LESSORS IN ELECTRICITY. 



305 



FIG. 19. 



yonr glass tube B, and bring it near one 

 of the balls ; test both : the near one, T', 

 is negative, the distant one, T, positive. 

 Touch the near one, the positive elec- 

 tricity, which had been driven along the 

 chain to the remotest part of the system, 

 returns along the chnin, passes through 

 (he negative, which is held captive by 

 the tube, and escapes to the earth. 

 When the tube R is removed, negative 

 electricity overspreads both chain and 

 balls. 



In fig. 8 you made the acquaintance of 

 the plate N, and the straw-index i i y , 

 shown on a smaller scale in fig. 19. By 

 their means you immediately see both 

 the effect of the first induction, and the 

 consequence of touching any part of tha 

 system with the finger. The plate K 

 rests over the ball or turnip T, the posi- 

 tion of the straw-index being that shown 

 by the dots. Bring the rubbed tube neat 

 T' : the end N of the index immediately 

 descends and the other end rises along 

 the graduated scale. Remove the glass 

 rod; the index 1 1' immediately falls. 

 Practice this approach and withdrawal, 

 and observe how promptly the index de- 

 clares the separation and rccomposition 

 of the fluids. 



While the tube is near T', and the end 

 S T of the index is attracted, let T' be 

 touched by the finger. The end N is im- 

 mediately liberated, for the electricity 

 which pulled it down escapes along the 

 chain and through the finger to the 

 earth. Now remove your excited tube. 

 The captive negative electricity diffuses 

 itself over both balls, arid the index is 

 again attracted. 



Instead of the chain you may interpose 

 between the balls 100 feet of wire sup- 



by silk loops. This is done m 

 fig. 20, which shows the wire w support- 

 ed by the silk strings 8 s s. For the bafl 

 or turnip T', fig. 19, the cylinder c, on a 

 glass support a, is substituted, the little 

 table M taking the place of the ball T. 

 Every approach and withdrawal of the 

 rubbed glass tube R is followed obedient- 

 ly by the attraction and liberation of N, 

 and the corresponding motion of the in- 

 dex N I. 



Repeat here an experiment, first 



