Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/69

Rh HEATING EFFECTS, j vapour and disintegrated matter. With a battery of 60 Grove s cells the arc onoe started has a certain persistence, for we may break the current for ^th of a second or so, and the light will start again when the current is turned on afresh. We may start the light without bringing the electrodes into contact by causing a spark from a Leyden jar or an inductorium to pass across the interval. If an image of the voltaic arc be thrown on a screen by means of a lens, its constitution can bo examined very readily. Four distinct parts at once strike us; first, the dazzling white positive carbon, which assumes a crater-like shape after the current has passed for some time ; second, the more pointed and if -anything less brilliantly-white negative carbon, which is in general strewed with little beads of melted or at least softened carbon; third, the central core or streak of glowing matter, which has a white appearance, though it is considerably less brilliant than the carbon; fourth, the globe-shaped aureole which surrounds the whole, whose brilliancy is greatly inferior to that of the other parts, and whose colour depends on the surrounding gas. If the electrodes be horizontal, the arc is in general curved upward by ascending air currents, its form is also affected in general by the earth s magnetic action. The hollowing out of the positive electrode is obviously due to a transfer of matter in the direction of the current. It is very easy to prove by a variety of conclusive experi ments that there is such a transfer, mainly in the direction of the current, but alao in part in the opposite direction. If we take a platinum point for positive, and a platinum plate for negative electrode, the matter carried to the plate forms a series of rings on it like the colour rings of Nobili. If, on the other hand, the platinum plate forms the positive elec trode, a series of slight excavations are formed where the matter has been torn away. There can be no doubt that the disintegration of the electrodes plays a very important part in the formation of the arc, for if we saturate the car bons with volatile matters, the brilliancy of the arc, the ease with which it forms, and its maximum length for given battery power are greatly increased. It is probably owing, in part at least, to the tearing away of matter at the posi tive electrode that the temperature there is in general highest. This effect is very marked in some cases. If we take a platinum point and a plate of the same metal for electrodes, the point glows through a considerable length when it is positive, but only at the end when it is negative. Again, when the light is generated between two platinum wires hsld crosswise at a small distance apnrt, the glowing portion is much longer on the positive electrode than on the negative. The electric light is the only artificial light whose bril liancy can compare with the sun. Measured by its actinic properties simply, it is not so very far behind the great luminary; its spectrum is longer towards the violet, and it has accordingly great advantages when it is required to pro duce fluorescence (see art. LIGHT). Its great chemical power is also shown by the readiness with which it induces the combination of hydrogen and chlorine; by means of it underground buildings, such as the catacombs, have been successfully photographed. Its illuminating powers have for a considerable time been employed in lighthouses, the current for its maintenance being furnished by powerful electromagnetic machinery, and it is now proposed to employ the Gramme machine and the electric lamp to light streets and public buildings, manufactories, &c. It was used for war purposes during the last siege of Paris, and in the Kusso-Turkish war on the Danube; and further applications to torpedo warfare have been contemplated. In most of these applications of the electric light it is important that the arc should be of constant length, and maintain a fixed posi tion. Owing to the unequal consumption of the carbons, special E L E C T B I C I T Y 59 appliances are required to secure the fulfilment of these conditions. The best known and perhaps the most efficient of the older electric lamps is that devised by Foucault. It consists of a piece of clock work, which moves the carbons towards each other with relative speed nearly equal to that at which they are consumed. The ma chinery is controlled by a detent worked by an electromagnet, which is excited by the current which feeds the arc. When the carbons are too far apart the electromagnet is weakened and releases tl&amp;gt; detent. The machinery then moves the carbons until the current in strong enough to enable the electromagnet to apply the detent again. This apparatus works well enough for lecture-room and other pur poses, but has not given perfect satisfaction in industrial applications. Accordingly many devices have been proposed, more especially of late years, to supersede it. One of the simplest and it would appeal- most effective of these is the electric candle of Jablochkoff. This consists simply of two carbons, separated from each other by a plate of kaolin. The arc passes between the carbons, and plays over the kaolin, which gradually melts away like the wick of a candle, and by its incandescence greatly helps the brightness of the light. 1 The heating powers of the electric arc are no less remark able; platinum and iridium melt in it like lead, and vola tilize. In this way the spectra of the glowing vapour of these metah can be projected on a screen. Almost nothing seems to resist the elevated temperature of the arc. Des- pretz generated it in vacuo by means of 500 to 600 cells of Bunsen, and observed pieces of carbon volatilize like a piece of heated iodine, while the carbon vapour condensed on the walls of the receiver in the form of a crystalline powder. Flint melted to a glassy mass, and boron behaved similarly, while cylinders of retort carbon softened and bent into an S-form. The voltaic arc behaves in many respects like an ordinary electric current. It is affected by the magnet, for instance, as an ordinary current would be. Owing, however, to the variety of transformations of energy taking place, it is difficult to estimate accurately the resistance and electro motive force of the arc. Edlund made experiments which seemed to show that a certain minimum electromotive force in every case was necessary for the maintenance of a con tinuous arc, yet the arc does not appear to consist of a series of disruptive discharges, for its image in a rotating mirror is a uniform band. It would seem, therefore, as if polar ization in some form or other were present. Edlund, in fact, found that when a galvanometer was substituted for the battery by which the arc was formed a considerable current was obtained, which might have an origin similar to that of electrolytic polarization, or be a thermoelectric effect. 2 For further details on these and other matters con nected with the electric light, we refer the reader to the admirable account of Wiedemann, Galvanumus, Bd. i. 701 $qq.), from which most of the above is taken. Disruptive Discharge, Light Effects, c. A. definite meaning has already been attached to the pheno- term disruptive discharge; the object of the present section mena or is to consider this phenomenon a little more closely in &amp;lt; isru P- several particular cases. The disruptive discharge proper tlve di * is in general accompanied by sound, heat, light, and me chanical effects, very often by all four. The attendant luminous phenomena have absorbed by far the greatest share of the attention of experimenters, partly, no doubt, on account of their great variety and wonderful beauty. It would be a hopeless task to endeavour, within the limits set us here, to give even a meagre summary, not to speak of a critical account, of all the experiments and observations that have been made on this subject. The scientific inves tigator will find sufficient guide for his reading in the three standard treatises of Riess, Wiedemann, and Mascart. Puess is particularly interesting in his account of the older 1 See Mature, Sept. 1877. 2 Wiedemann explains Edlund s results by means of an &quot; Uebergangs- widerstand.&quot; It is difficult to understand how in this way a return cxirrent could arise.