﻿A/echanism of Electrical Conduction, 67 



not exist. But since we suppose the fixed charge in each 

 molecule to be incompletely screened by the outer particles, 

 it follows that at external points in the immediate neigh- 

 bourhood of the metallic body the potential is raised above 

 the conduction-potential by the fixed central charges. If 

 these last were negative instead of positive, the potential 

 just outside the metallic mass would be lower than the 

 conduction-potential ; and we may suppose that at any 

 given temperature (such as the absolute zero with which 

 we are dealing) the difference between the conduction- 

 potential of a metallic body and the potential just outside the 

 body depends upon the nature of the metal. Thus, even 

 in vacuo, if two metals at the absolute zero of temperature be 

 connected together so as to have the same conduction- 

 potential, their induction-potentials may be different; and in 

 general, whatever the temperature of the metals in contact, 

 we may expect an inequality between difference of conduction- 

 potential and the difference of mdfzic£io?i-potential. 



Before attempting to devise a model of Peltier's phenome- 

 non and of electromotive forces of contact, I had held the 

 opinion — in common, I believe, with the majority of dis- 

 putants in the contact-force controversy — that the inductive 

 measurement of potential-differences in a sufficiently perfect 

 vacuum must conclusively decide the points at issue. But if 

 in reality there should be, as the model suggests, a difference 

 between conduction-potentials and induction-potentials, we 

 must not rely upon inductive experiments, even in a perfect 

 vacuum, to determine the seats of electromotive force in a 

 voltaic cell. For when we are dealing with the flow of cur- 

 rents through metals, it is the conduction-potential which 

 concerns us. 



11. The Transparency of Metals. 



A difficulty in connexion with this subject is stated by 

 Maxwell in the following well-known passage*: — "Gold, 

 silver, and platinum are good conductors, and yet, when 

 formed into very thin plates, they allow light to pass through 

 them. From experiments which I have made on a piece of 

 gold-leaf, the resistance of which was determined by Mr. 

 Hockin, it appears that its transparency is very much greater 

 than is consistent with our theory, unless we suppose that 

 there is less loss of energy when the electromotive forces are 

 reversed for every semi-vibration of light than when they act 



* 'Electricity and Magnetism,' 2nd ed. vol. ii. §800. Wien (Wiede- 

 mann's Annalen, xxxv. pp. 41-62) found a silver iilm to have only such 

 an opacity as would be deduced from about 1/440 of its actual con- 

 ductivity/ 



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