(a) Variation of the experimental α and β obtained in (3) as a function of the ratio measured independently with the sample holder grounded. From Figure 3 dI/dV spectra series, a and b were obtained going from Figure 3(a)ii and (b)ii series to Figure 3(a)iii and (b)iii series respectively. (b) For a set of dI/dV measurements with V_A = 30 mV and V_B = 0.0 mV, V_S is obtained from (3) using the corresponding experimental α and β values as a function of the ratio. (Each data point is corresponding to a 10 pm relative tip height change, Tip A in red and Tip B in blue).