On Contact Electricity of Metals. 



14. Intermediate states of polish give intermediate j values of 

 potential. For instance, if a zinc plate be filed it will bot be so 

 sharply scratched as it would be by glass-paper, and of course not so 

 smooth as if burnished. Its potential is also found to be between 

 those of scratched and burnished zinc. Each polishing, with any 

 material, was usually sufficiently thorough to efface the effects of all 

 previous polishings : the potential observed thus depended only ou 

 the state of surface produced by the polisher used just before the 

 observation. The results of experiments show that the smoother 

 the surface the more positive it becomes. 



15. The generalization given in 14 is supported by the results 

 of over 100 similar experiments, with a number of metals. 

 The actual results are given in Table I ; it will be well, however, 

 to discuss some of these in greater detail than is possible in tabular 

 form. 



16. In order to eliminate the use of different polishing agents, 

 I tried the effect of producing different states of surface by rubbing 

 the plate against another of the same metal. Two. copper plates 

 which had been polished on glass-paper gave 



(a) +0-01 volt, 



(6) +0-04 



with the standard plate. They were next gently rubbed together 

 until parts of each were shiny and now gave, with the same 

 standard, 



(a) +0-07 volt, 



(&) +0-07 



In another experiment two coppers which had been polished on 

 medium emery-cloth (Davies's No. !) gave, with the standard 

 plate, 



(a) -O'll volt. 

 (6) -0-06 



When slightly burnished by rubbing them together, they gave 



(a) 0-02 volt. 

 (6) -0-02 



Thus, smoothing by mutual friction made both more positive. The 

 amount of the change is not great, either in potential or in smooth- 

 ness, as it is very difficult to polish copper on copper, but the direction 

 of change is the same as previously found, namely, a smooth surface 

 is more positive than a sharply scratched one. 



17. On account of the difficulty of obtaining a burnished surface 

 by simply rubbing two pieces of the same metal together, I tried 

 another form of experiment in which the possible effect of the 



