342 Prof. A. LI. Hughes on the Contact 



Air was admitted to the rough vacuum of the molecular 

 pump to a pressure of 2 cm. 



{Zn electropositive ] .^ n ,. 

 5 mins. later. „ "66 ,, 



Rough vacuum pumped out as before. 



I 5 mins. later. 



Zn electropositive j '76 volt. 

 (e)\ toPtby /. 80 



Air admitted to the rough vacuum to a pressure of 10 cm. 

 and then pumped out. 



5 mins. later. „ *46 ., 



{ 



Air at atmospheric pressure admitted to apparatus. 

 (g) { 20 hours later. Zn electropositive j ^ ^ 



Apparatus pumped out completely. 



[_ 50 mins. later. „ -39 ,, 



Air at atmospheric pressure admitted to apparatus. 



,.J Zn electropositive 1 on - , , 



(*){ ~ toPtby f '39* volt 



The contact difEerence of potential between the zinc and 

 the platinum immediately after the distillation of the zinc is 

 evidently quite small and increases slowly with the time (a). 

 When the pressure of the air in the apparatus is slightly 

 raised, a great increase in the contact potential occurs (b). 

 It is necessary to point out here how small the increase of 

 pressure in the vacuum really is. The " rough vacuum " of 

 the molecular pump was maintained by a Gaede box-pump 

 giving a vacuum of about *01 mm. On passing a discharge 

 through the bulb F, the alternative spark-gap was 8 cm., 

 which indicates a good vacuum when one considers the size 

 of the bulb. When air at a pressure of 10 mm. was admitted 

 to the rough vacuum, the alternative spark-gap was reduced 



* Note.— It was observed that a surface of distilled zinc after exposure 

 to air was always more electronegative than a surface of polished zinc. 

 Hence the structure of the surface has, directly or indirectly, a con- 

 siderable effect on the contact difference of potential. 



