304: Prof. A. Anderson and Mr. H. N. Morrison 



on 



electrons from A to B and thence to earth ; and, in fact, A 

 will receive a positive charge. But the number of free 

 electrons per unit volume in A is still equal to that in B; 

 and, consequently, A must have lost some of its bound 

 electrons. Hence, when the number of bound electrons in 

 a metal increases it becomes more electronegative, and 

 when the number decreases it becomes more electropositive. 

 Different metals in contact have the same number of free 

 electrons per unit volume, but the number of bound electrons 

 varies, those metals which are more electronegative having 

 the greater number. 



Zinc, being electropositive to copper, has less bound 

 electrons per unit volume than copper when both are at the 

 same potential. This may be due to a difference in the atoms 

 of the two metals, on account of which the bound electrons 

 are more strongly held in the copper than in the zinc ; but 

 this does not imply that the positive atomic charges are 

 greater in the copper. It can be accounted for by sup- 

 posing a difference in the size, shape, or structure of the 

 positive atomic charges in different metals, just as the electric 

 field in the neighbourhood of a charged conductor depends 

 not only on its charge but also on its capacity. Indeed, 

 we might speak of the electric capacity of an atom 

 with perhaps as much propriety as we speak of its thermal 

 capacity. 



It is clear that in every case where a metal is rendered 

 more electropositive, work must be done on it, whether by 

 mechanical action or by some other agency; and, when it 

 becomes more electronegative, it does work at the expense 

 of its own energy, 



Here we may mention a difficulty referred to in a paper in 

 the i Proceedings ' of the Physical Society, vol. xxiv. part ii. 

 p. 105. It has to do with the energetics of a copper-zinc 

 uranium-oxide cell. Two plates, one of copper and the other 

 of zinc, connected by a wire and immersed in ionized air 

 form the cell. There will be a field of force producing a 

 current in the air between the plates, which we may suppose 

 to consist in the passage of electrons from the copper to 

 the zinc through the air between. If one effect of this 

 transference of electrons be to make the copper more, 

 and the zinc less, electropositive, the zinc will receive a 

 negative charge and the copper a positive one ; and one 

 effect will be a decrease of the contact difference of 

 potential. Whether this be so or not is a question to be 

 settled by experiment, which, however, presents some dif- 

 ficulty owing to the smallness of the currents and to the 



