On Contact Electricity of Metals. 123 



experiments, and it was found necessary to enlarge the scope of the 

 research in order to remove or explain them. 



22. The experiments detailed below are among the most definite 

 of those carried out. They show large changes in potential on 

 account of very thin films of oxide, and prove that the amount of 

 change is dependent on the thickness of the film when below a 

 certain limit. 



23. Two plates of cast zinc, which had been carefully polished 

 on medium glass-paper the day before, gave with one another 



+ 0-02 volt, 



the upper plate being positive. The upper plate was now taken out 

 and heated on the back by means of a very small blowpipe flame. Its 

 face became slightly blistered, but not much discoloured, and when 

 cold it gave with the other, which had not been altered in any 

 way, 



0-44 volt, 



the " minus " indicating that the upper is now negative. This shows 

 a permanent fall of O46 volt due to heating in air. 

 24. A plate of rolled zinc, which gave 



0-04 volt 



with another plate of the same material, was heated as in the last 

 experiment, but to a higher temperature, very nearly to its melting 

 point. Its face became a streaky yellowish-brown, slightly purple 

 towards the centre (the brown parts turn purple if moistened). 

 When compared, cold, with the clean plate it now gave 



-0-79 volt, 



;a f all of O75 volt. Thus this oxidized zinc plate has practically the 

 same potential as copper. This great fall of potential does not take 

 p!ace by exposure to the atmosphere at ordinary temperatures, unless 

 possibly after the lapse of many years, for the potential of zinc 

 plates which have been left unpolished for many months is only two 

 or three tenths of a volt below that of clean zinc (see 34). 



25. Similar experiments were made with copper. The film of 

 oxide was gradually increased in thickness by repeatedly heating the 

 plate, and the potential measured in each stage of oxidation, the plate 

 being cold. There was a change of about 0*03 volt in the negative 

 direction before the oxide-film became visible, and further increase in 

 the thickness of the film was judged by the gradual change of colour, 

 until the well-known dull purplish -black of massive copper oxide 

 was attained. The potential, measured each time when the plate 

 was cold, gradually fell until it reached a limit of about O30 volt 



