Light Yesterday and Tomorrow Page 84

sures inside the bulb. As pressures increase, the lines of the mercury-vapor spectrum tend to broaden and more long-wave or red color is added. Higher amperage, or more watts per unit length of the arc stream can be reached. An illustrative design, rated at about 5 kw, has an arc less than one inch long, and operates at a temperature of about 1000 degrees C and ten atmospheres pressure. An inseparable development has been the search for better refractory materials to hold these pressures at their corresponding high temperatures. So far, pure clear quartz has served well.

Because of their excellent efficiencies and high wattages, high-pressure mercury-vapor lamps have become popular in the larger industries, and — because of modest maintenance costs — for street and highway lighting. Where the “cold” color is undesirable, mercury lamps are used in combination with the more yellow-red tungsten lamps. For example, in the Statue of Liberty, lighting engineers achieve a realistic “flame” in the torch by mixing mercury and incandescent lamps. Modifications of the 3000-watt lamp also achieve magic-like photochemical transformations by means of the combination of ultraviolet and visible radiations emitted. Choice of glass bulb and control of pressure govern the percentage of ultraviolet in the emission. It can be much, or zero.

Fluorescent Lamps

The Columbian Exposition was the backdrop for the public introduction to the wholesale usage of incandescent lighting. At the Century of Progress forty years later, mercury-vapor lamps were introduced, although so modestly that not until

World War II did they come into consequential industrial use. And in keeping with the traditional unveiling of lighting achievements at huge expositions, the New York World’s Fair in 1938 provided the setting for the introduction of fluorescent lamps in the United States. This illuminant truly represents the leading light-source achievement of the 20th century!

Fluorescent lamps are essentially low-pressure mercury-vapor lamps, but with a new feature. Instead of producing visible light directly, the discharge or arc stream of a fluorescent lamp generates invisible ultraviolet radiations of chiefly 2537 Angstroms wavelength; these in turn are transformed into visible light by a thin coating of peculiarly crystallized inorganic compounds inside the bulb or tube, called phosphors. These phosphors provide the variety of tints and the soft, enduring quality of emission needed by any good light source. Constant research has uncovered a wide range of phosphor compounds, each of which converts 2537 Angstroms wavelength and similar radiations to some different wavelength, thus producing different colors. For example, zinc silicates transform to green, calcium tungstates to blue, and cadmium borates to pink. Further research is improving the qualities and extending the colors, sizes, and shapes.

Today there are tubular fluorescent lamps of circular and semi-circular shape, as well as one or two of bulb form. Others range in length from pencil-size tubes to those eight feet long and one and one-half inches in diameter.

Another goal of progress has been to simplify the auxiliary devices essential to the operation of all vapor lamps. One Westinghouse development, the glow-switch starter, sup-