

On Contact Electricity of Metals. 135 



the oxygen, the potential varied as shown below, the oxygenized 

 plate being positive to the other in all cases : 



Time. Contact-potential. 



Hrs. Mins. Yolt. 



3 0-12 



19 0-08 



30 0-06 



17 20 0-03 



Thus the effect of increasing the proportion of oxygen in the surface 

 film was to make the copper more positive. 



57. This variation is in the opposite direction to that found by 

 Lord Kelvin for platinum which has been soaked in oxygen. In 

 order to make sure that this difference was not the effect of some 

 impurity in the oxygen, I repeated his experiment, obtaining the 

 same result as he had obtained. This shows that the result given 

 above was not likely to be due to an impurity which had influenced 

 the action of the oxygen. 



58. Without any further treatment the same copper plate was 

 placed in the bell-jar and the oxygen admitted. After 45 

 minutes it was taken out and its potential again measured by com- 

 parison with the other plate. The results are given in the following 

 table, time being counted from the moment the plate was taken out of 

 the oxygen : 



Time. Contact-potential. 



Hrs. Mins. Yolt. 



2 0-13 



10 0-08 



21 0-07 



29 0-06 



The experiment was discontinued before the plate had returned to 

 its original value ; but it would no doubt have done so in a few hours, 

 for the amount of change, and its rate, are almost exactly the same as 

 in the former experiment (see 56). 



59. In the following experiment the conditions were somewhat 

 varied. A copper plate was polished on glass-paper. It gave, with 

 a standard copper plate, 



-0-05 volt. 



A jet of oxygen was now sent against its surface for 2 or 3 

 minutes, and with the same standard plate it now gave 



-0-06 volt. 



It was then left in oxygen for 25 minutes, and on being taken out 

 gave 



