18 ELEMENTS OF STATIC ELECTRICITY. 



the sides of which are too remote to be touched by the 

 leaves. 



A brass cap, covering the glass above, as shown at (7, 

 is also used to screen the leaves from external electric 

 influence, and wire screens are likewise used for the 

 same purpose. 



The use of the bottle, or glass shade, is to protect the 

 leaves from currents of air which would destroy them. 

 And the ebonite stopper is for better insulation, since 

 the glass generally used for bottles and shades is of 

 inferior insulating quality. The disc, or ball, and con- 

 necting-rod are for convenience in electrifying the 

 leaves, which are the efficient part of the instrument. 



The following experiment will illustrate its use : 

 Let the electrified sealing-wax touch the disc of electro- 

 scope A ; electricity is instantly transferred to the disc, 

 rod, and leaves, which are all good conductors, and the 

 leaves, being free to move, and at the same potential, 

 repel each other, and diverge. 



If the disc now be touched with the finger, the elec- 

 tricity escapes to the earth, and the leaves, being reduced 

 to zero, converge. 



The sensitiveness of this instrument is so great that a 

 chip of dry wood, less than a grain in weight, electrified 

 in cutting, and dropped on the disc, produces divergence 

 of the leaves. A wooden pen-holder, electrified by strik- 

 ing it lightly on the table, produces the same effect. 



Hence, care must be observed to prevent the leaves 

 from being torn by sudden, spasmodic movements, Avhich 

 are liable to occur when experimenting with highly elec- 

 trified bodies in their vicinity. 



MUTUAL EFFECTS OF FRICTION. Thus far we have 

 considered only the effect produced on the sealing-wax, 



