famous battery of the Royal Institution, which consisted of no fewer than 2,000 zinc-and-copper couples, exposing an aggregate surface of 128,000 square inches. Davy found that when the current from this pile was passed between two pointed pieces of wood-charcoal, attached to conducting wires, a light was produced of such dazzling brilliancy as to be comparable only with sunlight. Moreover, it was found that the intense light was accompanied by intense heat. Many substances which had previously been regarded as infusible were melted or even dissipated as vapour when introduced into the luminous focus. Similar effects were produced in a vessel exhausted of air, or in an atmosphere which is incapable of supporting combustion, such as nitrogen or carbonic-acid gas. Hence it was concluded that the light did not proceed from ordinary combustion.

If the electric light be carefully studied, it will be seen that glowing particles of carbon are constantly springing across the arc, from pole to pole. Although this transference of solid matter takes place in both directions, the prevalent course is from the positive to the negative electrode. In fact, the positive carbon becomes much more highly heated and wears away much more rapidly than does the negative carbon. Hence the extremity of the positive carbon at length presents a cup-like concavity, while the end of the negative carbon remains more or less sharply pointed. This erosion of the positive pole takes place even in vacuo, and is therefore due to the actual emission of solid particles, and not to the combustion of the carbon. It is indeed generally believed that the light is produced by the incandescence of particles of carbon which form a conducting chain between the two poles. It will, therefore, be seen that this mode of illumination is, after all, only a special modification of illumination by incandescence.

In order to produce the electric light, the carbon poles are momentarily brought into contact and then separated to a slight distance -the distance across which the light can leap depending on the power of the current. The luminous portion of the circuit, which forms a bridge between the two solid poles, is generally known as the voltaic arc. The colour and shape of this luminous arc depend upon the nature of the poles.

In the original experiments of Davy, wood-charcoal was employed. As long as this substance continued to be used, the light afforded nothing but a brilliant experiment for the lecture-table. But in 1844, M. Léon Foucault proposed to substitute for charcoal that carbonaceous matter which is deposited in the interior of gas-retorts, and is known as gas-graphite. This variety of carbon is produced by the decomposition of dense gaseous hydro-carbons at the high temperature at which the distillation of the coal is effected. The gas-carbon exhibits a laminated structure due to the mode of its deposition, layer after layer, upon the internal walls of the retort.

When this hard and dense carbon is sawn into rods, or pencils, it may be used with advantage in producing the electric light, since it is vastly more durable than the softer forms of carbon, such as woodcharcoal.

Considerable objection may nevertheless be urged against the use of gas-graphite. It is found, for instance, that its density and texture are not always uniform, and the light consequently fluctuates in brilliancy. To obtain perfectly homogeneous carbon various artificial processes have been suggested. It is unnecessary, however, to give details of any of these processes, though it should be mentioned that in France considerable success has been attained in this direction by M. Gaudoin and by M. Carré.

Since the carbon poles are slowly consumed during the production of the electric light, it becomes needful to move them together in proportion as they wear away. If the light is to be continuous, the distance between the carbons must be kept constant; and as the positive carbon is destroyed much more rapidly than the negative carbon, provision must be made for a corresponding increase in its velocity. This has always been one of the great difficulties in attempting to carry out electric illumination. Various automatic regulators have been devised to keep the poles separated by a definite distance, but most of these devices involve the use of delicate and complex mechanism. The electric lamp in common use for the illustration of scientific lectures and for theatrical effects is furnished with an ingenious regulator devised by MM. Foucault and Duboscq. A much simpler form of apparatus is due to Mr. John Browning, of London. In some of our lighthouses where electric illumination is employed, use is made of an automatic regulator constructed by Dr. Siemens. A favourite form of regulator in Paris is the ingenious but complex device of M. Serrin, and this arrangement has been used in some of the recent exhibitions of the electric light in this country. Some ingenious improvements of this regulator have been introduced by M. Lontin.

Little progress, however, was made in the industrial extension of electric illumination as long as mechanical regulators were needed. But the subject took a new departure when in 1876 M. Jablochkoff, an officer in the Russian army, brought forward an electric lamp in which the regulator was entirely abolished. Instead of placing the two carbon pencils one above the other in the same, or in nearly the same, vertical line, as had generally been done before, he placed them side by side. Between the two cylindrical rods of carbon, he interposed a layer of some insulating medium which kept them electrically distinct while mechanically in contact. For this purpose the inventor employed at first a composition containing kaolin, or china-clay, but this has since been displaced by common plaster of Paris. It is the presence of calcium in this plaster that gives an occasional reddish

tinge to the Jablochkoff light. So unimportant, however, is the insulating material that Mr. H. Wilde, of Manchester, has recently found it possible to dispense with it altogether, and obtains the light by simply mounting the two rods side by side, the carbons being merely coated with hydrate of lime. A small bridge of carbon, or a strip of plumbago and gum, at the upper extremity of the rods, serves temporarily to connect them with each other, and offers a passage to the electric current from pole to pole. But when once this passage has been established, the arc is afterwards self-maintained. The entire arrangement is suggestive of a double-wicked candle, in which the insulator plays the part of the wax-for it is melted and dissipated by the heat of the current-while the carbons represent the wicks: hence this device is commonly known as the electric candle.

If the two carbons wore away at an equal pace, the distance between them would be kept uniform by means of the interposed insulator, or even by their fixed position in the holders. But as a matter of fact the positive carbon is consumed twice as rapidly as the negative carbon. It is, therefore, necessary to resort to some device which will keep their extremities constantly opposite each other. The inventor first sought to overcome this difficulty by making the rapidly-burning carbon proportionally thick. If the sectional area of the positive carbon were twice as great as that of the negative it might be supposed that the ends of the two rods would be kept constantly at the same level. By such a compensating arrangement the light was indeed greatly improved, but it was still far from satisfactory. The difficulty was ultimately overcome by sending the electric current alternately through the two carbon rods, so that the pole which at one moment is positive becomes the next moment negative. The carbons are thus kept uniform in length, the wicks are always opposite each other, and the light becomes remarkably steady.

The Jablochkoff light is enclosed in a globe of opaline glass, which subdues the dazzling brilliancy of the electric arc, and converts it into a pure and soft light, though at the loss of about one half the illuminating power of the naked candle. Each opal globe contains four candles, but only one of these is alight at one time. As each candle burns for about an hour and a half, the four suffice for the entire evening. As soon as one candle has burnt down, the current is shunted, either manually or by an automatic commutator, to the next candle. Each candle in the lantern is mounted in a brass tube, securely held upright in a pair of jaws, and is connected with the electro-motor by means of a cable of seven tinned copper wires, which run down the hollow shaft of the lamp-post and are then carried underground in earthenware drainage-pipes.

It is the Jablochkoff candle which has hitherto been largely used

in Paris to illuminate not only some of the public thoroughfares such as the Place de l'Opéra, but also the interior of large buildings such as the Magasins du Louvre, and the new Hippodrome. It is this light also which has been introduced into England by the Société Générale de l'Electricité of Paris, and is already employed in Billingsgate Market; and it will be used on the Thames Embankment, where successful experiments have recently been made.

To obtain the alternating currents of electricity required for the Jablochkoff light, M. Gramme has devised a special modification of his well-known dynamo-machine. Little or no progress towards the practical extension of electric lighting could be made as long as the electricity was generated by a voltaic battery. But after Faraday's discovery of magneto-electricity, machines were constructed for the production of electricity by the rotation of an induction coil in front of the poles of a magnet. Here the mechanical force which is expended in the rotation of the coil, or armature, becomes transformed into electricity; while in the galvanic battery it is the expenditure of chemical force that gives rise to the electrical energy. Machines in which permanent magnets are thus used are generally termed magneto-electric machines. Perhaps the best known apparatus of this class is the Alliance machine,' which was devised by Professor Nollet and M. Van Malderen of Brussels, and is still employed for producing the electric light in some of the lighthouses on the French coast. A similar magneto-electric machine, constructed by Mr. Holmes of London, is used for electric illumination at the South Foreland.

By the substitution of electro-magnets for ordinary, or so-called permanent, magnets, a great step was made in the construction of these machines. When an electro-magnet has once been magnetised, it permanently retains a small amount of magnetism; and it was found almost simultaneously by Dr. Siemens, by Sir Charles Wheatstone, and by Mr. Varley, that if a coil be caused to revolve in front of an electro-magnet, this residual magnetism will induce a current in the revolving armature. The current thus produced is then used to increase the magnetism of the electro-magnet, by being sent through the wire surrounding the magnet. The stronger magnetism now reacts on the coil, and induces more powerful currents, which in turn strengthen the magnetism. And thus there is a continued action and reaction between the magnet and the armature, until ultimately very powerful currents are obtained. The machines which are constructed on this principle of mutual reinforcement are distinguished as dynamo-electric machines. At the last Paris Exhibition in 1867, Mr. Ladd, of London, exhibited a small machine of this type, which gave a continuous electric light. Machines of enormous power have since been constructed, in this country chiefly by Dr. Siemens and by Mr. Wilde, and in France chiefly by M. Gramme

and by M. Lontin. Siemens's machine and lamp are used at the Lizard Lighthouse. It is a special form of Gramme machine which is used in connection with the Jablochkoff candles in Paris and in London; while the Lontin machine feeds the electric light at the Gaiety theatre, and at some of the railway termini in Paris, as the Gare St.-Lazare. As it is not desirable to burden the pages of this Review with a mass of mechanical details, we forbear to enter into any minute description of the construction of these machines. With reference, however, to the motive power consumed in producing the light, it may be remarked that each separate light is said to require for its production one horsepower of an engine. Thus, every sixteen Jablochkoff candles in Paris are served by a single Gramme machine, which absorbs a motive force of about sixteen horse-power: for this expenditure of power a most brilliant light is obtained, but much of its intensity is lost by the opal globes which are necessary to soften and diffuse the light. Each Jablochkoff candle, representing one horse-power, is said to have the photometric value of 760 standard candles, but the globe reduces its effective lighting-power to something like 300 candles.

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Considerable attention has lately been attracted by the improvements of M. Rapieff, a Russian engineer, whose system has been introduced into this country by Mr. E. J. Reed, and is now employed in the machine-room of the Times office. The lamp is there worked by a Gramme machine, and its action is so successful that the light is also to be used in the composing-room. In this system of electric lighting, four carbon-points are employed, instead of two. These are arranged crosswise, in the form of the letter X, but the plane of the lower pair is at right angles to that of the upper: thus if the upper V of the X be in the plane of this page, the lower will be in a plane perpendicular to the page. The luminous focus is at the point of decussation, at the root of the four arms of the X. In proportion as the carbons are consumed, they are caused, by an ingenious arrangement of cords and pulleys, to approach each other; and thus the voltaic arc is always produced through a constant distance. With rods measuring twenty inches in length, and about six millimetres in diameter, a light may be uninterruptedly maintained for nine or ten hours. It should be noted that the current does not pass through the entire length of the carbon pencil; but it enters by means of curved metallic arms, at points near the luminous focus; hence the resistance offered to the current is kept constant whatever may be the length of the carbons. This constancy of resistance is obviously not obtained in some other systems, such as that of Jablochkoff. For when the current passes through the entire candle, or pencil of carbon, it experiences a resistance due to the length of the rod; but as the candle burns down, the length diminishes and therefore the resistance is decreased: hence the intensity of the current, and there