2 Newbery, The Theory of Overvoltage. 



have an electrolytic cell with an E.M.F. of about 112 

 volt. 



This cell is reversible, that is to say, if we allow it to 

 give a current, hydrogen and oxygen will unite to form 

 water, while if we pass a current through the cell in the 

 opposite direction, the same gases will be generated from 

 the water present. 



We should expect therefore that the same E. M.F., 

 I'i2 volt, when applied to the above cell, would be just 

 sufficient to decompose an acid or alkaline solution with 

 formation of oxygen and hydrogen. In 1899, Caspari 

 discovered that this was not the case, but that the E.M.F. 

 necessary to liberate these gases was always greater than 

 1*12 volt, and further that the excess E.M.F. above the 

 theoretical value was different when different electrodes 

 were used, and was not the same for anode and cathode. 



This excess E.M.F. he termed the ' Overvoltage ' of 

 the electrolytic cell. 



In any electrolytic cell, therefore, when a current is 

 passed through, a back E.M.F. is set up at each electrode 

 opposing the applied E.M.F., and this back E.M.F. is in 

 each case greater than that which would be obtained if 

 the electrode were merely surrounded by the substance 

 which is being liberated at that electrode. 



We may therefore call this excess back E.M.F. at each 

 electrode the Cathodic and the Anodic Overvoltage res- 

 pectively. 



Similar phenomena occur at the electrodes during 

 deposition or dissolution of a metal, a back E.M.F. being 

 set up at each electrode even w.hen the same metal is 

 being deposited at one electrode that is dissolved at the 

 other. We may speak of such overvoltages as ' metal 

 overvoltages,' while the term ' anodic or cathodic over- 

 voltage of an electrode' will imply the overvoltage when 



