In scanning Kelvin probe microscopy, a conducting tip (e.g. Fe, W) is vertically oscillated by piezo motors close to a substrate surface in air. The tip and the substrate function as two capacitor plates and a capacitive current flows. This current i is linearly dependent on the difference in working potential between tip and substrate material and non-linearly dependent on the distance between tip and substrate surface. During oscillation the potential is scanned to find i = 0. At this point the applied potential matches the difference in working potential.