Electric Lighting
illumination by means of electricity. Electric lights are of two sorts, the arc and the incandescent. As early as 1809 Sir Humphry Davy exhibited an arc light before the Royal Institute of London. He demonstrated that if two pieces of charcoal in a strong electric circuit be separated slightly, a brilliant glow is produced. In the modern application of the principle for street lighting, the pieces of charcoal are replaced by sticks of carbon. These are about ten inches in length, and vary in diameter from one-half to five-eighths of an inch. They are made of petroleum coke or ordinary gas coke, which is first ground thoroughly and mixed with a sort of paste. An automatic device is relied upon to hold the points of the carbon at the right distance apart and to bring them together at regular intervals. As these carbons waste away they are replaced by fresh ones.
The incandescent lamp was not brought to perfection until about 1879, when Thomas A. Edison devised the lamp now used in incandescent lighting. It consists essentially of a small carbon filament, prepared from a bamboo fiber, looped in a vacuum within a glass bulb. The filament is so slender that it offers great resistance to the passage of the electric current and converts the electricity into light. The vacuum is designed merely to prevent combustion. Otherwise the carbon of the filament and the oxygen of the air would unite; that is to say, the filament would burn out instantly. The strength of an incandescent lamp is expressed usually by comparing it with the light given by a standard candle. A very ordinary lamp gives sixteen times as much light as the standard candle. Inasmuch as there is no consumption, the incandescent lamp neither consumes nor pollutes the air. It requires no cleaning, produces very little heat and is more quickly available than any other kind of lighting known. The light is also steadier than that given by a candle, by gas, or by an oil lamp, and is on that account to be preferred by the reader.
Although ideal in the respects mentioned above, it must still be admitted that electric lighting is a wasteful method. Less than two per cent of the energy of coal consumed in the lighting plant reaches the consumer in the form of light. As stated, the earliest filament was a bamboo fiber. Other materials, particularly filaments of tungsten, are now in favor.
Although introduced as late as 1879, no one knows how many incandescent electric lamps there are in the world. One New York company lights at one time 1,400,000 sixteen-candle incandescent lamps. In Brooklyn there are no less than 800,000 lamps. At an anniversary dinner held by the American Institute of Electrical Engineers at the Hotel Astor in 1909, it was asserted that there were in the United States 5,111 electric light stations, supplying 555,921 arc lamps and 41,807,944 incandescent lamps. American factories turned out 100,000 lamps a day in 1910.
See Light; Electricity; Candle; Edison
Electro-magnet, a temporary magnet made by a current of electricity. If a wire carrying an electric current be wound about a rod of soft iron the core becomes a magnet. If the current be cut off, or if the iron be removed, the latter ceases to be a magnet. In physics, the wire so wrapped is called a helix, the iron rod is called the core, and, so long as the core is magnetized, it is an electro-magnet. Any body attracted by a magnet is called its load. Wrap the wire of an electric lamp around a soft nail, turn on the current, and the nail becomes a magnet. Turn off the current, and the nail loses its magnetism. By turning the current off and on, the electro-magnet may be made and unmade at will. When a telegraph operator works the lever of his key, he is making and unmaking the electro-magnet of the receiving instrument miles and even thousands of miles away. The rapid clicking indicates that the magnet is seizing and releasing its load.
A huge magnet, possibly shaped like a cheese, and possibly larger than a wagon wheel, suspended from the arm of a crane and controlled by a powerful electric current, is capable of doing marvels. Lowered over a pile of scrap iron, such a magnet picks up metal large and small,--an iron filing or a ton casting, it is all the same to the magnet. The crane swings, the load is dropped, and the magnet swings back for another load. Foundries find that in this way they can handle scrap iron at a fraction of the former cost. Pig iron, too,--bars under which a man staggered,--are handled like matches. A fifty-ton car of pig iron can be unloaded in half an hour.
By arranging an overhead track for the hangings to travel on, the lifting magnet is used to carry kegs of nails. The contrivance trundles to and fro, lifts a dozen kegs of nails, carries them to their destination, and drops them in place. All the operator has to do is push a lever when the load is in place to be dropped. The lift magnet is a cheerful worker. It will drop a huge bar of iron into the hottest flame, or pull it out again without a word of complaint. It is never tired, never burns its fingers. A fifteen ton burglar-proof steel safe is a hard article to get hold of,--a hard article to handle. It is impossible to get men enough around it even to budge it, but the magnet lifts it and carries it away without an apparent effort. Huge steel ship plates forty feet long are carried to the frame and are held in place until they are riveted safely. The uses of the lift-magnet are so manifold, and its use does away with so much grimy, exhausting labor, that large machine shops, foundries, shipyards, and wholesale hardware stores are not considered up to date without an equipment of electrical lift-magnets.
The electro-magnet is used for small articles as well. Bits of metal too small for the fingers may be held against a polishing wheel. In the manufacture of small metal articles like screws and pins, where it is desirable to secure speed and automatic accuracy without employing hand labor, there are a thousand applications of the electro-magnet.