The power-law E(J) characteristic of a superconductor may be determined by ac susceptibility (χ=χ′−jχ″) measurements from the field amplitude at which χ″ takes maximum, Hm(χm″), and frequency f. The relation between E vs J and Hm(χm″) vs f obtained from direct calculations of χ for a long cylinder and a thin disk with a power-law E(J) provides a firm base for the ac susceptibility technique of Jc determination of advanced high-temperature superconductors.