CHAP. I,] ELECTRICITY AND MAGNETISM. 13 



protect them from draughts of air and to support them 

 from contact with the ground. Through the cork, which 

 should be varnished with shellac or with paraffin wax, is 

 pushed a bit of glass tube, also varnished. Through this 

 passes a stiff brass wire, the lower end of which is bent 

 at a right angle to receive the two strips of gold-leaf, 

 while the upper supports a flat plate of metal, or may be 

 furnished with a brass knob. When kept dry and free 

 from dust it will indicate excessively small quantities of 

 electricity. A rubbed glass rod, even while two or three 

 feet from the instrument, will cause the leaves to repel 

 one another. The chips produced by sharpening a pencil, 

 falling on the electroscope top, are seen to be electrified. 

 If the knob be even brushed with a small camel's hair 

 brush, the slight friction produces a perceptible effect. 

 With this instrument all kinds of friction can be shown 

 to produce electrification. Let a person, standing upon 

 an insulating support, such as a stool with glass legs, 

 or a board supported on four glass tumblers, be briskly 

 struck with a silk handkerchief, or with a fox's tail, or 

 even brushed with a clothes' brush, he will be electrified, 

 as will be indicated by the electroscope if he place one 

 hand on the knob at the top of it. The Gold-Leaf 

 Electroscope can further be used to indicate the kind of 

 electricity on an excited body. Thus, suppose we rubbed 

 a piece of brown paper with a piece of indiarubber and 

 desired to find out whether the electricity thus excited on 

 the paper was + or - , we should proceed as follows : 

 First charge the gold leaves of the electroscope by 

 touching the knob with a glass rod rubbed on silk. 

 The leaves diverge, being electrified with + elec- 

 tricity. When they are thus charged the approach of 

 a body which is positively electrified will cause them to 

 diverge still more widely ; while, on the approach of one 

 negatively electrified, they will tend to close together. 

 If now the brown paper be brought near the electroscope, 

 the leaves will be seen to diverge more, proving the 



