ELECTRIC LIGHT (ante). The machines which are now used for generating the light-producing current are described in the article MAGNETO-ELECTRIC MACHINE. The more recent inventions of electrodes for producing light are those of Jablochkoff, Lodyguine, Kohn, Sawyer, and Edison. The first of these produces the light by the electric arc; the others by the incandescence of some refractory substance, as carbon or platinum.
The principal difference between the Jablochkoff light and the ordinary arrange ment with carbon points is in a provision by which the current is reversed from time to time so that the more rapid consumption of the positive electrode is made to take place with one and the other point alternately. When the apparatus is started, there is also a slight bridge of carbon between the two points through which the cur rent passes before the are is established. The carbon rods are placed parallel and near together, so that a uniform distance may be maintained during their consumption. A pair of carbon points constitutes a "candle," and four candles are usually placed in a globe of opalescent glass. Each candle burns about one hour and a half, so that the set of four will give light about 6 hours, the change of electric action from one candle to another being accomplished by an automatic switch. The motive power required in the Jablochkoff lamp is about one-horse power applied to a magneto-electric machine for each candle, and each such candle is said to have a light value of 700 standard candles; but this, from the absorption of light by the opalescent glass, is reduced to that of 300 candles. In 1873, M. Lodyguine, a Russian as well as Jabloch koff. invented a lamp which gave light by rendering carbon incandescent by the electric current. A portion of the conducting rod of carbon was made much thinner than the rest, so that the increased electrical resistance in that part would cause it to become intensely incandescent. The carbon rod was inclosed in a glass vacuum chamber, but the apparatus was not practically successful, as the carbon wasted t6o rapidly, and required too frequent replacement within the vacuum chamber. In 1875, M. Kohn of St. Petersburg patented an arrangement intended to obviate this difficulty by a device having the same object as that in the Jablochkoff lamp, viz., to supply the place of the consumed carbon with a new piece. This lamp has been used with considerable success. The Sawyer lamp has the following characteristics: It employs the resist ance of a small piece of carbon, placed in an air-tight glass cylinder filled with pure nitrogen,which, being a non-supporter of combustion, protects the carbon in a manner like that of a vacuum, the advantage claimed being that it is easier to keep a vessel full of pure nitrogen than to maintain a vacuum, because of the equality of the inward and outward pressure. The heat produced is prevented from reaching the mechanism at
the base of the apparatus by having the copper standards present a great radiating surface. Diaphragms are also placed so as to cut off much of the downward heat rays, and a switch device is employed to prevent the too sudden turning on of the current, and thereby prevent crumbling by too sudden heating.
The experiments of Mr. Edison on the electric light have been in progress about two years, in which time he has used various substances for the incandescent material. He commenced with platinum, and employed a device by which the galvanic current would be reduced when the metal approached the melting point. This device consisted chiefly in placing within the' fine platinum spiral a rod of the same metal which would be moved, on the principle of the pyrometer, one way or the other by a lever, and thus cool by its presence the incandescent spiral when it became too hot. But this device did not prove successful. Another arrangement employed heated air acting upon a diaphragm as the regulating power. The various metals which he used soon became oxidized and rendered useless. He then gave his attention to perfecting the vacuum employed by Lodyguine in 1873. Edison's platinum lamp as perfected consists of a long coil of platinum coated with calcined magnesia, supported by a platinum rod within a glass vacuum tube, which rests upon a metal frame containing the regulating apparatus. It is claimed that Edison has produced a vacuum more perfect than any other, so that only one millionth of an atmosphere remains. His attention was called from the use of platinum to that of small threads of carbon, made by charring cotton thread in a vacuum with the electric current. Light of great intensity was obtained in this way. He experimented with various forms of woody fiber, but finally found that nothing gave more satisfactory results than charred paper. Bristol card-board cut in the shape of a small horse-shoe, the strips being about 2 in. long, and an eight of an inch wide, and laid upon one another in an iron mold, being separated by tissue-paper. When the mold is packed, it is placed in an oven and gradually heated to afterwards,_in a furnace, to.a white heat., The carbonized product is then care fully removed and placed in a small glass globe, and made the resisting portion of the galvanic circuit; the globe is then exhausted and sealed air-tight.