FRICTION AL ELECTRICITY. 201 



ball is attracted by a body brought near it, the body is electrified. 

 To determine the sign of the electricity of the body thus shown to 

 be electrified, the pith ball is allowed to touch it and be repelled. 

 If now the ball be repelled by a glass rod rubbed with silk (or by 

 any other body known to be positively charged), the pith ball and 

 the body in question manifest + electricity. If the pith ball, after 

 repulsion by the body whose electricity is under examination, be 

 repelled by sealing-wax rubbed with flannel (or by any other body 

 known to be negatively charged), the pith ball and the body in 

 question manifest electricity. 



(&.) One way of testing with the gold leaf electroscope is to 

 touch the knob or plate with the electrified body. The knob, rod 

 and leaves are thus charged with the same kind of electricity, 

 and the leaves diverge. If the leaves be rendered more diver- 

 gent by holding a positively electrified body near the knob, the 

 original charge was + ; if this effect be produced by a negatively 

 charged body, the original charge was . This method is objec- 

 tionable for the reason that if the original charge be at all intense 

 it is likely to tear the gold leaves. A safer method is as follows 

 Cautiously bring the electrified body near the knob ; the leaves 

 will diverge. Touch the knob with the finger ; the leaves will fall 

 together. Remove first the finger and then the electrified body ; 

 the leaves will diverge again. If now the divergence of the leaves 

 be increased by bringing a positively charged body near the knob, 

 the original charge was ; if the divergence be thus diminished, 

 the original charge was + . (See Appendix, K.) 



333. Conductors. From a horizontal glass rod or 

 tightly-stretched silk cord, suspend a fine copper wire,, a 

 linen thread- and two silk threads, each at least a meter 

 long. To the lower end of each attach a metal weight of 

 any kind. Place the weight supported hy the wire upon 

 the plate of the gold leaf electroscope. Bring the electri- 

 fied glass rod near the upper end of the wire ; the gold 

 leaves instantly diverge. Repeat the experiment with the 

 linen thread ; in a little ivliile the leaves diverge. Repeat 

 the experiment with the dry silk thread ; the leaves rlo not 

 diverge at all. Rub the rod upon the upper end of the 

 silk thread ; no divergence at all. Wet the second silk 



