88 Lord Kelvin on 



the water arc still remains unbroken. You see the spot of 

 light, at the instant of metallic contact, suddenly leaves E 

 and moves to the right, and settles quickly at the metallic 

 zero after a few vibrations through diminishing range. 



(5) Lastly, I break the metallic contact, and hold the zinc 

 disc again parallel to the copper (fig. 4) with the water con- 

 nexion still remaining unbroken between them ; the spot of 

 light shows no sudden motion ; it creeps to the left till, in 

 half a minute or three-quarters of a minute, it reaches its 

 previous steady position on the left. This is the now well- 

 known phenomenon (never known to Yolta) of the recovery 

 of a voltaic cell from electrolytic polarisation after a metallic 

 short-circuit. 



§ 6. The succession of experiments described in § 5, inter- 

 preted according to elementary electrostatic law, proves the 

 following conclusions : — 



(1) When the dry and polished discs of zinc and copper 

 are metallically connected and held parallel, their opposed 

 faces are oppositely electrified, the zinc with vitreous elec- 

 tricity, and the copper with resinous electricity, in quantities 

 varying inversely as the distance between them when this is 

 small in comparison with the diameter of each. 



(2) The opposed polished faces are non-electrified when 

 polished portions of the zinc and copper surfaces are connected 

 by water, and when there is no metallic connexion between 

 them. Or, if not absolutely free from electrification, they 

 may be found slightly electrified, zinc resinously or vitreously, 

 and copper vitreously or resinously, according to differences 

 in respect to cleanness, polish, or scratching or burnishing, 

 as explained in § 1(3 below ; and according to polarisational 

 or other difference in the wetted portions of the surfaces. 



If instead of pure water we take a weak solution of common 

 salt, or carbonate of soda, or sulphate of zinc or ammonia, 

 Ave find results but little affected by the differences of the 

 liquids. 



§ 7. But if the polished surface of either the copper or the 

 zinc is oxidized, or tarnished in any way, notably different 

 results are found when the experiments of § 5 are repeated 

 with the disc or discs thus altered. 



For example, hold the copper disc, with its polished side 

 up, over a slab of hot iron, or a spirit-lamp, or a bunsen- 

 burner, till you see a perceptible change of colour, due to 

 oxidation of the previously polished face. Then allow the 

 copper to cool, and repolish a small area near one edge : 

 place a little mound of water upon this area, and operate as 

 in § 5 (2), (3). The water connexion between polished zinc 

 and polished copper brings the spot of light to the same elec- 



