28 Newbery, The Theory of Overvoltage. 



free atoms and molecules. If these are gaseous, they will 

 be held against the electrode surface partly by the attrac- 

 tion of the metal due to its electrical charge and partly 

 by the pressure of incoming ions. The latter will find 

 the nearest portions of the electrode already occupied, 

 and therefore will have to get further and further in to 

 the electrode before finding a vacant spot where they can 

 make electrical contact and give up their charges. 2 When 

 they have given up their charges they will unite to form 

 molecules and then escape as they are knocked off by the 

 general disturbance taking place. 



The electrode surface probably compares with an ion 

 much as a bundle of sticks or a pile of loose stones does 

 with a bullet. Hence there will be many cases where ions 

 will enter a hole too small to allow molecules to escape. 

 Enormous pressures are thus set up by the imprisoned 

 molecules which can only escape by re-ionising or breaking 

 the metal surface. The deeper the hole, the greater the 

 pressure. The overvoltage of an amalgamated zinc 

 cathode in dilute sulphuric acid can rise to over 087 

 volt under certain conditions. The calculated pressure to 

 produce this is io 30 atmospheres. While it is evident that 

 the pressures in the electrode are very great, such a 

 pressure as this passes the bounds of credibility. It is 

 necessary therefore to consider further what happens to 

 the gas at the electrode before it is liberated. 



(It has of course been assumed that hydrogen behaves 

 as a perfect gas, an assumption which cannot be true at 



2 Rapid stirring of the electrolyte reduces overvoltage considerably by 

 ' brushing' away some of the adhering surface gas. Also during overvoltage 

 measurements, cases are frequently observed where the overvoltage remains 

 perfectly steady but the current passing through the experimental cell is 

 erratic, the jerky variations being sometimes greater than 10% of the main 

 current although the applied E. M.F. is quite constant. This variation of 

 resistance between the electrode and electrolyte is evidently due to distur- 

 bances in the eas fii m adhering to the external surface of the electrode. 



