[en] The dynamic resistance as a function of transport current in a superconducting YBa₂Cu₃O_x (YBCO) sintered powder sample depends on its thermal surroundings. Plots of V, dV/dI, and d²V/dI² versus I are markedly different for the sample in vapor nitrogen compared to those measured in liquid nitrogen at 81 K. Assuming the usual power law dependance of V = V_o (I/I_o)ⁿ then n = (I x d²V/dI²)/(dV/dI) + 1. Therefore, by measuring dV/dI and d²V/dI² at a given current, the n factor can be determined for the V-I curve at that current. Plots of (I x d²V/dI²)/dV/dI as a function of I and dV/dI quantify the curvature of the V-I characteristics of the sample. At 81 K, it is found that at the onset of detectable flux flow in the sample, the n factor determined from the dynamic derivatives of the V-I curve is 15 in the vapor versus 5 in the liquid. This phenomena could be the basis for low power cryogenic flow meters, bolometers, level detectors, or other types of thermal environment sensors