Page:All the Year Round - Series 2 - Volume 1.djvu/630

 primary wire by the sudden rushing into it of a stream of electricity.

The first endeavours to work the new coil were frustrated by its own powers of destruction. It melted the platinum, and burnt up the brass of the original contact breaker. When used with a small amount of condenser surface, it burst the primary wire into fragments, and escaped from it laterally. When these difficulties were overcome, and the whole apparatus was in order, it afforded a spark, or rather a flash of lightning, twenty-nine inches in length, and apparently about a third of an inch in width. The length was measured, of course, by the distance between the terminals, and when this exceeded twenty-nine inches, no distinct flash was given. For a distance within its power to cross, it would almost seem that the electricity, like a strong leaper, makes an effort proportionate to the resistance to be overcome. When the terminals are distant, but still within the twenty-nine inch limit, the flash strikes upon the disc with a heavy shock and a loud report. When they are near together, or within two or three inches, the flash gushes forth without noise, and lazily, like a spurt of molten metal, or of dense flame; and from this "flaming spark," as it is called, the flaming portion can be blown aside by bellows, leaving the actual course of the electricity distinctly visible. Either the flaming spark or the longer one will perforate considerable thicknesses of glass, and five inches of solid plate glass have already been pierced by it. At one visit we chanced to see a remarkable illustration of the way in which metallic surfaces may serve to attract lightning. The outer covering of the coil displays the name and address of Mr. Apps, its maker, in gold letters of considerable size. In taking a long spark, the stands that support the terminals were placed nearer to the coil than usual; and the attraction of these gold-leaf surfaces was sufficient to divert the spark from its course, and visibly to break it up into portions. In the darkened theatre at the Polytechnic, the long flash lights up the room and the audience with the peculiar lurid glare so well known as an effect of brilliant lightning at night, and displays the features and action of every one present. But it is curious to note that, the flash being of instantaneous duration only, it allows no motion to be seen. We should think, if guided by our consciousness, alone, that the flash lasted an appreciable time; but this would be an error, due to the persistence of the impression on the eye, after the flash itself had ceased. If the room be made perfectly dark, and if the spectators all raise their arms and wave their hands to and fro as quickly as they can, the flash will display the position of the arms, but not the movements of the hands. While the flash lasts, the hand has no time to move, and is consequently seen, as if motionless, in the position in which the flash finds it. It is in contemplation to exhibit the same effect in a more complete way by affixing a picture to a revolving disc. When the disc revolves so rapidly that no outlines of the picture can be distinguished by means of any ordinary light, they will be perfectly seen in a darkened room by the light of the flash. It lasts so short a time, that the revolving disc does not change its position in the brief period.

It is the smallest part of the advantage expected from the new coil, that it allows all the luminous and all the destructive phenomena of chamber electricity to be exhibited, in hitherto unapproached beauty and intensity. Men of science anticipate from it new discoveries of high importance. In the intervals between the public exhibitions of artificial lightning, the effects of the coil are being closely studied by those who are best able to appreciate them; and we believe no long time will be required in order to prove that Mr. Pepper, in his ever zealous catering for the entertainment and instruction of his especial public, has laid the foundation of real and solid scientific progress.