METHODS OF ELECTRICAL STIMULATION 33 



formed in equivalent quantities, so that there is 

 no difference in electrical pressure. 



When metallic zinc is placed in dilute sulphu- 

 ric acid, it dissolves, and positively charged zinc 

 ions enter the solvent, but no negative ions are 

 formed at the same time. The solution next 

 the metal becomes statically charged with posi- 

 tive electricity, and in consequence the zinc itself 

 becomes negatively charged. The electrical stress 

 thereby produced compensates the difference 

 between the osmotic pressure of the zinc ion and 

 the electrolytic solution pressure of the zinc. 



When copper is placed in a solution of copper 

 sulphate, the osmotic pressure of the metal ion 

 exceeds the electrolytic solution pressure of the 

 metal and metallic copper is deposited on the 

 surface of the electrode. The metal becomes 

 positively charged and the solution negatively 

 charged through the sulphanions that gather at 

 the layer of solution next the metal. This pro- 

 cess continues until the electrical stress balances 

 the difference between the actual osmotic pres- 

 sure and the electrolytic solution pressure. 



The electrical double layer about each elec- 

 trode is of only molecular thickness, so that the 

 solution or deposition of an extremely small 

 quantity of metal is sufficient to establish equi- 

 librium. 



