This paper discusses the application of an ionizing source coupled with galvanic differences between metals in a measure of the work function difference between the metal surfaces. The electrical field generated from the contact potential difference (CPD) between two electrodes will cause the gaseous ions to discharge at both surfaces, creating a measurable current. The current depends on the surface size, spacing, and ionizing source power. One of the surfaces (probe) can vary in shape and size, and if inert, can be used to obtain the work function or surface potential of the second surface. The ionic current is proportional to ion mobility, ion generation rate, CPD, and the probe size, but inversely proportional to the spacing between the probe and the sample. It is found, as expected, that there is an approximate linear relationship between the ionization probe signal and the work function of the surfaces of metals.