Superconducting transition temperature ($T_c$)

The superconducting transition temperature $T_c$ is the temperature at which the material enters superconductivity on cooling in zero applied magnetic field and with negligible measuring electric current. A measured transition at nonzero magnetic field or appreciable electric current need not occur at this zero-field $T_c$. If a resistance-versus-temperature experiment for superconductivity gives a transition spread over several temperatures, report whether its quoted value is the onset, midpoint, or disappearance of detectable electrical resistance, together with the experimental conditions. This operational distinction matters when comparing samples. ^[ref]