Difference of Potential of Distilled Metals. 34$ 



by less than 1 cm. Another way of considering the vacuum 

 is as follows : — According to the information supplied with 

 the pump, a pressure of 10 mm. in the rough vacuum means 

 a pressure of '0005 mm. in the high vacuum when the pump 

 makes 8000 revolutions per minute. As the speed in these 

 experiments was about 9600 revolutions per minute, it is- 

 probable that the pressure in the high vacuum was below 

 *0005 mm. It is, therefore, clear that the introduction of a 

 very small quantity of air into the apparatus causes a great 

 change in the contact difference of potential when one of the 

 metals has never been exposed to air before. At this stage, 

 the contact difference of potential is sensitive to changes in 

 the pressure of the air. The new zinc surface is absorbing 

 gas slowly and, up to a certain point, the contact potential 

 difference increases with the amount of gas absorbed. The 

 absorption of more gas caused a reduction from *92 to 

 "80 volt (observation (b)). From observations (c) (and also* 

 (d), (e) 9 and (f)), it may be inferred that the gas absorbed is 

 not yet held very firmly, for, on reducing the pressure again, 

 the contact difference of pressure increases. From obser- 

 vations (g), (A), and (i), we see that when the distilled metal 

 has been in contact with air for several hours, its contact 

 potential relative to platinum is no longer altered by changing 

 the pressure of the gas around it, and probably the absorbed 

 gas has become firmly attached to the metal and is not 

 appreciably affected by the pressure of the gas in the 

 surrounding atmosphere. 



The evidence that the change in the contact potential on 

 varying the pressure of the gas in the vacuum is localised at 

 the surface of the distilled metal is straightforward. The 

 deflexion of the electrometer on admitting air into the 

 vacuum was observed, (1) when the platinum surface of the 

 disk was opposite to the electrode E, and (2) when the newly 

 distilled deposit of zinc was opposite to E. No appreciable 

 alteration in the deflexion was observed in case (1), while 

 very considerable changes in the deflexion always occurred 

 in case (2). 



Part of another series of observations made under similar 

 conditions is given below. 



( 1 min. after Zn electronegative \. m lf 



distillation. to Pt by J U 



2 mins. later. ,, *00 



, Zn electropositive "I . n .. 



1 J » » to Pt by J UJ 



1 2 „ „ ;, -06 



2 „ „ „ -09 



1 2 „ „ „ -11 



