Fig. 6

Estimation of resistance between current electrodes. The coordinates z and θ are as defined in Fig. 2a. C₁ and C₂ are current electrodes, and P₁ and P₂ are potential electrodes. \({V}_{{\mathrm{C}}_{1}},\) \({V}_{{\mathrm{C}}_{2}}\), \({V}_{{\mathrm{P}}_{1}}\), and \({V}_{{\mathrm{P}}_{2}}\) are electrical potential at C₁, C₂, P₁, and P₂. R_measured, R_sample, and R_contact are, respectively, the measured resistance between the current electrodes, the sample resistance between the current electrodes, and the contact resistance at the current electrodes. I is measured current. The left diagram depicts the measurement area (i.e. the sample’s side surface), and the right diagram represents the numerical model of the measurement area with its calculation grids. Performing numerical calculations under various sample resistivities searched for a potential distribution that mostly explains the observed \({V}_{{\mathrm{P}}_{1}}\)–\({V}_{{\mathrm{P}}_{2}}\). The calculations assumed the sample to have a homogeneous resistivity structure. \({V}_{{\mathrm{C}}_{1}}\)–\({V}_{{\mathrm{C}}_{2}}\) is extracted from the determined potential distribution, and used to obtain R_sample by dividing it by I