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with slight modifications, was used. A few such lamps are even now

in use. In 1906 the carbon filament was replaced by the more economical

tantalum and tungsten filaments, which can be heated to higher

temperatures and yield a whiter light. The early tungsten-filament

lamps were not very successful because the tungsten filament was so

brittle and fragile that it broke easily and because it vaporized so

rapidly that the lamps had very short lives. W. D. Coolidge, of the

Research Laboratory of the General Electric Company, worked for

many years trying to accomplish the "impossible" feat of making

tungsten ductile. At last his indomitable perseverance, backed by

excellent facilities, brought the triumph that made the modern, inexpensive,

economical electric lamp possible. This invention did not

solve all of the lamp manufacturer's problems. The bulbs began to

blacken soon and grew blacker with use.

It was found that the blackening

produced by the vaporization of the tungsten filament and its

subsequent deposition on the glass could be greatly decreased by adding

an inert gas like nitrogen or argon. This caused a decrease in the

efficiency of the lamps due to the heat transmitted by the gas, but this

was more than overcome, in turn, by using concentrated filaments or

coils of filament. On the other hand, higher temperatures were made

possible, and a better, whiter light was produced. The modern gasfilled

tungsten-filament lamp uses about one-fifth to one-sixth as much

power as the carbon-filament lamps for the production of the same

amount of light. Progress is still being made in the production of

lamps with longer lives which produce more light at lower cost. For

example, a simple change in the construction of 60-watt lamps in the

United States alone gave users $12,000,000 worth of additional light

for their money in a single year.

Incandescent lamps of considerable power have been developed so

that 500- to 1000-watt lamps have now largely replaced arc lights in

projection machines. Lamps of even greater power are used for certain


Two recent types of lamps are the photoflood lamp and the photoflash

lamp. The photoflood lamp gives an intense white light, with

some ultraviolet radiations, and it can be used as a source of such

radiations. When photoflood lamps are used in the ordinary lighting

circuits, the filaments are heated to an intense white heat.

In this case

the intensity of the light is more important than the length of life of the

lamp, which is only a few hours. The photoflash lamp contains thin

aluminum foil or wire, which burns with a flash in the oxygen gas in

the bulb when the current is turned on; the current from a small dry

cell is all that is required.

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