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THE POPULAR SCIENCE MONTHLY. 



of the attraction of neutral bodies by our excited tubes. In reality, 

 the attracted bodies are not neutral : they are first electrified by in- 

 fluence, and it is because they are thus electrified that they are at- 

 tracted. 



This is the place to stamp upon your mind the following considera- 

 tions: Neutral bodies, as just stated, are attracted, because they are 

 really converted into electrified bodies by induction. Suppose a body 

 to be feebly electrified positively, and that you bring your rubbed 

 glass-rod to bear upon the body. You clearly see that the induced 

 negative electricity may be strong enough to mask and overcome the 

 weak positive charge possessed by the body. We should thus have 

 two bodies electrified alike, attracting each other. This is the clanger 

 against which I promised to warn yon in Section 10, where the test of 

 attraction was rejected. 



We will now apply the principle to explain a very beautiful inven- 

 tion, made known by the celebrated Volta in 1775. 



Sec. 14. The Electrophorus. Cut a circle, T (Fig. 15), six inches 

 in diameter, out of sheet-zinc, or out of common tin. Heat it at its 



Fig. 15. 



centre by the flame of a spirit-lamp or of a candle. Attach to it there 

 a stick of sealing-wax, H, which, when the metal cools, is to serve as 

 an insulating handle. You have now the lid of the electrophorus. A 

 resinous surface, or what is simpler a sheet of vulcanized India-rubber, 

 P, or even of hot brown paper, will answer for the plate of the elec- 

 trophorus. 



Rub your " plate " with flannel, or whisk it briskly with a fox's 

 brush. It is thereby negatively electrified. Place the "lid" of your 

 electrophorus on the excited surface : it touches it at a few points only. 

 For the most part lid and plate are separated by a film of air. 



The excited surface acts by induction across this film upon the lid, 

 attracting its positive and repelling its negative electricity. You 

 have in fact in the lid two layers of electricity, the lower one, which 

 is " bound," positive ; the upper one, which is " free," negative. Lift 





