It was found that as the charcoal points in Davy’s lamp in process of time became oxidized and burnt away, it was necessary to have some arrangement by which they should be maintained at a constant distance from one another. This problem was first solved by Duboscq, a French _savant_, who by the combined action of the electric current and a system of clockwork, succeeded in obtaining a constant and steady light. Gas carbon, as found incrusted on the inside of gas retorts, was at the same time substituted for the charcoal employed by Davy, as it was found to burn more equally and to last much longer. In July, 1877, a new form of electric light apparatus was introduced into France and elsewhere, which, from its practical simplicity, attracted a large amount of attention. This invention is due to Mr. Jablochkoff, a Russian engineer, and is known as the Jablochkoff candle. In this form of regulator all clockwork and mechanism are avoided; the two carbons are placed side by side, in parallel lines, and are separated by some substance which, though readily fusible, at the same time offers so enormous a resistance to the passage of the electric current as practically to prevent its passage through it at all. Kaolin clay and plaster of Paris have both been employed for this purpose with success. The current not being able to pass through the insulating material, can only pass between the two carbons at the extremity of the candle, where the arc is therefore formed. As the carbons burn away, the insulating material melts, and an uninterrupted light is obtained. As it is found that one carbon burns away more quickly than the other, in this form of lamp the electric current is supplied alternately in different directions, which makes the carbons burn equally, the reversions of the electricity being so rapid that the arc is to all appearances continuous. This lamp has been largely employed in Paris, and is at present in actual operation on the Thames embankment. Its chief defects are its great expense and the unsteady character of the light, which, owing to the oxidation of the insulating material, flickers and changes color. Another lamp, and one which has been largely used in Europe and in America, is the Brush regulator, called after its inventor. In this form the carbons are vertically one above the other, the upper one being controlled by an electro magnet, which supports it, allowing it to descend of its own weight when, through the distance between the carbons becoming too great, the current is weakened, and the magnet unable to support its load, thus keeping the arc of a constant length. There are a large number of other arc regulators, some of which work very well, and are largely employed; but they are most of them based on a very similar principle to that of the Brush lamp, and therefore they need no special description.