1897.1 on Contact Electricity of Metals. 533 



polishing the metal plate to be tested on clean glass paper or emery 

 cloth, and then measured its difference of potential from the standard 

 gold plate. After that the plate was subjected to some particular 

 treatment, such as filing or burnishing ; or polishing on leather or 

 paper ; or washing with water, or alcohol, or turpentine, and leaving 

 it wet or drying it ; or heating it in air, or exposing it to steam or 

 oxygen, or fumes of iodine or sulphuretted hydrogen ; or simply 

 leaving it for some time under the influence of the atmosphere. 

 The plate as altered by any of these processes was then measured 

 for potential against the standard gold. Very interesting and in- 

 structive results were found ; only of one can I speak at present. 

 Burnishing by rubbing it firmly with a rounded steel tool, or by 

 rubbing two plates of the 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 . . . . -f- 1*02 volt. 



It was then scratched by polishing on glass paper, 

 and its surface potential returned to its original 

 value of + -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 homogeneous interior) 



* 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 fai-t 

 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 deficite 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 ether or 

 air outside the metal at distances from the frontier small or moderate in com- 

 parison 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 equilibrium and motion 

 of electricity through metals with outer surfaces bounded by ether or air or other 



Vol. XV. (No. 91.) 2 n 



