By A. A. CAMPBELL SWINTON.

It seems at present that electricity is to be the illuminating agent of the future, and that, as gas has now all but superseded candles and oil, so in turn gas will soon be superseded by electricity. The reasons for this change are several and various, and follow that most immutable of natural laws, the law of the survival of the fittest.

About the commencement of the present century, Sir Humphry Davy, the eminent chemist, succeeded in producing at the Royal Institution the most brilliant light then known. By passing the electricity derived from an enormous battery of four thousand plates through two charcoal points separated from one another, he obtained in air a continuous electric discharge four inches in length, which was increased to seven inches when the experiment was repeated in vacuo.

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.

One of the greatest objections to gas as an indoor illuminant is the fact that not only does it burn a large amount of the oxygen of the air, but it also gives off during combustion carbonic acid gas and other poisonous vapors, besides a great amount of heat, thus vitiating the atmosphere. In public buildings where there is much gas burnt and little ventilation, this is seen to advantage, the air becoming in a short space of time hot and unwholesome. Now in the case of the incandescent electric light, this is altogether altered, the incandescent filament which produces the light, although in itself enormously hot, is too small in point of size to radiate much heat, and the fact of its being hermetically enclosed in a glass globe, which is impervious to the atmosphere, entirely prevents the escape of any noxious gases. The same circumstance prevents there being any consumption of oxygen.

These facts make the electric light far more wholesome than gas for the illumination of music-halls, churches, or other places of concourse. In a recent trial in the Town Hall at Birmingham, the employment of gas raised the temperature of the atmosphere thirty-eight degrees in three hours, while the building was equally well lighted with electricity for seven hours with a rise in temperature of only two degrees. Thus, after a period of lighting by electricity 2.33 times as long as by gas, the temperature at the ceiling was increased by only 1-19th of the amount due to gas.

Another great advantage consequent to the employment of incandescent lighting, is the greater immunity from accidental fire; for as the carbon filament is instantly entirely consumed, the moment the glass envelope is broken it is impossible for the lamp to ignite anything in its vicinity however inflammable. The experiment has been tried of breaking a lighted incandescent lamp in a vessel containing gunpowder, with perfect safety. As these lamps may be placed in any position, they lend themselves very readily to ornamental and decorative purposes. At the recent electrical exhibition at the Crystal Palace a very beautiful chandelier of Edison lamps was shown, in which the lamps, which were of very small size, formed the petals of finely worked glass and brass flowers. This chandelier had a really magnificent effect when lighted.

These and other facts too numerous to mention, demonstrate that electricity, when properly applied, will be a far more elegant, safe, and wholesome agent for illuminating purposes, than coal-gas as now employed. But in order to have the full benefits of its use, a system is required by which the electric current shall be produced and conveyed to the lamps.

Not only has Mr. Edison invented an incandescent lamp, but he has also identified his name with a very complete system for producing the light on a large scale to suit both domestic and commercial requirements. In the first place he has invented a peculiar form of dynamo-machine, which when driven at great speed by powerful steam or water engines, produces the electricity in great quantity at some central station. From this centre the current is conveyed by copper wires laid under the streets or over the roofs of the houses, these conductors being tapped of their electric fluid by smaller wires which convey the electricity into the houses, in a way similar to that in which gas is conveyed by small pipes from the larger street mains. In each house is an electric meter, a special invention of Mr. Edison’s, which measures the quantity of electricity which passes through it. This meter is very ingenious, and therefore the principle on which it is based may be described. If a current of electricity be passed through a solution of sulphate of copper, contained in a copper jar, the sulphate solution is decomposed and metallic copper is deposited on the inside of the jar. Now it has been proved by experiment, that the amount of copper deposited is always directly proportional to the strength and duration of the electric current. Mr. Edison’s meter consists of such an arrangement, and he finds that by weighing the copper jar, so as to determine exactly what it has gained in weight through the metallic deposition of the solution it contains, he can accurately calculate in units the amount of electricity that has passed through the meter. By means of this beautiful discovery electricity can be supplied and paid for in a manner very similar to that employed in the case of gas at the present time.

Within the building to be illuminated, the electric fluid reaches the lamps along small copper wires, about the thickness of ordinary bell wire, which are covered with a coating of gutta percha to prevent the escape of the electricity, which might cause sparks or even fire, or in any case seriously injure any one who might come in contact with the bare metal, by giving him a very violent if not fatal electric shock. The lamps themselves may be fixed to ordinary gas brackets. Mr. Edison has designed some special ones, and the light can be turned on and off, by means of a tap or button, with as great or even still greater facility than gas.

Mr. Edison has recently established a central station in New York, from which he proposes to light the houses included in an area of a wide radius from the center. In part of this area the installation of the lamps and wires is now complete, and the light is giving every satisfaction, the cost being considerably below that of gas, which in the United States is very expensive. It must be remembered that electric lighting is comparatively a new science, and not yet fully understood. There is very little doubt that, by practice, it will before long approach more nearly to perfection, and sooner or later entirely supersede gas, the arc form of lamp being employed for the illumination of streets and large areas, while the incandescent pattern meets domestic requirements.—Good Words.