April 8, 1911

Superconductivity

Kamerlingh Onnes's team first observes mercury's near-zero resistance

The day's actual goal was to see whether liquid helium could be moved into a new cryostat. At exactly four o'clock, the notebook recorded mercury's resistance as practically zero.

8 April 1911: a transfer test at Leiden

The Leiden University experiment of 8 April 1911 began at 7 a.m., and its principal objective was the first successful transfer of liquid helium into a separate experimental cryostat — not the discovery of a new state. Kamerlingh Onnes arrived when helium circulation began at 11:20 a.m.

The apparatus already contained gold and solid-mercury resistors. The mercury conductor was seven U-shaped glass capillaries connected in series, each with a reservoir intended to keep the frozen mercury from breaking apart during cooling.

Practically zero, at about 3 K

Gilles Holst made the precision resistance measurements, while Gerrit Flim, Cornelis Dorsman and other laboratory specialists handled the cryogenic and temperature work. After measuring mercury and gold at 4.3 K, the team reduced the helium vapor pressure to about 197 mmHg, corresponding to roughly 3 K.

At exactly 4 p.m., the notebook recorded that mercury's resistance was practically zero. The sharply defined transition at 4.20 K came from later experiments — on 26 October the resistance changed from below 10⁻⁶ ohm to 0.1 ohm within 0.01 K.

Two anomalies seen, only one understood

Late that afternoon the team also noted that the helium stopped boiling near the lowest temperature and evaporated instead from a visibly shrinking surface. Historians interpret this retrospectively as the helium lambda transition; the team did not identify superfluid flow.

The often-repeated story of a sleepy apprentice letting the helium warm and causing a dramatic galvanometer swing is contradicted by the notebooks. Kamerlingh Onnes's first report on the resistance result was dated 28 April 1911.

Sources

Researched 23 Aug 2026 7 sources 2 audit passes, 1 issue open

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What the sources leave uncertain

Notebook 56 says “8 April” but omits the year, and a nearby entry carries an erroneous year. Van Delft and Kes reconstructed 1911 securely from the experiment sequence and the April 28 report.
On April 8 the decisive reading was made at about 3 K. The now-famous abrupt transition curve at 4.20 K comes from the October 26 experiment.
The team retrospectively crossed the helium lambda transition that day, but did not recognize or measure zero-viscosity flow. Credit for discovery of superfluidity conventionally belongs to Kapitza and, independently, Allen and Misener in 1937–1938.