Conditions necessary to get meaningful measurements from the Kelvin method

In this work, the authors emphasise the difficulties encountered in determining the contact potential difference (CPD) using the Kelvin method with a small area reference electrode. A new theoretical analysis of this problem based on a plane capacitors scheme has been tentatively worked out. It gives a rather good explanation of the various experimentally observed phenomena. They also show that it is misleading to attempt to measure a CPD corresponding to a perfectly defined surface without setting a guard shield around the reference electrode, and that experiments carried out with a large sample guard spacing (>0.2 mm) could not give significant results.