Manchester Memoirs, Vol Ix. (1916), No. 11. 19 



produce the flattened polished surfaces which are obtained 

 during metal deposition in presence of colloids. 



c. General. 



The effect of current density has been referred to 

 several times already. It varies greatly with different 

 electrodes and is sometimes so great that any statement 

 as to the overvoltage of a particular metal is useless 

 unless the current density is also stated. In the case of 

 mercury the overvoltage may vary from a very high value, 

 07 volt, to a negative value, — C05 volt, when the current 

 density is raised from a low to a high value. 



In almost all cases the overvoltage rises with the 

 current density to a certain maximum and then falls 

 with still greater current density. 



The current density required to produce this maximum 

 varies to a remarkable extent and is affected to a greater 

 degree than overvoltage itself by the nature of the 

 electrodes, impurities in the electrolyte, time, etc,, etc. 



With some electrodes (Hg, Ag, W for example) it is 

 difficult to apply a current density low enough to obtain 

 the true maximum, specially when time effects are 

 present ; with others (gas carbon, amalgamated zinc, 

 tantiron, etc.) the maximum is barely attained with the 

 enormous current density of 200 amperes per sq. dm. 



Generally speaking, the harder and the more porous 

 the surface of the metal, the higher will be the current 

 density required to produce maximum overvoltage, but 

 this rule has exceptions. 



The effect of time is similar to and largely dependent 

 on current density. 



The general rule to which there are few, if any, ex- 

 ceptions is that at low current densities the overvoltage 

 increases with time up to a certain maximum and then 



