166 THE POPULAR SCIENCE MONTHLY. 



a little ocean encompassing the sjjhere, 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 

 stixmger charge than its fiat 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 cer- 

 tain 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 wood- 

 en 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 dis- 

 tinct. 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 sub- 

 ject. Two cones, Fig. 16, are placed together base to base. Calling 



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/7 <M 



'yntn> / ' ^_ 771 171 



04/ ^--^W 



Fig. 16. 



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 sharp- 

 ness 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 



