ELECTRIC LIGHT. All known methods of generating electricity can produce light of greater or less steadiness and brilliancy. Cavallo, in his treatise published a century ago, refers to a light, different from the electric spark, produced by frictional electricity. A needle or wire presented to an insulated person, at the distance of about 1 in. from his body, while he is actually rubbing the tube, will, he says, exhibit a lucid pencil of rays, seemingly issuing from the point and diverging towards the person; and other like experiments are described by Watson and other early writers. Light from battery electricity was first discovered by sir Ilumphry Davy at the royal institution, London, in 1810, when, on the continuity of a current from 2,000 cells being broken, a brilliant light was seen. To this the name of the " voltaic arc " was given, and the points where the current was broken were termed " electrodes." An early and simple arrangement for producing this light consisted of two carbon-points fixed into hollow brass reds, which are connected with the battery by wires entering at binding screws. The rods slide in the heads of glass pillars. The wires from the battery being connected, the points are made to touch, and are then withdrawn a line or two, when a dazzling light appears, approaching the light of the sun in purity and splendor. Its intensity prevents the naked eye from examining its form, but this may be ascertained by projecting the images of the points on a screen, when it is no longer painful to the eyes. The light is partly due to the incandescence of the tips of the carbon, and partly to an arch of incandescent particles extending from the one to the other. The positive pole is brightest and hottest, as may be proved by intercepting the current, when the positive pole continues red for some time after the negative pole has become dark. During the maintenance of the light, a visible change takes place in the condition of the poles, and the positive pole becomes blunt by the loss of particles of carbon. The wasting of the poles renders the distance between them too wide to allow of the passage of the current, and the light is thus suddenly extinguished, until again renewed by contact and removal. The heat of the voltaic arc is very intense. Quartz, the sapphire, magnesia,
lime, and other substances equally refractory, are forced by it into a state of fusion. The diamond when placed in it becomes white hot, swells up, fuses, and is reduced to a black mass resembling coke. The electric light can be produced in a vacuum, and below the surface of water, oils, and other non-conducting liquids, and is thus quite independent of the action of the air.
In 1820, Oersted proved the identity of electricity and magnetism; but it remained for Faraday, in 1831, by his great discovery of induced currents, to render practicable the application of electricity to the production of good artificial light. It was not, however, until 1853, that the magneto-electric machine was actually applied to the purpose ;. and, in 1857, the first great practical trial took place, when Faraday had the satis faction of seeing his conception carried into effect. This trial of Holmes' machine resulted in the E. L. being introduced into the South Foreland light-house on 8th Dec., 1858, and later the light was adopted at Dungeness. The French gov ernment adopted the light for two light-houses near Havre, in 1863.
The problems to be solved in the production of E. L., are to supply a constant and equal current (which battery electricity does not yield), and to provide a form of elec trode which will not cause the light to blink or go out by wasting away. The first gen erating machines were "magneto-electric," revolving coils in front of permanent steel magnets (or contrariwise, revolving magnets in front of coils), but some later machines are "dynamo-electric," based on a discovery simultaneously made by Werner Siemens, Varley, and Wheatstone, that by revolving coils in front of soft iron eleetro-magnets, the residual magnetism in the iron would gradually be augmented, dynamic force being thus converted into electricity. The currents created by machines of either sort are alternate, but where, as in the case of some forms of lamp, the current must proceed in one direction, the alternate currents are made continuous by the use of a commutator. There are a large number of machines in use for generating currents for producing the E. L.; and for details reference may be made to Shoolbred's Electric Lighting (1879).