Contact Potential Phenomena between Metals. 253 



the different amounts of work to take an electron across the 

 surfaces of different metals. 



(3) The influence of the dryness of the air on the charging 

 curve is shown in fig. 5 for zinc-brass. 



Fio-. 5. 



*-(d) 



35 150 165 180 195 Seconds 



Curve (a) is taken with laboratory air in the vessel A 

 (contact potential 0*564 volt). 



(6) is taken after passing dry air through the apparatus 

 for four hours (contact potential 0'518 volt). 



(c) is taken after passing dry air through for three days 

 (contact potential 0'495 volt). 



Upon allowing ordinary air to enter the apparatus at the 

 end of this time the contact potential returned to 0*545 volt, 

 hence showing that part of this decrease is due to the usual 

 slow change in contact potential when using polished plates. 

 It will be seen that this latter change is in agreement with 

 the magnitude of change recorded in (1). 



After this preliminary decrease was obtained it was found 

 that the contact potential became far more constant and 



