The invention of methods of using this powerful light received a great stimulus in 1876, when Jablochkoff's " candle" was invented. This electrode consists of two car bon pencils, A inch in diameter, separated by a narrow strip of kaolin or plaster of Paris. The current passes alternately up each carbon, forming the arc at the extremity of the candle, the alternate currents causing each point.to be in turn positive and nega tive, and thus to waste equally. The "Wilde candle" has no separating medium, the inventor- having found that the light would run to the points spontaneously. By means. of a magnet introduced into the circuit, the points are drawn separate when the cur rent passes, and the light springs into existence. Should the light go out, the pencils fall together, and as the circuit is thereby completed, the points are at once separated again and the light is re-established. Rapieff's candle has the same merit of spontaneous lighting and regulation.
One of the most successful endeavors to solve the question of regulation of the light produced from the "arc" is the Wallace lamp. In this apparatus two carbon plates slide in a vertical frame. A rod extends from the upper plate by means of which it. is drawn up on the passage of the current, and the light springs out at the point between the plates that occupies the proper position. When this point wastes, the light travels to another, passing thus slowly backwards and forwards along the carbons. It is claimed for this lamp that it will burn continuously for 100 hours, and by its durability it has solved one difficulty in the production of the electric light. The Serrin, Lontin, and Rapieff lamps are different arrangements for the production of light by means of the "arc," the adjustment of the carbons being effected by a combination of wheels and magnets through an electro-magnet.
The upper rod bearing a carbon pencil rests upon a second pencil, but, upon the pas sage of a current, it is raised by an electro-magnet which separates the carbons the distance necessary to produce the "arc." When the carbons waste so that the light goes out, the magnet fails to act, the points conic together, and the original process is repeated. The interval is so short, that only an almost imperceptible blink was seen in this lamp. In the later lamps great steadiness has been achieved.
The above described lamps and candles produce light in the form of the "voltaic arc," but a number of lamps have been produced which work entirely by incandescence of the electrodes. Werdermann's lamp produces light by the incandescence of a pencil of carbon pressed against a block of the same material, the latter forming the negative pole. In Reynier's lamp, a fine rod of carbon presses on a disk of carbon placed verti cally and free to rotate. Edison's lamp depends for its action on the incandescence of fine particles of iridium, ruthenium, or other metal, held in solid form by magnetic oxide of iron or other substances not easily fused. The question of subdivision of the light is still to be solved. To produce the current, a very steady engine is required, the endeavor to drive an electric machine by a large engine at Woolwich arsenal having shown that an engine on which the strain varies by machines being thrown into or out of gear does not give satisfactory results. During the year 1878, great progress was made in the adaptation of the electric light to street and workshop illumination; but it remains dearer than gas, and cannot at present (1879) be looked on as further than the stage of successful experiment.