On Contact Electricity of Metals. 



143 



may vanish, and that a plate of any metal at that temperature would 

 be about 0'4 volt positive to a standard gold plate at 16 C. 



78. The approximate numerical values of the temperature - 

 variations are given in the following table : 



Table V. 



It must be clearly understood that these are true temperature- 

 variations and not permanent changes in the plate caused by exposure 

 to a high temperature. 



X. Elimination of Metal- Air Potentials by Solid Non-conducting Films 

 on the Metallic Surfaces. 



79. As very great differences of opinion seemed to exist as to 

 the part played by the layer of air which is close to the metallic 

 surface, I devised a method in which it should be removed and a film 

 of solid non-conducting material of a very different chemical nature put 

 in its place. A copper plate which had been polished on glass-paper 

 was filed with a clean dry file which had not been used for any other 

 metal. Its potential with the standard gold plate being 



+ 0-045 volt. 



A zinc plate was prepared in an exactly similar way, and with the 

 copper plate gave 



+ 0-655 volt. 



