Influence of Friction upon a Voltaic Current. 375 



rent passing from the platinum. The effect of the friction 

 was to greatly increase the strength of the current. 



I then faced the brass arm with zinc instead of platinum. 

 On connecting the instrument to the galvanometer a small 

 deflection was again observed ; but this time the + current 

 flowed^ as I expected, not from the zinc-faced arm, but from 

 the brass core of the cylinder. The substitution of the zinc 

 for the platinum had the effect of reversing the current, brass 

 being positive to platinum and negative to zinc. In this case, 

 too, rotation of the cylinder largely increased the current. 



I now laid aside the motograph, and connected to the two 

 wires of the galvanometer a sheet of brass and a sheet of pla- 

 tinum. Between these metallic plates I placed a thin slice of 

 dry chalk; the galvanometer indicated nothing. I rubbed the 

 metals successively against the chalk ; still the galvanometer 

 remained motionless. For the pure chalk 1 substituted a 

 chalk plate which had been soaked in a saturated solution of 

 phosphate of soda and thoroughly dried ; again there was no 

 result. Neither was there when I used dry blotting-paper 

 which had been saturated with phosphate of soda. But with 

 a piece of blotting-paper which had been saturated with a 

 solution of caustic potash and made as dry as possible, the 

 results were just the same as with the electro-motograph : 

 brass and platinum gave a + current from the platinum ; 

 brass and zinc gave a + current from the brass ; and in both 

 cases the current was much increased by rubbing. 



This experiment was repeated with the following pairs of 

 metals — brass and platinum, zinc and lead, zinc and copper, 

 lead and copper, tin and copper, zinc and tin — covering one of 

 every pair of metals successively with a wet cloth and rubbing 

 the one so covered with the other. In every case the friction 

 seemed to considerably increase the current which was gene- 

 rated on mere contact. If, for instance, a piece of lead is 

 covered with a wet cloth and a piece of copper is pressed upon 

 it, a + current will of course flow from the copper. On rub- 

 bing, this current is very greatly increased. If the copper be 

 the metal covered, and the lead rubbed against it, the current 

 from the copper will again be greatly increased, but apparently 

 not quite so much so as in the former case. And I believe 

 that this difference in the effect produced according to the 

 metal covered, occurs in the case of all the other pairs of 

 metals which I tried. In some cases it is very notable ; in 

 others it is small, and extremely difficult to detect without a 

 machine for producing uniform friction. But, as far as I can 

 judge, the effect on the current is always greatest when the 



