Conversely, as the tip approaches the substrate (L < 2.5), both it and is rapidly increase indicating the onset of strong positive feedback. FEM simulations of the approach curves presented in the Supporting Information are in good agreement with experimental curves for this simple experiment, and demonstrate that, as expected, essentially 100 % of the Ru(NH3)6 2+ generated at the C tip is oxidized at the Pt substrate when L < 2.5 (i.e., d < 8 µm). before it reaches the substrate electrode, and/or that the anodic current due to oxidation of Ru(NH3)6 2+ is precisely canceled out by the cathodic current due to reduction of SO4 •-.