376 Influence of Friction upon a Voltaic Current. 



positive element is covered and the negative element exposed 

 to friction. 



It seems, therefore, that Prof. Barrett's experiment is only 

 an illustration of the effect of friction on one of the elements 

 of a voltaic couple in increasing the current. This effect is, 

 however, so very remarkable that 1 was induced to make 

 further experiments. 



The most curious result at which I arrived is this : — Take 

 two plates of the same metal, cut from the same sheet, and 

 connect them with a galvanometer ; cover one of them with 

 a wet cloth and bring the other down upon it. If the two 

 pieces of metal are in the same plrysical condition there will 

 be no material deflection of the galvanometer. Now rub the 

 covered metal with the bare metal. As long as the rubbing 

 continues, the galvanometer indicates a current of electricity, 

 which ceases as soon as the rubbing is stopped. And this 

 current invariably flows from the plate which is covered by 

 the wet cloth, 



I have made the experiment with plates of tin, lead, copper, 

 brass, and zinc, with the same result in every case. 



I do not suggest an explanation of the above-mentioned 

 phenomena. When the two plates of a single metal are used, 

 it appears that friction renders the one rubbed relatively 

 electro-positive. But in the case of two metals, friction seems 

 generally to have a greater effect upon the negative than the 

 positive element, and it makes the negative element not more 

 positive but more negative. Any possible explanation that 

 occurs to me of the one case is inconsistent with the other. 



Certain speculations led me to try the effect of passing a 

 battery-current through a pair of metallic plates, separated by 

 a piece of wet rag or paper, while they were being rubbed 

 together ; and I found that when the current passed from the 

 covered to the uncovered plate, a remarkable diminution in 

 the friction occurred. A current in the opposite direction 

 produced no such result. I exhibit a little apparatus for ren- 

 dering this effect visible to a large audience. 



This experiment seems to show conclusively that the gene- 

 rally received theory of electrolytic action is sufficient to ex- 

 plain the phenomena presented by the electro-motograph ; for 

 it is difficult to conceive any other possible effect of the current 

 than the liberation of hydrogen on the surface of the plate 

 which is connected with the negative pole of the battery, the 

 layer of hydrogen having the effect of diminishing the friction. 



Riverstone Lodge, Wandsworth, S.W. 



