CHAP. I.] ELECTRICITY AND MAGNETISM. 27 



insulated conductor, the - electricity is " bound," being 

 attracted, while the + electricity at the other end, being 

 repelled, is " free " ; and if the insulated conductor be 

 touched by a person standing on the ground, the " free " 

 electricity will flow away to the earth through his body, 

 while the " bound" electricity will remain, no matter 

 whether he touch the conductor at the far end, or at the 

 near end, or at the middle. 



25. Inductive method of charging the G-old- 

 leaf Electroscope. The student will now be prepared 

 to understand the method by which a Gold-leaf Electro- 

 scope can be charged with the opposite kind of electricity 

 to that of the excited body used to charge it. In Lesson 

 II. it was assumed that the way to charge an electro- 

 scope was to place the excited body in contact with the 

 knob, and thus permit, as it were, a small portion of the 

 charge to flow into the gold leaves. A rod of glass 

 rubbed on silk being + would thus obviously impart + 

 electricity to the gold leaves. 



Suppose, however, the rubbed glass rod to be held a 

 few inches above the knob of the electroscope, as is 

 indeed shown in Fig. 7. Even at this distance the gold 

 leaves diverge, and the effect is due to induction. The 

 gold leaves, and the brass wire and knob, form one con- 

 tinuous conductor, insulated from the ground by the 

 glass jar. The presence of the + electricity of the 

 glass acts inductively on this " insulated conductor," 

 inducing - electricity on the near end the knob, and 

 inducing + at the far end, *.*., on the gold leaves, 

 which diverge. Of these two induced charges, the - 

 on the knob is " bound," while the + on the leaves is 

 " free." If now, while the excited rod is still held above 

 the electroscope, the knob be touched by a person 

 standing on the ground, one of these two induced charges 

 flows to the ground, namely the free charge not that 

 on the knob itself, for it was " bound," but that on the 

 gold leaves which was " free " and the gold leaves 



