chap. I.] GOLD-LEAF ELECTROSCOPE. 7 



they reappear. That is to say, the charged conductor 

 s has by its influence decomposed the neutral elec- 

 tricity of AB. Since unlike electricities attract, s being 

 charged positively, negative electricity will be at- 

 tracted to the end A, while the negative electricity of 

 AB will be repelled to the end B. At a place near the 

 centre there will be a neutral zone. The neutral zone 

 will be on the A side of the centre, because, owing to 

 the difference of distance, negative electricity will be 

 more strongly attracted towards A than positive re- 

 pelled to B. As soon as the influence of B is removed, 

 the separated electricities re-combine. This pheno- 

 menon is called induction, or electrification by in- 

 fluence. If, while s is in the neighbourhood of AB, 

 connection between the earth and AB be made, say 

 by touching AB with the finger, the repelled electricity 

 ( + ), being free, will escape. The negative electricity 

 will remain attracted, and the pith-balls will collapse, 

 indicating the absence of free electricity. If now, 

 AB being again insulated, s be removed, the bound 

 electricity will become free, and the pith-balls will 

 again diverge. AB will thus have received by in- 

 duction a charge of negative electricity. 



It is by induction that an electrified body attracts 

 one unelectrified. It induces opposite electricity to 

 its own on the end near to it, and the two unlike elec- 

 tricities attract one another. The similar electricity is 

 repelled to the end farthest from the electrified body ; 

 and as the repellent force has thus to act through a 

 greater distance, the attractive power has the advantage. 



The gold-leaf electroscope, for indicating the 

 presence and kind of electricity in any body, has been 

 constructed by taking advantage of the fact of induc- 

 tion. It consists of a metallic stem BB (Fig. 3), from 

 one end of which hang, parallel to one another, two 

 very fine gold leaves a b. The other end terminates 

 in a metallic knob. The rod is fixed in the tube of a 



