Incandescent lamp

"I'll leave the light burning in my room", you say. And with these words you close the door behind you, going out into the street, say, to spend an evening with your friends.

Yes, you have left the light burning. But what do you really mean? Not a fire left burning in the stove, not a candle or kerosene lamp smoking at the moment when you were closing your door, nor a gas flame. No, it is just the habit of centuries reappearing in that one word "burning". Strictly speaking, the incandescent lamp you leave behind to light your room, is not "burning" at all. The only fire that does actually burn is kilometres away, at a power-station, where the electric current that feeds the lamp is being generated.

When the conductor of an electric current is so hot that it radiates light rays, it is said to be incandescent. Conductors used for making lamp filaments, when heated, soon become so hot that they radiate a white light. In other words, the light is radiated by a metal filament heated to incandescence by the electric current. The percentage of luminous radiation from a hot body is low but it rapidly increases with the increase in temperature of the radiating body.

The filament must be placed in an air-tight place for it will burn out unless oxygen is removed.

Incandescent lamps of older types had carbon filaments prepared in various ways. To make a suitable lamp filament was an extremely difficult thing. Lodygin, for instance, had to experiment several years before he was able to make the first incandescent lamp burn successfully. By the way, he was the first to use tungsten for filament. Some idea of the work this problem involved may be gathered from the fact that Edison had tried 6,000 different materials before he hit upon the carbonized filament.

As the efficiency of an incandescent lamp greatly depends upon the temperature its filament can be heated to, tungsten is now mostly used for lamp filaments. Its melting point reaches about 3,300° C. The energy to be expended per candle power is less than half that used by carbon lamps of the same type.

Generally speaking, tungsten and tantalum are the very metals that are used for lamp filament production, at present. The resistance of these metals is considerably less than that of carbon. The light radiated is whiter and it requires less current than with the carbon filaments.

The modern tungsten lamp filament is a coil whose ends are fastened to the ends of supports connected with leading wires.

The incandescent lamp efficiency is greatly increased provided it is used at a voltage above that for which it was manufactured but under such conditions the life of the lamp is shortened, of course.

Two great inventors played a prominent part in the development of the incandescent lamp. Their names are Lodygin and Edison. Lodygin was not only the inventor of the lamp under consideration, but also the first scientist to use it for lighting purposes. When Lodygin's lamp appeared first and replaced the arc lamp, it attracted Edison's attention. For several years the American inventor worked hard at its improvement. At last, in 1879, he created an improved incandescent lamp suited for practical purposes and finally solved the problem of cheap electric lighting on a large scale.