626 Lord Kelvin ^ [May 21, 



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 connection 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 Volta) of the re- 

 covery of a voltaic cell from electrolytic polarisation after a metallic 

 short-circuit. 



§ 6. The succession of experiments described in § 5, interpreted 

 according to elementary electrostatic law, proves the following con- 

 clusions : — 



(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 electricity, 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 connection between them. Or, if not 

 absolutely free from electrification, they may be found slightly elec- 

 trified, zinc resinously or vitreously, and copper vitreously or resi- 

 nously, according to difierences in respect to cleanness, polish, or 

 scratching or burnishing, as exj)lained in § 16 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, we 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 

 oxidised, 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 connection between polished zinc and 

 polished copper brings the spot of light to the same electrolytic zero 

 E as before. But now, when we lift the zinc disc and break the water 

 connection, the spot of light moves to the right, instead of remaining 

 steady as it does when both the dry opposed surfaces are polished. If 



