Figure shows time courses of (a) the electric current I and (b) the potentials U of five Pt probes vs. CA, which were simultaneously measured after the application of a potential of 200 mV between the AN and CA. The redox electrolyte contained 0.1 M H2SO4 and three redox couples of Fe 2+ /Fe 3+ (0.77 V), Fe(bpy)3 2+ /Fe(bpy)3 3+ (1.03 V), and Fe(phen)3 2+ /Fe(phen)3 3+ (1.12 V), where bpy and phen represent bipyridyl and ophenanthroline ligands, respectively, and the numbers in parentheses refer to the standard electrode potentials. The concentration of each reductant Ri or oxidant Oxi was chosen to be 0.1 M in the absence of an electron-transfer reaction between the redox couples. The electric current initially decayed and became constant, indicating that a steady state was attained. The potentials of the five Pt probes, which measured the redox potential of the redox electrolyte at each position, also initially changed and became constant with approximately the same potential difference between each other, again indicating that a steady state was formed. These results can be explained as follows. In the absence of an applied potential between the AN and CA, the redox electrolyte is in equilibrium and the concentrations of Ri and Oxi are homogeneous throughout the electrolyte. Under an applied potential, the concentrations of Ri and Oxi at the AN and CA surfaces are determined by the applied potential. In a steady state, the electric current density does not change along a line of electric current. Thus, the redox potential in the redox electrolyte changes linearly with spatial distance along a line of electric current. This behavior explains why the potentials of the five Pt probes became constant with approximately the same potential difference in the steady state.