LIGHTING

—oOne of the earliest _ methods of obtaining artificial light was - that of burning oil. The oils in use up to the mid.-XIX. cent. were principally - of animal or vegetable origin, mineral oils being brought into use for lighting purposes about 1853. Early lamps, which consisted of a shallow, containing- ' ve el and a short wick dipping into the - oil, were very unsatisfactory, for they gave off unpleasant vapors, and the flame was smoky. The first real im_ prove ment in construction was made in 1784, when the Argand lamp was introduced. In this lamp a cylindrical wick was placed between two concentric metallic tubes, the combustion of the oil being made more complete by a constant stream of air pa ing through the inner tube to the flame, and by an additional draught, got by placing round the burner a gla chimney resting on a ' perforated base. The flame in this lamp varied with the level of the oil in the reservoir, and in the Carcel lamp, invented about 1800, this defect was - remedied by a clockwork arrangement, _ which kept the burner supplied with an abundant supply of oil. The Carcel lamp is still used to a small extent in ' France. The moderator lamp, invented about 1836, is based upon the Carcel lamp, the oil being forced through a tube to the burner by the pre ure of a spring ‘\upon a disc floating in the oil. In the ‘tube is placed a tapering rod called the} moderator, which regulates the flow of oil in accordance with the pre ure of the spring. The above-mentioned lamps were only suitable for vegetable or animal oils, for the principle of forcing a super- - abundance of oil to the burner cannot be applied in the burning of petroleum, aft in, or other mineral oils. In lamps tended for mineral oil, only as much ofl as the wick is able to suck up reaches the burner, in order to prevent smoke, and a constant and abundant supply of air is provided. Petroleum may ‘(be burned in the ordinary way or e may be vaporized and used in conjunction with an incandescent mantle. When coal gas was first tried as an illuminant it was burned as it i ued from an open iron tube, but this was soon found to be unsatisfactory, as a large quantity of gas was consumed for 2 small amount of light. The end of the tube was then closed up, and three small holes bored in it, giving three small jets. Various arrangements of holes and sawcuts followed, Jeading up to the fishtail burner, in which two holes are bored at such an angle as to give two jets impinging upon one another, and producing a flat flame. In the regenerative burner the heat of the flame is used to raise the temperature of the air supply. Incandescent gas lighting rose from the fact that if certain incombustible substances were raised to a high temperature a brilliant light was produced. Platinum mantles were first tried, but their illuminating power decreased rapidly, owing to the erosive action of the gases in the flame. The discovery of the Bunsen burner about 1855, in which a non-luminous flame is obtainéd by mixing the coal gas, before combustion, with a certain proportion of air, paved the way to the modern incandescent burner. Welsbach hit upon the idea of a mantle consisting of a cotton fabric soaked in a solution of a metallic salt, and took out his first patent in 1805, but the Welsbach mantle was not perfected until 1893. Various methods of increasing the efficiency of the incandescent light have been introduced such as supplying the gas or the air, or both, at a high pre ure, or producing suction upon the gas and air by means of a special form of chimney used in combination with a burner adapted to utilize the augmented supply. The shape of the ordinary type of mantle is such that the light is given off at an angle above the horizontal, and this is a drawback for most domestic purposes, in which the light is required chiefly below the level of the mantle, This defect is remedied in the inverted type of mantle, which was introduced about In 1801 Sir Humphry Davy discovered that if two carbon rods were connected with the terminals of a powerful battery, and the points of these rods first brought into contact, and then slightly separated, the current did not cease, but continued to cro the gap, producing what is called the electric are. This discovery led to the construction of the arc lamp, the first practical means of employing the electric current for purposes of illumination. The carbon points in the arc lamp become incancesccat, and the intervening space is LIGHTING SYSTEMS > filled with incandescent particles carbon. The temperature of the are is very much higher than an ordinary flame, and therefore light has a higher efficiency.