In SKP the probe and sample are held parallel to each other and electrically connected to form a parallel plate capacitor. The probe is selected to be of a different material to the sample, therefore each component initially has a distinct Fermi level. When electrical connection is made between the probe and the sample electron flow can occur between the probe and the sample in the direction of the higher to the lower Fermi level. This electron flow causes the equilibration of the probe and sample Fermi levels. Furthermore, a surface charge develops on the probe and the sample, with a related potential difference known as the contact potential (V c). In SKP the probe is vibrated along a perpendicular to the plane of the sample. This vibration causes a change in probe to sample distance, which in turn results in the flow of current, taking the form of an ac sine wave. The resulting ac sine wave is demodulated to a dc signal through the use of a lock-in amplifier. Typically the user must select the correct reference phase value used by the lock-in amplifier. Once the dc potential has been determined, an external potential, known as the backing potential (V b) can be applied to null the charge between the probe and the sample. When the charge is nullified, the Fermi level of the sample returns to its original position. This means that V b is equal to -V c, which is the work function difference between the SKP probe and the sample measured.