^LESSOXS IN ELECTRICITY. 



303 



electrified it. 



You cannot do better lierc than repeat 

 Stephen Gray's experiment. Suppoit :i 

 small plank or lath, L L', Fig. 14, upon a 

 wnnn tumbler, G, and bring under one of 

 its ends, L, and within four or five inches 

 of that end, scraps of light paper or of 

 gold leaf. Excite your glass tube, R, 

 vigorously, and bring it over the other 

 end of the plank, without touching it. 

 The ends may be six or eight feet apart ; 

 the light bodies will be attracted. The 

 experiment is easily made, and you are 

 not to rust satisfied till you can make it 

 with case and certainty. 



This is a fit place to repeat that you 

 must keep a close eye upon the tumblers 

 you employ for insulation. Some of 

 thorn, made of common glass, are hardly 

 to be accounted insulators at all. 



14. Experimental Researches on 2Jlcc- 

 tric Induction. 



Our mastery over this subject of in- 

 duction must be complete ; for it under- 

 lies all o';r subsequent inquiries. "With- 

 out reference to it nothing is to be ex- 

 plained ; possessed of it you will enjoy 

 not only a wonderful power of explana- 

 tion, but of prediction. We will attack 

 it, therefore, with the determination to 

 exhaust it. 



Arid here a slight addition must be 

 made to our apparatus. We must be in 

 a condition to take samples of electricity, 

 and to convey them, with the view of 

 testing them, from place to place. For 

 this purpose the little "carrier," shown 

 in fig. 15, will be found convenient. T 

 i.5 a bit of tin-toil, two or three inches 

 square. A 9*. raw stem is stuck on to it 

 by sealing-wax, the lower end of the stem 

 being covered by sealing-wax. To make 

 the inflation sure, the part between R 

 and s' is wholly of sealing-wax, You 

 v,an have sterns of ebonite, which are 

 stronger, for a few pence ; but you can 

 have this one for a fraction of a penny. 

 The end u' is to be held in the hand ; the 

 electrified body is to be touched by T, 



id the electricity conveyed to an clec- 



osoopo to be tested. 

 Touch your rubbed glass rod with T, 



id then touch your electroscope : the 

 leaves diverge with positive electricity. 

 ~?oacli your rubbed gutta-percha or seal- 

 ing-wax \vitIiT, and then touch your elec- 



troscope : the leaves diverge with nega- 

 tive electricity. If the electricity of any 

 body augment the divergence produced 

 by the glass, the electricity of that body 

 is positive. If it augment the divergence 

 produced by the gutta-percha, the elec- 

 tricity is negative. And now we ar^ 

 ready for further work. 



Place an egg, E, fig. 16, on its sid* 



FIG. 16. 



upon a dry wine-glass \ bring your ciciV- 

 cd glass tube, o, within an inch or so of 

 the end of the egg. What i.s the condi- 

 tion of the egg 1 Its electricity is decom- 

 posed ; the negative fluid covering the 

 end a adjacent to the glass, the positive 

 covering the other end b. Remove the 

 glass tube : what occurs ? The two elec- 

 tricities flow together and neutrality is 

 restored. Prove this neutrality. Neither 

 a carrier touching the egg, nor the egg it- 

 self, has any power to pffect your elec- 

 troscope, or to attract your balanced lath. 

 Again, bring the excited tube near tho 

 egg. Touch its distant part b with your 

 carrier. The carrier now attracts the 

 straw (fig. 2) or the balanced lath (fig. 4). 

 It also causes tho leaves of your electro- 

 scope to diverge. What is the quality 

 of the electricity ? It repel* and i-i rc- 



rlled by rubbed glass ; the electricity at 

 is therefore positive. Discharge tho 

 carrier by touching it, and bring it into 

 contact with the end a of the cg.-j nearest 

 to the glass tube. The electricity you 

 take away repels and \s> repelled by gutta- 

 percha. It is therefore negative. Test 

 the quality also by the electroscope. 



While the tube o is near the egg touch 

 the end b with your finger ; now try to 

 charge the carrier by touching b : you 

 cannot do so the positive electricity has 

 disappeared* Has the negative disap- 



