Contact Potential Phenomena between Metals. 



249 



It can now be seen at once why the curves (a) will differ 

 from the curves (6) and (c). During (a) there is no per- 

 manent change in tho layer on the brass, while in (b) and (c) 

 this layer does change, and as the passage of electrons would 

 be retarded with the formation of the surface layers, it is 

 apparent in the case of (a) that with one layer already in its 

 final state, there will be a slower transfer than in the case 

 of (b) and (c), where both layers change. A perceptible 

 time factor in the formation of the layer is thus definitely 

 indicated, and some capacity clue to the layers should be 

 detectable. This conclusion was verified by another test as 

 follows : — 



All parts of the apparatus were earthed for more than five 

 minutes, then the brass plate and brass vessel were suddenly 

 charged to the contact potential, P, and the plate C was 

 disconnected from earth immediately after. The time of 

 charging the brass itself was, however, negligible in compari- 

 son with the time necessary for -completing the subsequent 

 disconnexion of the plate C from earth. In ordinary 

 circumstances the electrometer would remain stationary, but 

 in this experiment we should expect a temporary change in 

 the potential of the plate C, while the layer on the brass was 

 assuming its new potential. The plate C should first assume 

 a slightly lower potential (i. e. if Y 2 + a 2 >V 1 + a 1 ) and then 

 return to V 2 . Observations showed that this was actually 

 the case; the electrometer needle moved rapidly in the right 

 direction, and then slowly returned to the zero value. An 

 enumeration is given in Table III. of the various chancres 

 that take place in the metals and in their layers. 



Table III. 





Potential immediately 



after connecting brass 



to potential divider. 



Potential a short 



time after C is 



insulated. 



Final 

 potential. 



Brass 



V x +P 

 ▼i+«i 



Y l +a l — a 2 



V x +P 



V 1 + a 1 +P-E 1 



V 1+ ' ai +P-E 2 



V 1 4-a 1 — a 2 ~ ^3 



V\+P 



V.+ai+P 

 v i+ a i— a 2 



Layer on Plate ... 

 Plate C 





E l5 E 2 , E 3 represent the effects due to the fact that it takes 

 over a minute for the surface layers of the brass to follow 

 the change of the brass itself. E x alters steadily in value 

 from P to zero, and E 2 , E 3 first grow from zero to a small 

 value and then return to zero. The magnitude of the effect 



Phil. Mag. S. 6. Vol. 25. No. 116, Feb. 1913. S 



