Who first observed the sudden vanishing of electrical resistance and at what approximate temperature?

Answer

Heike Kamerlingh Onnes at 4.2 K.

The Dutch physicist Heike Kamerlingh Onnes first observed this phenomenon in 1911 when he cooled mercury down to $4.2 ext{ K}$, noting the abrupt drop in resistance.

Who first observed the sudden vanishing of electrical resistance and at what approximate temperature?
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