[en] It is well recognized that conventional and high-T_c superconductors exhibit zero Ohmic resistance in magnetic fields below the lower critical field, H_c1. The picture is not as clear when the applied field penetrates the material. Data in this region are often interpreted in terms of thermally activated motion of flux lines out of pinning sites. The prediction is that there should be a finite resistance at all non-zero temperatures. The authors have measured current-voltage curves of polycrystalline YBa₂Cu₃O_7-δ as a function of magnetic field over a wide range of temperatures. The results indicate the presence of a second order phase transition which exhibits the scaling behavior characteristics of a vortex glass and cannot be explained by conventional flux creep models. The Ohmic resistance vanishes at low current at a non-zero temperature which implies that polycrystalline YBa₂Cu₃O_7-δ is a true superconductor in the mixed state