Nonlinear magnetic behavior in high‐Tcsuperconductors as reflected by the appearance of harmonics in ac magnetic response, yields valuable information about transition temperature, critical fields and the dynamics of fluxons. In this paper we discuss the specific applications of the third harmonic measurements in developing (i) a high resolution spectrometer for determining Tc’s and (ii) a microscope which reveals local variation of Tc and the other superconducting properties.