The steady-state current of the scanning electrochemical microscope (SECM), which can be used to convert approach curves to topographical information, is considered for positive feedback on the substrate surface with an infinite (case I) and finite (case II) glass insulator around the disc electrode. A very simple and accurate approximation (case I) is derived from short and long distance expressions. Moreover approximate expressions (case II) for different RG values are suggested. Tabular compilations of dimensionless current are reported. (C) 2003 Elsevier B.V. All rights reserved.