The potential in induced polarization (IP) is proportional to impressed current density. Overvoltage is the extra potential caused by an electrochemical and electrokinetic barrier set up at an electrode-to-electrode interface. Activation overvoltage is caused by current passage stimulating an electron-transfer reaction such that the electrode potential deviates from its reversible potential without appreciably changing the ion concentrations at the electrode surface. Concentration overvoltage is brought about by a depletion or accumulation of oxidized and reduced ion species at the electrode surface, causing a change in the reversible potential of the electrode. See induced polarization.