376 Prof. 0. Lodge on the Controversy 



I have never been able to see why these philosophers hesi- 

 tate to complete the process and give the charge actually to 

 the conductor. Why do they stop short outside ? The 

 completion of the process is so easy that precaution is needed 

 to avoid it. 



It is usually an easier matter to measure experimentally 

 the potential of a conductor, a thing which can be connected 

 directly by a wire to an electrometer, than it is to measure 

 the potential of a point in air or other insulator; the latter 

 operation requiring a water-dropper or a smoking match or 

 an infinitely sharp point. 



It has been authoritatively stated that the heterogeneous 

 molecular structure of a metal makes it difficult to affix any 

 meaning to the potential of an inaccessible point inside it ; 

 but if this difficulty were a real one it would surely apply 

 still more forcibly to a heterogeneous molecular insulator. 

 The difficulty, however, seems to me purely imaginary from 

 any practical point of view ; we need have nothing to do with 

 the molecular structure of a metal in this connexion ; by 

 hypothesis it is a conductor, its molecules are all effectively 

 connected, and the potential of one point is the potential 

 of all. Nothing can be simpler than the statement that in 

 electrostatics every part of a conductor is at one potential. 

 We need not put on what Lord Kelvin has somewhere called 

 a molecular microscopic binocular, and examine the atoms on 

 all occasions. 



If asked to define the potential of a metal with reference 

 to some arbitrary standard conductor given as zero, I have 

 only to say — the work needed to transfer a small unit charge 

 from one to the other : or the potential energy of each unit 

 of charge existing on the metal, provided the removal of that 

 charge does not appreciably affect the potential : or, better, 

 dW/dQ. Where is the difficulty ? The process of charging a 

 conductor is experimentally easy, why not introduce the 

 process into the definition *. 



* Nevertheless, Lord Kelvin says : — u There has been much of wordy 

 warfare regarding potential in a metal, but none of the combatants has 

 ever told us what he means by the expression. In fact, the only defini- 

 tion 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 inter- 

 molecular space," &c. (Footnote to R. I. discourse, p. 13). 



He goes on, however, '' It would also vary intenselv from point to 

 point in the ether 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." And with this sentence I can quite agree ; as 

 well as with the bare fact stated at the end of the previous sentence, 



