LESSOST3 m ELECTRICITY. 



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 cor- 

 ners and points of such bodies, when elec- 

 trified, to be greater than that communi- 

 cated by the gently rounded or flat sur- 

 faces. The difference may not be great, 

 but it will be distinct. Indeed an egg 

 laid on its side, as we have already used 

 it in our experiments on induction (fig. 

 1C), yields a stronger charge from its 

 ends than from its middle. 





Fia. 25. 



Let me place before you an example of 

 this distribution, taken from the excellent 

 work on " Frictional Electricity" by Pro- 

 fessor Riess of Berlin. Two cones, fig. 

 24, are placed together base to base. 

 Calling the strength of the charge along 

 the circular edge where the two bases join 

 eacli other 100, the charge at the apex of 

 the blunter cone is 133 ; and at the apex 

 of the sharper one 202. The other num- 

 bers give the charges taken from the 

 points where they are placed. Fig. 25, 

 moreover, represents a cube with a cone 

 placed upon it. The charge on the faco 

 of the cube being 1, the charges at fho 

 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. 



Iliess found that he fould deduce with 

 great accuracy the sharpness of a point, 

 from the charge which it imparted, lie 

 compared iu this way the sharpness of 

 various thoins, with that of a firio Knolish 

 Rcwing'needle. The following is the re- 

 sult : -Euphorbia thorn was si mi per than 

 the needle ; gooseberry thorn of ilu same 



sharpness as the needle ; while cactus, 

 blackthorn, and rose, fell more and more 

 behind the needle in sharpness. Calling, 

 i^r example, the charge obtained from 

 c iphorbia 90 ; that obtained from the 

 needle was 80, and from the rose only 

 53. 



Considering that each electricity is 

 self-repulsive, and that it heaps itself up 

 upon a point, in the manner here shown, 

 you will have little difficulty in conceiv- 

 ing 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 theoret- 

 ically important : Attach a stick of 

 sealing-wax to a small plate of tin or of 

 wood, 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 horizontally a carrot. You 

 have thus an insulated conductor. Stick 

 into your carrot at one of its ends a sew- 

 ing needle ; and hold for an instant your 

 rubbed glass tube in front of this needle 

 without touching it. AVhat occurs? The 

 negative electricity of the carrot is imme- 

 diately discharged from the point against 

 the glass tube. Remove the tube, test 

 the carrot : it is positively electrified. 



And now for another experiment, not 

 BO 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 icpe'llcd 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, paraffin, gutta-percha 

 or any other insulator. Pass your rubbed 

 glass tube once downwards or upwards, 

 the insulating plate being between the ex- 

 cited tabe and the point. The point will 

 discharge its electricity against the insu- 

 lating plate, which on trial will be found 

 negatively electrified. 



17. The Electrical Machine. 



An electrical machine consists of two 

 principal parts : the insulator which is 



