Contact Electricity of Metals. 



83 



other, which is kept metallically connected with the sole 

 plate of the electroscope by a thin flexible wire. 



To commence the experiment I place one disc resting on the 

 other, and lift the two till the 

 upper touches a brass knob 

 connected by a stiff metal wire 

 with the lower plate of the 

 Volta condenser. I break this 

 contact and then lift the upper 

 plate of the condenser ; you see 

 no divergence of the gold leaves. 

 This proves that no disturbing- 

 electric influence sufficient to 

 show any perceptible effect 

 on our gold-leaf electroscope 

 is present. Now I repeat what 

 I did, with only this change — 

 I hold the lower disc with the 

 upper disc resting on it two 

 or three centimetres below the 

 knob. I then with my right 

 hand lift the upper plate of the 



•Volta-condenser ; you see a very slight divergence between 

 the shadows of the gold leaves on the screen. I can just see 

 it by looking direct at the leaves from a distance of about 

 half a metre. Still holding the lower plate firmly in my left 

 hand in the same position, and holding the upper plate by 

 the top of its glass stem in my right, at first resting on the 

 lower plate I lift it and let it down very rapidly a hundred 

 times, so as to produce one hundred cycles of operation — 

 break contact between discs, make and break contact between 

 uppar disc and knob, make contact between discs. Lastly, 

 I lift the upper plate of the condenser ; you see now a great 

 divergence of the gold leaves, many of you can see it direct 

 on the leaves, while all of you can see it by their shadows on 

 the screen. Now, keeping the upper plate of the condenser 

 still unmoved, I bring a stick of rubbed sealing-wax into the 

 neighbourhood of the electroscope ; you see the divergence 

 of the leaves is increased. I remove the sealing-wax and 

 the divergence diminishes to what it was before. This proves 

 that the gold leaves diverge in virtue of resinous electricity 

 upon them, and therefore that the insulated plate of the 

 condenser received resinous electricity from the copper disc. 

 If now I interchange the two discs so that the upper is zinc 

 and the lower copper, and repeat the experiment, you see 

 that the rubbed sealing-wax diminishes the divergence as it 



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