1877.] 



Contact of different Substances. 



303 



result of an experiment described by Professor P. Jenkin in his ' Text 

 Book of Electricity and Magnetism but this experiment seems to leave 

 something to be desired on the score of sensitiveness, and I was anxious 

 to put the conclusion to a somewhat severer test. With this object the 

 following experiment was performed : — 



The condenser was furnished with carefully cleaned plates of zinc and 

 copper, which were connected as before by insulated copper wires with 

 the respective terminals of the electrometer. Into a vessel containing 

 distilled water two insulated plates of zinc and copper respectively (both 

 carefully cleaned) were plunged and connected with the key above men- 

 tioned by copper wires. By means of this key the zinc plate in water 

 was connected with the zinc condenser-plate, and the copper plate in 

 water with the copper condenser-plate, the two condenser-plates being 

 separated by an interval certainly not more than 0*2 millim. The needle 

 of the electrometer was at once deflected, and in a few minutes became 

 stationary in a position showing a considerable deflection due to the 

 difference of potential of the terminals of the zinc-water- copper ele- 

 ment, while the zinc and copper plates of the condenser are necessarily 

 at the same potentials as the zinc and copper plates in the water. 



The key was now opened, so that the condenser-plates and electro- 

 meter, still in connexion as before, became insulated, and no change in 

 the deflection of the electrometer-needle was perceptible. 



The condenser-plates were now quickly separated, but the electrometer- 

 needle remained absolutely undisturbed. It appears, then, that the zinc 

 and copper plates in water are so nearly at the same potential, that the 

 apparatus employed fails to show the difference between their potentials. 

 Considering that if the zinc and copper condenser-plates are connected 

 by a wire when at the same distance apart, and are then insulated and 

 separated in the same way, the needle is so strongly deflected as to be 

 turned completely round, I feel justified in stating that clean zinc and 

 copper plates when first plunged in distilled water, if not absolutely at 

 the same potential, certainly do not differ in potential by the thousandth 

 part of the difference of potential produced by the contact of zinc and 

 copper. 



The electromotive force, then, of a voltaic element composed of zinc and 

 copper plates dipping in distilled water, and connected by a copper wire, 

 is at first, for all practical purposes, due entirely to the difference of po- 

 tential produced by the contact of zinc and copper, the water having no 

 perceptible effect. 



This was assumed by Volta, on less conclusive evidence, and forms the 

 basis of his reasoning with respect to the action of the pile devised by him. 



I cannot, however, agree with Professor Jenkin that the water is also 

 at the same potential as the zinc and copper plates immersed in it. The 

 above-mentioned experiments seem to me to show that the water is 

 negative to both plates, but to the same extent, 



VOL. XXVI. Y 



