The two grand essentials now being at command, invention of electric light appliances went on rapidly upon two lines, eventuating in two systems, which became known as arc lighting and incandescent lighting. By 1879–80, the arc light was sufficiently advanced to meet with favor as an illuminant for streets, railway stations, markets, and any large spaces, in which places it became a substitute for gas and other lights. The essential features of the arc light are: (1.) The dynamo machine, situated in some central place, for the generation of electricity. (2.) Conducting wires to carry the electricity throughout the areas or to the places to be lighted. (3.) The arc lamp, which may be suspended upon poles in the streets, or upon wires in stores and other covered places. Its mechanism consists of two pencils or candles of graphite carbon, very hard and incombustible, adjusted above and below each other so that their tips or ends are very close together, but not in contact. By means of a clockwork or simple gravity device these carbon tips are brought into contact at the moment the electric current is turned on, and then are slightly separated as soon as the current has heated them. The air between the heated tips, having also reached a high temperature, becomes a conductor, and the electricity leaps in the form of an arc or curve through it, rendering it brilliantly incandescent. Should the current be diminished in strength for any reason, the above-mentioned clockwork or gravity device brings the carbons a little closer together; and should the current be increased, the carbons are separated a little wider; thus the steadiness of the light is regulated. There are also various automatic devices for thus regulating the proximity of the carbons and maintaining the evenness of the glow. The power of an arc light is measured by candles. An ordinary arc light under two amperes of current gives a light equal to twenty-five candles, while under fifty amperes of current it gives a light equal to twenty thousand candles. In searchlights on board vessels, and where very large areas are to be lighted, both heavier currents and larger carbons are used than in the arc lamps for ordinary street purposes. No light surpasses the arc light in brilliancy, excepting the magnesium light. There are few cities in this country and Europe that do not employ the arc lamp as a means of street, station, and large-area lighting, owing to its superiority as an illuminant and the wonderful policing effect it has upon the slum sections.