30 Newbery, The Theory of Overvoltage. 



The hydrogen in the alloy is probably in the mon- 

 atomic condition, as has been proved in the case of 

 palladium/ and this would explain the so-called catalytic 

 effect of palladium, platinum, etc., in promoting the com- 

 bination of hydrogen and oxygen. When the current is 

 stopped, the alloy will tend to decompose spontaneously, 

 thus liberating gas within the electrode, which will further 

 increase the pressure beneath the surface. 



In the case of anodic overvoltage, which can rise to 

 much greater values than cathodic, the alloy or solid 

 solution is probably formed in many cases, not from the 

 gas itself but from a compound of the gas and the elec- 

 trode material. With certain metals, the presence of the 

 alloy produces passivity. 



(iv.) Lastly, when the gas comes out of the electrode 

 in the form of a tiny bubble, it acquires an electrostatic 

 charge on its surface as soon as it comes into contact 

 with the electrolyte. This charge is very close to the 

 electrode and therefore acts inductively on the electrode 

 reducing the measured overvoltage. The reduction is 

 directly proportional to the current density and also 

 varies inversely with the size of the escaping bubbles. 

 Experiment shows that the size of the bubbles varies with 

 the electrode chosen but is practically constant with a 

 given electrode. Hence at high current density there 

 may be a considerable lowering of the overvoltage from 

 this cause, in fact with platinum and mercury the over- 

 voltage can be reduced to a negative quantity at very 

 high current densities. 



An interesting experiment which serves to confirm 

 some of these views was made with a mercury electrode 

 I sq. cm. surface area. This was used as cathode in N/i 

 H S0 4 for a short time until its overvoltage was o*6 volt. 



:; Winklemann : Drudes Ann., 6, 1 04 (1901). 



