We developed a new method to measure the stiffness ρ_s of a superconductor without applying magnetic field. The method is based on the London equation J=-ρ_sA, where J is the current density and A is the vector potential. Using a rotor free A and a J measurement we determine ρ_s. We apply the method to La_2-xSr_xCuO₄ and find some surprising results.