96 Lord Kelvin on 



same metal together, increased the potential in every case ; 

 that is to say made the metallic surface more positive if it 

 was positive to begin with ; or made it less negative or 

 changed it from negative to positive, if it was negative to 

 begin with. Thus : — 



Zinc immediately after being scratched 

 sharply by polishing on clean glass- 

 paper was found -f- '70 volt. 



After being burnished with hard steel 



burnisher it was found + *94 volt. 



After being left to itself for 2 hours it 



was found + "92 volt. 



After further burnishing + 1*00 volt. 



After still further burnishing .... +1*02 volt. 



It was then scratched by polishing on 

 glass-paper, and its surface potential 

 returned to its original value of . . -f "70 volt. 



§ 16. This seems to me a most important result. It cannot 

 be due to the removal of oxygen, or oxide, or of any other 

 substance from the zinc. It demonstrates that change of 

 arrangement of the molecules at the free surface, such as is 

 produced by crushing them together, as it were, by the 

 burnisher, affects the electric action between the outer surface 

 of the zinc and the opposed parallel gold plate. It shows 

 that the potential * in zinc (uniform throughout the homo- 



* There has been much of wordy warfare regarding potential in a 

 metal, but none of the combatants has ever told what he means by the 

 expression. In fact, the only definition of electric potential hitherto 

 given has been for vacuum, or air, or other fluid insulator. Conceivable 

 molecular theories of electricity within a solid or liquid conductor might 

 admit the term potential at a point in the interior ; but the function so 

 called would vary excessively in intermolecular space, and must have 

 a definite value for every point, whether of intermolecular space or within 

 the volume of a molecule, or within the volume of an atom, if the atom 

 occupies space. It would also vary intensely from point to point in 

 the aether or air outside the metal at distances from the frontier small or 

 moderate in comparison with the distance from molecule to molecule in 

 the metal. 



But when, setting aside our mental microscopic binocular which shows 

 us atoms and molecules, we deal with the mathematical theory of equi- 

 librium and motion of electricity through metals with outer surfaces 

 bounded by aether or air or other insulating fluids or solids, we find 

 it convenient to use a mathematical function of position called potential 

 in the interior of each metal. This function must, for the case of equi- 

 librium, fulfil the condition that it is of uniform value through each 

 homogeneous portion of metal. Its value must, as a rule, change gra- 



