce a wide variety of colors. The application of these lights, commonly called neon lights, introduced a minor revolution in advertising that is still in evidence in some cities, such as Las Vegas.

In the 1930’s, another major development in the low-pressure mercury-vapor lamp took advantage of the fact that mercury emits ultraviolet light when stimulated. Materials known as phosphors undergo fluorescence when they absorb ultraviolet light. In the process of fluorescence, the ultraviolet light energy absorbed by a phosphor is subsequently emitted in the form of visible light. This discovery prompted development of the fluorescent light, in which the interior of a gas discharge tube, very similar to that developed by Hewitt, is coated with a phosphor. The result is a uniform emission of light from the phosphor when the light is turned on. Manipulation of the chemical nature of the phosphor allows for variation in the light produced. Since their initial development, fluorescent lights have become widely used for illumination in office and factory environments, while their domestic use has been somewhat restricted to utility areas in the home. As further refinements to mercury-discharge lamps are introduced, such lamps may become the predominant source of external and internal artificial lighting.

Bibliography

Bowers, Brian. Lengthening the Day: A History of Lighting Technology. New York: Oxford University Press, 1998. Presents a concise history of lighting technologies, including fluorescent lighting. Discusses both technical aspects of different lighting technologies and the societal effects of advances in such technologies. Well illustrated, with exploded diagrams and reproductions of etchings.

Burke, James. Connections. London: Macmillan, 1978. Excellent volume describes how different innovations are often related to one another through subsequent development. Of interest in the context of lighting is an excellent discussion of gas lighting and the production of fuel for gas lighting.

Coaton, J. R., and A. M. Marsden, eds. Lamps and Lighting. 4th ed. London: Edward Arnold, 1997. Comprehensive textbook includes chapters on low-pressure mercury lamps, fluorescent lamps, sodium lamps, and the like. A wealth of theoretical material makes this a challenging work for the layperson, but most chapters are well written. Includes diagrams of the designs of various types of lamps, specifications, and construction techniques.

Hall, Stephen S. “The Age of Electricity.” In Inventors and Discoverers, edited by Elizabeth L. Newhouse. Washington, D.C.: National Geographic Society, 1988. Presents a history of the development of electricity, including discussion of the monumental battle concerning alternating versus direct current. Also offers a good biography of Edison. Well illustrated, with both photographs and artwork.

Kane, Raymond, and Heinz Sell, eds. Revolution in Lamps: A Chronicle of Fifty Years of Progress. 2d ed. New York: Fairmont Press, 2001. Written for designers, engineers, and architects as well as lay readers, this book provides a history of the progress made in lamps and lighting as well as information on new lighting technologies. Includes a chapter on lamp phosphors and one on fluorescent lamps.

Schroeder, Henry. History of Electric Light. Washington, D.C.: Smithsonian Institution, 1923. Written only a few years after Hewitt’s discovery, this source includes an excellent drawing of Hewitt’s lamp. Also provides an excellent history of the development of other types of lights in use at the beginning of the twentieth century. Although obviously dated, contains a wealth of information that cannot be found in more recent texts.