Light Yesterday and Tomorrow Page 83

Such are some of the highlights of lamp-making progress. The evolution of lamp-making machines gains stature when it is considered that each of today’s machines is capable of producing 8000 to 9000 lamps a day compared with the 300 turned out by the 1910 equipment.

Later Incandescent Lamp Developments

While machine designers were developing automatic lamp-making equipment, lamp engineers were not idle. Filament arrangement, a problem from the beginning, began to take new forms. From the unimaginative double loop of the carbon-filament lamp, the filament developed to a draped uncoiled tungsten wire in 1917, to the single-coil filament (a helix) in 1927, and finally the coiled-coil or double helix design in 1936. This compacting of the filament metal offers the best method of conserving heat and thus of getting the most light per watt from a gas-filled incandescent lamp.

Many different filaments — some of them unusual in formation — have also been developed for special services. High-wattage projection lamps, for example, have a “floating” filament, necessitated by extreme temperature changes and corresponding expansion and contraction.

The Westinghouse biplane-type filament for motion picture and stereopticon projection lamps, developed in 1932, made it possible to double the wattage of the light source without increasing the effective filament area. The net result was a 75 percent increase in the brightness of the projected picture.

A leader among special-purpose incandescent lamps is the “sealed-beam” automobile headlight. An all-glass unit with sealed-in reflector, it requires a precisely positioned filament for accurate focusing. Around this design has grown up a host of sealed reflector bulbs — for fog and spotlights, bicycles, locomotive and aircraft headlights, and the spotlighting of merchandise in stores.

In a special lamp for deep-sea diving, engineers took advantage of the cooling effect of water and put a super-quantity of wattage within a bulb sufficiently small for convenience in handling under water. These tough, spherical glass bulbs, capable of withstanding crushing pressures in excess of 300 pounds per square inch, simplified salvage operations on several submarines and other sunken vessels; they have aided in deep-sea photography and fish capture, and assisted the emergency repairs of many ships.

Such has been the progress of the incandescent lamp in its short but illustrious development period since the turn of the century. But though it covers a broad range, from glow lamps to searchlights, from “grain-of-wheat” lamps to those rated at 50 000 watts, this great mosaic of brilliant successes is by no means the whole picture.

Vapor Lamps

The production of visible light by the incandescence of tungsten filaments likely will continue to be the chief basis of our lighting for many years because of its standardized simplicity and flexibility; nevertheless, the production of light by an electric discharge through a gas or a metallic vapor actually is more efficient intrinsically. For this reason, scientists down the years have pursued vigorously the studies of metallic vapors, fluorescent phenomena and electron performance, and have sought all the other avenues of vapor-lamp research.

Strictly speaking, the carbon-arc lamps that lighted Paris streets in 1878 were vapor lamps operating on the electric discharge principle. Energy in the stream of free electrons was transformed to light, and luminous vapor was a source of radiation. At the dawn of the new century, Mr. Westinghouse

was solidly engrossed in this new method of generating light, although undoubtedly his engineers fell short in their imaginative evaluations of vapor illuminants because electronics was then an unexplored wilderness.

Nevertheless, he agreed in 1900 to finance and exploit the new low-pressure, liquid-pool, mercury-vapor lamp, which had been developed two years before by Dr. Peter Cooper-Hewitt. Mercury-vapor lamps did not reach wide prominence until many years later when higher pressures and efficiencies were attained, but Cooper-Hewitt lamps were familiar industrial light sources until, shortly after World War I, large wattage gas-filled incandescent lamps proved simpler and more economical. Because of their richness in actinic light, mercury-vapor lamps continued to be used in early portrait photographers’ studios and in blueprinting.

In 1918 Dr. Arthur E. Compton, working in the Westinghouse Research Laboratories, decided that sodium and neon were the most likely materials at that time for gas-discharge lamps, because of their rich emission lines in the visible spectrum. Other noted pioneers — Dr. McFarland Moore working with carbon-dioxide gas and Dr. Claude with his neon tubes — were extending the art. Finding that sodium was capable of producing light with at least double the efficiency of good filament lamps, Compton made a special glass to withstand the corrosive effect of heated sodium vapor. This was exciting progress and Dr. Compton’s work materially aided the later development of the lemon-yellow sodium lamps, still sometimes used in highway lighting.

Recognizing that the efficiency ceiling of the tungsten-filament lamp was well below 50 lumens per watt, and that some vapor illuminants could exceed this, lamp designers and scientists pressed further in their studies of vapor lamps. They found that a high-pressure, high-efficiency version of the old Cooper-Hewitt lamp produced a more balanced color of light than the yellow sodium lamp.

A 400-watt mercury-vapor lamp of this new design (type AH-1) was introduced at the Century of Progress Exposition in Chicago in 1933. Since that time higher wattage lamps, up to 3000 watts, have been developed, with lives of approximately 4000 hours and excellent efficiencies. The 1000-watt size, for example, produces 60 lumens per watt, and a water-cooled super-high-pressure type may do slightly better.

Research and engineering on mercury-vapor lamps have been aimed primarily at improving color and efficiency, partially accomplished by using higher and higher vapor pres-