Prof. Franz Exner on Contact Electricity. 293 



If two bright copper plates be in contact in air, they show 

 no difference of potential, — according to the contact theory, 

 because they are the same substance ; according to the che- 

 mical theory, because the action of the air is equal on both. 

 If now this action be removed from one plate, e. g. by oxida- 

 tion in a flame, it is well known that a great difference of 

 potential is obtained — which, according to the chemical theory, 

 is because only one of the plates now continues to oxidize in 

 the air and the other not ; according to the contact theory, 

 because now two substances are brought in contact, viz. 

 copper and copper oxide. Which of the two explanations is 

 correct is decided by the above experiment with silver in 

 chlorine. In the case where one of the silver plates is in air 

 and the other in chlorine, there is a certain difference of po- 

 tential so long as the action of the chlorine continues. As 

 soon, however, as both plates are surrounded by air, the dif- 

 ference of potential disappears altogether, since now the 

 action on both plates is again the same. This experiment 

 with silver and chlorine succeeds well, because the silver takes 

 but a short time to consume the film of chlorine clinging 

 to it. 



According to the chemical theory, the explanation of this 

 experiment is perfectly clear ; but according to the contact 

 theory, not so. According to the latter, the difference of 

 potential should continue, since one plate is silver and the 

 other silver chloride, or at least is covered with it. In fact, 

 I see no way of bringing this experiment in accord with the 

 contact theory. 



While I was engaged in carrying out the foregoing work, 

 there appeared two papers by Brown * on the same subject 

 which I cannot here pass unnoticed. Unfortunately Brown 

 was not in a position to make quantitative measurements with 

 his apparatus ; still it is always remarkable when investiga- 

 tors are led from quite different sides and quite independently 

 to the same result. Though priority at least in carrying out 

 the idea qualitatively certainly belongs to Brown, yet I believe 

 I am the first to have placed the matter by quantitative proof 

 in the true light. 



Reviewing now the results of all researches on contact 

 electricity up to the present time, we shall find, as it appears 

 to me, no argument capable of being held at all against the 



* Phil. Mag. [5] vi. August 1878, and vii. February 1879. A short 

 abstract describing the results obtained by Brown is omitted here ; as the 

 two papers can easily be referred to. 



