Discoveries and Inventions of the Nineteenth Century

the field magnets that are made to revolve, and in these cases, not even a sliding contact is required, but the end of the armature coils are directly and permanently connected with the main circuit. But as these dynamos are self-exciting, the electricity induced in a few of the armature coils is collected apart 526from the main circuit, and passed through the electro-magnets of the machine itself, after the alternate currents have, by means of a commutator, been converted into one direct continuous current. Fig. 280g.—Electric Lamp. The arc electric light, as used for the illumination of streets and public places, is too intense and concentrated to be pleasant to the eye, and therefore it has been found necessary to surround it by globes of enamelled glass, or of porcelain, or of ground glass, or of frosted glass. By these expedients for diffusing and softening the light, it is rendered much more acceptable, but this advantage is gained at the cost of a considerable loss of the whole illuminating power, a loss which is, probably, never less than 10 per cent., but is usually much greater. The globes used in Paris, with the Jablochkoff candles, were of enamelled glass, and the apparatus was arranged, as shown in Fig. 280g, where it is partly represented in section, and with a part of the globe broken off, in order to show one of the candles placed in the holder which connects it with the circuit. In each lamp several candles were mounted, in some cases four; but the lamps in the Place de l’Opéra held twelve. At first there were mechanical arrangements, automatic and otherwise, by which, when the candle was burned down the current could be turned on to another. But M. Jablochkoff afterwards discovered that there was really no need for such a mechanism. For when the whole of the candles are simultaneously and equally connected with the circuit conductors, it is found that one of them will 527more easily transmit the current than any one of the rest, and when that particular one has once been lighted by the heat developed, the current will pass almost entirely through the arc, any loss through the connecting strip of carbon, at the tops of the other candles, being quite insignificant. When the first of the candles has burnt down completely, until the insulating porcelain holder separates the carbons, the current will at once re-establish itself at the top of one of the remaining carbons, and so on, while one is left. The arc electric light has not been brought to its present position without the expenditure of much care and ingenuity in the preparation of the carbons used for its production. When Davy first produced the voltaic arc, the electrodes he used were simply sticks of charcoal. These were very quickly consumed, and a more durable form of carbon was sought for. This was found by Foucault, who made use of rods sawn out of the carbonaceous residue left in the retorts in the process of making coal-gas. This substance was, however, by no means uniform or sufficiently pure, and the light obtained was consequently unsteady. Many experiments were made in preparing special carbons. Pounded coke, coke and charcoal, were mixed with syrup or tar into a paste, which was moulded and compressed, and then the sticks were kept in covered vessels at a high temperature for many hours. Acids were used for purification, and also alkalis, to remove silica. At the present time there are several manufacturers of electric light carbons who carry on extensive operations by processes which probably are very similar one to another, and which may well be represented by M. Carré’s, whose carbons have the highest reputation. M. Carré prefers a mixture of powdered coke, calcined lampblack, and a syrup made of sugar and gum. The whole is well mixed and incorporated, water being added from time to time to make up for loss by evaporation, and to give the paste the proper degree of consistence. The paste is then subjected to compression, by which it is forced through draw-holes, and the carbons, having been piled up in covered crucibles, are exposed for a certain time to a high temperature. As a practical illuminant for lighthouses, the arc electric light came into use many years ago (1862) as we have already seen. This was when the generator of the current was the magneto-electro machine; but, now, when this generator has developed into the modern dynamo, the cost of the electric supply has been enormously reduced, so that, power for power, electric lights may be worked at half the former cost, and with greater convenience and certainty. Light for light, electrical illumination is said to be far cheaper than gas. Again, the arc electric light has properties which have caused it to be employed, not only in every important lighthouse in England, France, Russia, America, and elsewhere, but most ships of war are provided with means of projecting a beam of electric light in any direction, in order that the presence of torpedo boats, etc., may be discovered at night, or harbours entered and signals made under circumstances when such operations would be otherwise impossible. It was by the use of the electric light that, in 1886, one of the Peninsular and Oriental Company’s steamers passed safely through the Suez Canal, at night, and the experiment was so satisfactory, that the canal authorities placed beacons and light-buoys to guide such vessels, as, being provided with electric apparatus, were enabled to hold their proper course between its banks. The use of projected beams for watching the movements of enemies, and for signalling to great distances in time of war, has been 528recognized by all the great military powers. The advantage of the electrical light in some mines, in subterranean and submarine operations and generally, in work that has to be carried on at night by large bodies of men, is constantly finding illustration. Few readers are unacquainted with the brilliant effect of the arc lamps in exhibitions, parks, &c.; at out of door fêtes, or applied to the illumination of fountains, such as those at the Paris Exhibition of 1889. The arc lamps are used in series; that is, where there are a certain number of lamps to be supplied, the same electrical current circulates through the whole of them, and this, of course, must have force enough to overcome the resistance of the whole circuit. Thus, at each lamp, the intensity of the illumination must necessarily be very great. A solution was long sought to the problem of so dividing the current energy, that it might be made to produce lights, of moderate intensity, at a greater number of points. When Mr. Edison, shortly after having invented the phonograph, announced that he had solved the problem of the electric light division, there was a great panic amongst the holders of shares in gas companies, and a heavy fall in this kind of stock immediately occurred. As it turned out, the alarm was unnecessary, for gas was not to be superseded, immediately and definitely, by electricity. Nevertheless, it is by virtue of the principle that was contained in Edison’s invention, that electric lighting has assumed the wide-spread importance it has at the present day, and that it is now actually ousting gas as an illuminant in the business and domestic premises of our large towns, and in theatres, libraries, and other places of resort. The principle which has brought about this great development of electric illumination is that shown in a simple form in Fig. 261. It appears, however, that as early as 1841, a platinum wire, made incandescent with a battery current, was proposed as a source of light, and in 1845, carbon was used in the form of slender rods, by King, and also by J. W. Starr, in the United States. Both inventors inclosed their carbons in glass tubes, from which the air was exhausted, so that the carbon might not burn away. In the following year, Greener and Staite turned their attention to lamps of this kind, and, again, in 1849, Petrie worked on the same subject. After that, the problem ceased to engage attention, until, in 1873, a Russian man of science, named Lodyguine, took the matter up and patented a carbon incandescent lamp, which did not, however, prove a practical success, and although the idea was worked out in various ways by Konn, Reynier, Trouvé, and others, the apparatus they designed was, in every case, lacking in simplicity, and certainty of action. The Edison incandescent lamp, the announcement of the discovery of which so fluttered the gas companies, about 1878, was a reversion to the plan of an incandescent metallic wire. This wire was made of an alloy of platinum and iridium, which was adopted by Edison on account of the very high temperature required for its fusion. And in order to prevent the temperature from quite reaching that point, the wire was arranged in a spiral within which was a rod of metal that, by its dilatation with a certain temperature, caused a contact to be made which diverted part of the current through a shorter circuit, and thus lowered the temperature of the spiral to within the assigned limits. But the advantages presented by carbon over metallic conductors led Edison to attempt the formation of filaments by charring first slips of paper, afterwards slips of bamboo. About the same time Mr. J. W. Swan, of Newcastle-on-Tyne, was experimenting in 529the same direction, and, in the latter part of the year 1880, he exhibited the first incandescent lamps shown in England. Swan’s carbon filaments were prepared from cotton threads which had previously been steeped in dilute sulphuric acid, washed, and passed through draw holes to give them an uniform section. They are thus made perfectly homogeneous throughout, and, after having been wound on pieces of earthenware to the required shape, they are carbonized by packing in powdered charcoal and heating. These filaments are very thin, but solid and elastic. The arrangement of the lamp (see Fig. 280h) is extremely simple: the filament of carbon bent into a horse-shoe form, or turned so as to form a loop, is inclosed in a glass bulb of a globular or egg shape, about two inches in diameter. The extremities of the filament are connected in an ingenious manner to two platinum wires that pass outward through the glass into which they are fused, and terminate either in binding screws or in two small loops. The bulb is exhausted first by an ordinary air-pump, and then by a Sprengel mercurial pump, the current of electricity being sent through the filament during the last stages of the process, and finally the bulb is hermetically sealed. The light yielded by these lamps is mild and steady, and its intensity depends on the electric current sent through them; but this may, it is said, be carried as high as to make the light equal to that of twenty candles. Each horse power of force expended on the dynamo suffices to maintain ten of these lamps. At the Exhibition of Electrical Apparatus at Paris in 1881, the Swan lamp received the gold medal as being the best system in its class. The Swan and the Edison patents are now worked together by one Company, and the productions of this Company are very largely used, although there are several more or less modified systems of glow lamps prepared by other manufacturers. Fig. 280h. The great advantages offered by electric glow lamps over gas-lights caused them to be speedily adopted by the most enterprising managers of theatres and places of amusement. Mr. D’Oyly Carte had the Savoy Theatre, in London, completely fitted up with these lamps in 1881. The light was soft and agreeable, it did away with the risks of fire both for the audience and the performers: for the footlights and scene-lights were also electric glow lamps, and the coolness of the house and greater purity of the air were at once appreciated. Several other London theatres have since