Page:Popular Science Monthly Volume 9.djvu/187

Rh the needle; while cactus, blackthorn, and rose, fell more and more behind the needle in sharpness. Calling, for example, the charge obtained from euphorbia 90, that obtained from the needle was 80, and from the rose only 53.



Considering that the electricity is self-repulsive, and that it heaps itself up upon a point in the manner here shown, you will have little difficulty in conceiving that, when the charge of a conductor carrying a point is sufficiently strong, the electricity will finally disperse itself by streaming from the point.

The following experiments are theoretically important: Attach a stick of sealing-wax to a small plate of tin, so that the stick may stand upright. Heat a needle and insert it into the top of the stick of wax; on this needle mount a carrot. You have thus an insulated conductor. Stick into your carrot at one of its ends a sewing-needle, and hold for an instant your rubbed glass rod in front of this needle without touching it. What occurs? The negative electricity of the carrot is discharged from the point against the glass rod. Remove the rod, test the carrot: it is positively electrified.

And now for another experiment, not so easily made, but still certain to succeed if you are careful. Excite your glass rod, turn your needle away from it, and bring the rod near the other end of the carrot. What occurs? The positive electricity is now repelled to the point, from which it will stream into the air. Remove the rod and test the carrot: it is negatively electrified.

Again, turn the point toward you, and place in front of it a plate of dry glass, wax, resin, shellac, paraffine, gutta-percha, or any other insulator. Pass your rubbed glass tube once downward or upward, the insulating plate being between the excited tube and the point. The point will discharge against the insulating plate, which on trial will be found negatively electrified. These experiments, I may say, were discussed, and differently interpreted by the two philosophers, during an important correspondence between Faraday and Prof. Riess.