﻿Mechanism of Electrical Conduction. W.\ 



Let us now turn to the case of a considerable mass of metal, 

 made up entirely of such molecules as that sketched in fig. 2; 

 every molecule having the same structure and the same 

 internal charge. Let us suppose also that the mass of metal 

 is at the same temperature and in the same physical condition 

 throughout, so that the average distance between adjacent 

 molecules is the same in the neighbourhood of each point. 

 If the metallic body is subjected to electrostatic induction, it 

 is easy to see that no electric influence from without can 

 penetrate far beyond the surface ; for the outer particles 

 of the superficial molecules, owing to their intermittent 

 contacts with one another, will screen the interior of the 

 metal as a network of continuous wires might do, and at a 

 depth of a very few molecules beneath the surface the 

 screening will be practically complete. Thus any charge 

 communicated to the conductor will be confined entirely to 

 the superficial layers of molecules; and it follows that if 

 between any two points well within the metal there were a 

 difference of average conduction-potential, there would be a 

 general flow of electrification from the higher towards the 

 lower potential. The average conduction-potential within the 

 metal does not therefore vary from point to point, and account 

 being taken of the equality of the internal charges of the 

 molecules, it follows that the average potential of the inter- 

 molecular ether is similarly free from variation. This implies 

 that the total electrification of any considerable assemblage 

 of molecules within the metal is sensibly zero; so that the 

 average charge on the outer conductive particles of each 

 molecule is equal and opposite to the fixed charge on the 

 central particle. The conclusions of this paragraph have 

 been necessarily confined to molecules not too near the 

 surface; for although the general tendency of an encounter 

 between two molecules is always to equalize their conduction- 

 potentials, these potentials change somewhat after the mole- 

 cules have become separated, and near the free surface of the 

 body the change will be systematically greater for the 

 molecule which moves outwards after collision than for that 

 which moves inwards. 



We may now try to realize what will happen when contact 

 takes place between two metals whose molecules are con- 

 structed on the general plan of fig. 2, while those of the one 

 metal are not identical with those of the other. The most 

 simple and intelligible view will be obtained by supposing the 

 molecule of the one metal to have a positively charged central 

 particle, while the molecule of the other has its central 

 particle negatively charged (fig. 3). Consider what would 



