concerning Volta' s Contact Force. 375 



itself if pressed, by using a lead electrode or a copper one 

 with an intense current). 



But if told that this cannot happen always,, and that the 

 energy of combination with something (say sulphur) must 

 be very different, I fully admit it ; but then I say that in 

 an atmosphere of which the active ingredient is sulphur (say 

 sulphuretted hydrogen) the Volta force will be different too. 

 Mr. J. Brown * has shown that it is, but the conclusiveness 

 of the experiment is evaded by pointing to films of sulphide 

 which introduce fresh contact-forces and complicate the circuit. 



So here begins a conflict of experimental testimony and 

 interpretation. Lord Kelvin depends on the experiments of 

 Mr. Bottomley in a hydrogen vacuum f. I am now able 

 to appeal to the still more elaborate and to me satisfactory 

 experiments of Mr. Spiers (Phil. Mag. Jan. 1900). In so 

 far as they are not conclusive, and I must assume that they 

 are not yet rigorously conclusive, they should be repeated 

 and improved. The research is bound to be laborious and 

 troublesome if properly executed. I will only say, in case 

 any young and energetic experimenter takes the matter up, 

 that Mr. Spiers seems to me in method and motive extremely 

 on the right track. 



The question of Expression in terms of Potential. 



Superposed upon the controversy as to the junction at 

 which the main contact force really exists, ther^ has arisen 

 another controversy, chiefly concerned with words or modes 

 of expression, concerning the most convenient method of 

 defining the potential of a conductor, and especially the 

 difference of potential between two metals. 



It has been customary with certain writers, among them 

 Professor Perry, to define the potential of a conductor as the 

 potential of a point in the air near it, — the work that must be 

 done to convey unit electric charge from the earth (or any 

 place of zero potential) to a point very near the conductor ; 

 or, better, to a point in a hollow surrounded by the conductor. 



* Phil. Mag. August 1878 et seq. 



t Mr. Bottomley sums up thus (British Ass: iciation Report, 1885, pp. 901- 

 903) :— " The result of my investigation, so far as it has gone, is that the 

 Volta contact-effect, so long as the plates are clean, is exactly the same 

 in common air, in a high vacuum, in hydrogen at small and full pressure, 

 and in oxygen. My apparatus, and the method of working during these 

 experiments, was so sensitive, that I should certainly have detected a 

 variation of 1 per cent, in the value of the Volta contact-effect, if such a 

 Variation had presented itself/' 



