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At what temperature did mercury first become superconducting in Kamerlingh Onnes’s experiment?

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Figure 1: Specific heat in the normal (Cen) and superconducting (Ces) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (Tc) and above Tc. — superconductivity Figure 1: Specific heat in the normal (C_en) and superconducting (C_es) states of a classic superconductor as a function of absolute temperature. The two functions are identical at the transition temperature (T_c) and above T_c.

In his 1911 experiment, the Dutch physicist Heike Kamerlingh Onnes discovered that the electrical resistivity of a mercury wire disappears suddenly when it is cooled below a temperature of approximately 4 K (−269 °C).1