106' Lord Kelvin on 



15 times that of zinc. We are therefore quite sure that the 

 increase of attraction according to the inverse square of the 

 distance is not continued to such small distances as 10~ 9 of 

 a centimetre ; and at distances less than this the electric 

 attraction merges into molecular force between the two 

 metals. 



§ 29. Consider, now, a large number of discs of zinc and 

 copper, each of 1 square centim. area and thickness D, and 

 polished on both sides. On one side of each disc attach three 

 very small columns, of length D, of glass or other insulating- 

 material, and place one disc on top of the insulators of 

 another, zinc and copper alternately, so as to make a dry 

 insulated pile of the metal discs separated by air spaces each 

 equal to the thickness D. If in the building of this pile each 

 disc is kept metallically connected with the one over which it 

 is placed while it is being brought into its position, work will 

 be done upon it by electric attraction to the amount shown in 

 column 3, and the total work of electric attraction during the 

 building of the pile will be the amount shown in column 3 

 multiplied by one less than the number of discs. 



But if each disc, after being metallically connected with the 

 one on which it is to be placed till it comes within some con- 

 siderable distance — say 300 D, for example, from the disc 

 over which it is to rest — is then disconnected and kept insu- 

 lated while carried to its position in the pile, no work will be 

 done on it by electric attraction. And if now, lastly, metallic 

 connexion is made between all the discs of the pile, currents 

 pass from each copper to each zinc disc, and heat is generated 

 to an amount equal to that shown in column 4, multiplied by 

 one less than the number of discs ; and if this heat is allowed 

 to become uniformly diffused through the metals, they rise in 

 temperature to the extent shown in column 6. 



All these statements assume that the electric attraction 

 increases according to the inverse square of the distance 

 between opposed faces of zinc and copper. We have already 

 (§ 28) seen that this assumption cannot be extended to such 

 small distances as 10~ 9 of a centimetre. We have now 

 further proof of this conclusion beyond the possibility of 

 doubt, because the large numbers in columns 5 and 6 for 

 10~° are enormously greater than any rational estimate we 

 can conceive for the heat of combination of equal parts of 

 zinc and copper per gramme of the brass formed. (See § 32 

 below.) 



§ 30. When, on a Friday evening in February 1883 — 

 fourteen years ago — quoting from an article which had been 

 published in Nature* in 1879, I first brought these views 



