O-GEIS measurements are conducted by applying a small-amplitude sinusoidal current excitation, which is superimposed about a large applied direct current. The applied current serves to bias the kinetics into the non-linear region, but does not directly contribute to the impedance measurement; the impedance is calculated only from the sinusoidal voltage response to the current excitation, and O-GEIS therefore measures the differential charge-transfer resistance (𝑅 𝑐𝑡 ′ ) .