308 Mr. J. Frenkel on the Surface Electric 



Let us take two metals A and B whose external potentials 

 are zero, the intrinsic ones being equal to V A and Y B 

 respective^, and put them in contact with each other, so 

 that their surface electric double-layers partly overlap, the 

 negative half of the one coinciding with the positive half of 

 the other. The electric field in the residual double-layer 

 will be directed from A to B if V A >V B , and will drive from 

 B to A a number of electrons sufficient for its annihilation, 

 i. e. for the equalization of the internal potential in the 

 system AB. Denoting by (£ A and </> B the variations of the 

 internal potential of A and B respectively, we see that they 

 will be equal to the external potentials of the metals in the 

 final state and will be connected by the equation 



We shall thus observe an external electric field, due to a 

 positive charge on B (<£ B >0) and a negative charge on 

 A (</> A <0), corresponding to a potential difference 



as if the latter really existed between the metals. As a 

 matter of fact, however, their internal potentials are the 

 same and will remain equal after their separation. Measuring 

 their mutual attraction we shall obtain the external potential 

 difference, which is equal and opposite to the difference of 

 the intrinsic potentials. If A and B were connected with 

 two other bodies C A and C B of the same material character- 

 ized by the intrinsic potential V c , then the internal potentials 

 of these bodies being equal to the common internal potential 

 of A and B, the external potentials <£ AC and <£ BC must be 

 equal too. There will be, consequently, no electric field 

 between A and B , and if the latter are the quadrants 

 of an electrometer, there will be no deflexion when A and B 

 are disconnected. But if the surrounding gaseous medium 

 is ionized, the surface charge of A and B, acquired during 

 the contact, will be dissipated ; the external potential 

 difference between them will disappear, while the internal 

 one will resume its initial value V A — V B , equal, of course, 

 to the difference between the external potentials of C A and 

 G B , which will be measured by the deflexion of the needle. 

 This is the principle of the ionization method for the deter- 

 mination of contact electromotive forces. It will be noticed 

 that the electrometer always measures the internal potential 

 difference of the metals considered. 



At ordinary temperatures but very few electrons can 



