 This difference, when expressed in terms of a voltage, is called the contact potential difference, "VCPD" This causes the apex of the parabola to rest at "ΔV = Vtip − Vsample − VCPD = 0".Typically, the value of "VCPD" is on the order of a few hundred millivolts.Forces as small as piconewtons can routinely be detected with this method.