162 



THE POPULAR SCIENCE MONTHLY. 



straw-index I T, shown in Fig. 13. By its means you immediately 

 see both the effect of the first induction and the consequence of 

 touching any part of the system with the finger. The plate N rests 

 over the ball or turnip T, the position of the straw-index being that 

 shown by the dots. Bring the rubbed tube near T': the end Not the 

 index immediately descends and the other end rises along the grad- 

 uated scale. Remove the glass rod ; the index II' immediately falls. 

 Practise this approach and withdrawal, and observe how promptly 

 the index declares the induction and recomposition of the fluids. 



While the tube is near T, and the end iVof the index is attracted, 

 let T' be touched by the finger. The end JVis immediately 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, and 

 the index is again attracted. 



Instead of the chain you may interpose between the balls one 

 hundred feet of wire supported by silk loops. This is done in Fig. 

 14, which shows the wire w supported by the silk strings S S JS, and 

 where, for the ball or turnip, the cylinder C, on a glass support G, is 

 substituted. Every approach and withdrawal of the rubbed glass 

 tube H is followed obediently by the corresponding motion of the 

 index. 



&& 



Fia. 14. 



Or, substituting a carrot, a cucumber, or other elongated conduct- 

 or for the ball T', Fig. 12, you cause your rubbed glass tube to act 

 upon a greater extent of surface. You thus decompose more elec- 

 tricity and produce a greater attraction. 



Repeat here an experiment, first made by a great electrician named 

 iEpinus. I wish you to make these grand old experiments. Support 

 an elongated metal conductor, or one formed of wood coated with 

 tin-foil even a carrot, cucumber, or parsnip, so that it will be insu- 



