PHYSICO-CHE.MICAL BASIS OF TRANSMISSION' 405 



the surface is in both cases attended with chemical 

 change. The chief peculiarity of the chemical reactions 

 occurring under the inlluence of electric currents in 

 electro-chemical circuits is that they are confined to the 

 boundary region, where the current passes between the 

 electrode and the electrolyte solution. Xo chemical 

 change occurs in the interior of either ])hase; the reactions 

 are surface reactions. Since the region of transition from 

 the metallic conductor to the electrolytic conductor is 

 the only region in the circuit where the passage of the 

 current involves chemical change, the fundamental 

 question relates to the general ])hysical nature of the 

 conditions in this region while the current is llowing. 

 According to modern i)hysico-chemical theory, the 

 carriers of the current in the electrolyte solution arc the 

 anions and cations of the dissociated electrolyte; in the 

 metal the carriers are free electrons. At the surface of 

 contact there is a transfer of electrons between the 

 electrode and the ions of the solution. For example, at 

 the anode, ferrous ions are o.xidized to ferric ions; on 

 the electron theory this implies the transfer of an electron 

 from each ferrous ion to the electrode; conversely, at the 

 cathode electrons are transferred from the metal to the 

 ions in solution, H ions becoming uncharged H atoms. 

 To effect tliis transfer a certain })otential gradient is 

 required; it would appear therefore that the region of 

 transition represents that portion of the circuit where the 

 potential gradient is steepest and where the forces acting 

 to displace electrons are greatest. That the contact 

 of two dissimilar conductors, respectively metallic and 

 electrolytic, is not the essential condition, but rather the 

 existence of a large fall of potential across a short distance, 



