Manchester Memoirs, Vol. Lx. (191 6), No. 11. 13 



excess back E.M.F. or over-voltage is obtained, free 

 from any complications due to internal resistance of the 

 cell, gas films, etc. The relative positions of the elec- 

 trodes in the cell is immaterial since no current is passing 

 through the cell while measurements are being taken, and 

 no current passes through the galvanometer when the key 

 K„ is in position. 



By reversing the current through P 2 and P lt anodic 

 over-voltages may be measured in the same way. 



The standard hydrogen electrode H 1 may be replaced 

 by more convenient electrodes — calomel, mercurous sul- 

 phate, mercuric oxide, etc. — allowance being made for the 

 difference of single potential in each case. 



The experimental electrodes are best prepared from 

 small strips or rods of metal, the surface of which is 

 carefully cleaned with fine glass paper and coated with 

 sealing wax or paraffin wax, leaving 1 sq. cm. only un- 

 covered. The current density per sq. cm. is then obtained 

 by doubling the reading of the milliammeter M, since the 

 current is passing for only half the time. 



2. Phenomena and Controlling Factors of 

 overvoltage. 



The number and variety of the factors affecting over- 

 voltage is so great that the merest outline of the most 

 important is all that can be given here. 



We may discuss them under three heads : — 



a. In the electrodes — 

 Nature of electrodes. 

 Impurities in electrodes. 

 Nature of surface. 

 Previous history of electrode. 



