[en] The time dependence of electric field E at the surface of HTSC ceramics samples of the critical state is studied both theoretically and experimentally. The superconductors are placed in external longitudinal magnetic field and irradiated by a finite amplitude radio wave. An expression for E(t) is derived with an arbitrary dependence of critical current density in magnetic inductance. The function E(t) contains a comprehensive information on critical state of the superconductor and on peculiar penetration of magnetic flux deep into the sample. A simple contact-free method based on measuring E(t) is proposed to determine the HTSC sample critical parameters