We developed a new method to measure superconducting stiffness ρ_s and critical current density J_s without subjecting the sample to a magnetic field or attaching leads [1]. 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 measuring J via the magnetic moment of a superconducting ring, we determine ρ_s. By increasing J until the London equation does not hold anymore, we determine J_c. The technique is sensitive to very small stiffness, which translates to penetration depth on the order of 1 mm. Naturally, the method does not suffer from demagnetization factor complications or the presence of vortices. Therefore, the absolute value of the stiffness can be determined. We apply this method to La_2-xSr_xCuO₄ and compare with Low Energy μSR (LEM) of the same sample. We show that the Stiffnessometer can measure stiffness where LEM, and in fact all other techniques, fail. This leads to new conclusions on the cuprates phase transition.