This discharge, or arc, as it is called, consisted of very minute particles of charcoal, which being raised to white heat by the resistance offered by the points to the electric current, were also by its means conveyed with great rapidity from one charcoal point to the other, emitting during their passage a light of dazzling brilliancy. The discharge of heated particles being continuous, the arc could be maintained for a considerable length of time.
This light, however, was entirely impracticable for any but purely experimental purposes. A battery of four thousand plates is not easily maintained in working order, and besides, the expense of such an arrangement puts it entirely out of the question. Of late years, however, a new method of producing electricity on a large scale has been discovered in the dynamo-electric machine, by means of which currents of great volume and intensity can be obtained from the power generated by a steam engine, water wheel, or other prime motor.
This great discovery instigated scientific men to try and bring the electric light within the range of practical utility, in which end they have already been eminently successful.
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.
It has been found, however, that, adapted as some of the arc regulators are for the illumination of streets and large areas, none of them are at all able to compete with gas in the lighting of private houses. Not only do they require the constant attention of skilled workmen to renew the carbons and to clean the mechanism, but they give far too strong and dazzling a light for any but very large apartments.
For domestic lighting we therefore come to quite a new departure in electric lamps; instead of the arc we have the incandescent regulator.
If an intense electric current be transmitted through a fine platinum wire, the latter will, in a very few seconds, become white hot, and give a considerable amount of light. If such a platinum wire be enclosed in a glass globe, from which the air has been extracted, we have one kind of incandescent lamp, so called because the light is produced through the incandescence or intense heating of a platinum or other conductor. It was a lamp such as this that, when brought out by Mr. Edison two years ago, produced such a scare among holders of gas shares. It was not, however, a practical invention; it was found that the electric current constantly melted the platinum, or broke the glass envelope, after which the lamp was of course entirely useless. In vain Mr. Edison tried various alloys of platinum and iridium; nothing of that nature was found that could resist the intense heat produced by the electricity. While, however, the incandescent lamp was not progressing very rapidly in America, in England Mr. Swan, of Newcastle, who had been experimenting with the electric light for some time, brought out another kind of regulator, which has given rise to great expectations. The Swan lamp consists of a pear-shaped globe, blown out of glass, and from which all the air, or at least as much as can be, has been exhausted. In this globe there is a tiny carbon filament, manufactured of carbonized thread, in the form of a loop, which is attached to two platinum wires which project through the glass bulb. On an electric current being passed through the carbon, by means of wires attached to the platinum projections, a soft yet brilliant light is obtained. These lamps, which give a light corresponding in power and color to an ordinary gas flame, can now be obtained for five shillings each, and it is probable that this price may yet be still further reduced.
Mr. Edison also, having abandoned his earlier platino-iridium regulator, has brought out another lamp very similar to Mr. Swan’s. In his case the carbon filament is formed of carbonized bamboo, and the glass bulb is of an elongated form. Incandescent lamps have also been invented by Maxim, Crooks, Fox Lane, and others; but they only differ in details of manufacture from those of Swan and Edison.
Among edifices now entirely illuminated by the Swan system may be mentioned twenty-one steam vessels, including several war ships—the City of Rome, an Anchor Liner, which is second only to the Great Eastern in point of size, and several passenger boats in the Cunard and White Star Lines.