Scanning Electrochemical Microscopy: Fitting Rg with Negative Feedback Curve
In scanning electrochemical microscopy (SECM), Rg is defined as the ratio between the insulator thickness and the radius of the microdisk electrode. This Demonstration shows how to manually or automatically fit Rg by using the approximation for negative feedback current, which is a function of the tip-substrate distance and Rg.
The analytic approximation (blue line) for calculation of Rg  is derived by the finite element method. In this Demonstration, experimental data (the red points) can be fitted manually by changing Rg, the distance , and the function scale. Alternatively, the experimental data can be automatically fitted by checking the "fit" checkbox.
Use the function scale to fit the experimental distance to the theoretical value. It is difficult to reach the value in an experiment so we need to use the function scale to detemine the expected experimental data.
 R. Cornut and C. Lefrou, "New Analytical Approximations for Negative Feedback Currents with a Microdisk SECM Tip," Journal of Electroanalytical Chemistry, 604, 2007 pp. 91–100.