244: Mr. Norman Shaw on Charged Surface Layers in 



§ 2. Measurement of Contact Potential betiveen 

 the Plates. 



The contact potential between a given plate C and the 

 brass plate B could be measured with this apparatus in the 

 following ways: — 



(1) The Kelvin method: — The ionizing source is removed,, 

 all parts of the apparatus are earthed, and B and C are 

 placed a few millimetres apart. L is then connected to a 

 potential divider, and B is charged to such a potential that,. 

 if C be insulated after this charging of B, it will be impossible 

 to alter the electrometer reading by raising or lowering B,, 

 providing that metallic contact is not made with 0. The- 

 applied potential necessary to produce this condition is equal 

 to the contact potential difference between the plates. 



(2) Ionization Method, No. 1 : — All parts are earthed at 

 first, then C is disconnected from earth by raising the key, K„ 

 If the gas between the plates is ionized, then the electro- 

 meter reading will immediately change, and after a short 

 interval become steady at a new position. C is then con- 

 nected to the potential divider, and it is found that the 

 potential necessary to maintain the second reading is equal 

 to the required contact potential difference. 



(3) Ionization Method, No. 2 :— The plate B and the 

 vessel A are both connected to the potential divider. If A 

 and B are charged so that the electrometer reading remains 

 unchanged when C is disconnected from earth, then this 

 charging potential is also equal to the contact potential 

 difference between the metal, C, and brass. 



Aluminium, zinc, lead, iron, copper, and brass plates wer& 

 tested in this way, and in every case there was complete 

 agreement between the three methods *. The two ionization 

 methods were, however, about five times as sensitive as the 

 electrostatic method, and gave results agreeing with each 

 other to within two-thousandths of a volt. 



§ 3. The Direct Electrical Detection of Charged 

 Surface Layers. 



This section presents an account of some experiments which 

 were performed with the object of obtaining direct evidence 

 of charged surface layers by determining whether there was 

 any appreciable time lag between the ordinary charging of 

 a metal and the formation of the charged layers. It is 



* An exposure of only a few millimetres of the wire in TIH to the- 

 ettects of ionization will vitiate this agreement. Especial care must bo 

 taken in the preparation of the insulating sulphur. 



