Page:Popular Science Monthly Volume 9.djvu/186

166 a little ocean encompassing the sphere, and of the same depth every-where.

But supposing the conductor, instead of being a sphere, to be a cube, an elongated cylinder, a cone, or a disk. The depth, or as it is sometimes called the density of the electricity, will not be everywhere the same. The corners of the cube will impart a stronger charge to your carrier than the sides. The end of the cylinder will impart a stronger charge than its middle. The edge of the disk will impart a stinger charge than its flat surface. The apex or point of the cone will impart a stronger charge than its curved surface or its base.

You can satisfy yourself of the truth of all this in a rough but certain way, by charging, after the sphere, a turnip cut into the form of a cube; or a cigar-box coated with tin-foil; a metal cylinder, or a wooden one coated with tin-foil; a disk of tin or of sheet-zinc; a carrot or parsnip with its natural shape improved so as to make it a sharp cone. You will find the charge imparted to the carrier by the sharp corners and points, to be greater than that communicated by gently-rounded or flat surfaces. The difference may not be great, but it will be distinct. Indeed, the egg laid on its side, as we have already used it in our experiments on induction, yields a stronger charge from its ends than from its middle.

Let me place before you an example of this distribution, taken from the excellent work on "Frictional Electricity," by Prof. Riess, of Berlin, who is probably the greatest living exponent of the subject. Two cones, Fig. 16, are placed together base to base. Calling



the strength of the charge along the circular edge where the two bases join each other 100, the charge at the apex of the blunter cone is 133, and at the apex of the sharper one 202. The other numbers give the charges taken from the points where they are placed. Fig. 17, moreover, represents a cube with a cone placed upon it. The charge on the face of the cube being 1, the charges at the corners of the cube and at the apex of the cone are given by the other numbers; they are all far in excess of the electricity on the flat surface.

Riess found that he could deduce with great accuracy the sharpness of a point, from the charge which it imparted. He compared in this way the sharpness of various thorns with that of a fine English sewing-needle. The following is the result: Euphorbia-thorn was sharper than the needle; gooseberry-thorn of the same sharpness as