On this page
In current electricity you learn that every wire has resistance, and that resistance falls as a metal cools. In 1911 a Dutch physicist found a metal whose resistance did not just fall but vanished. This post covers how Heike Kamerlingh Onnes reached temperatures no one had reached before, what he actually measured, a short calculation using his numbers, and what students can learn from his way of working.
Who was Heike Kamerlingh Onnes?
The Nobel Prize biography gives the outline. He was born in Groningen on 21 September 1853, studied under Bunsen and Kirchhoff in Heidelberg, and earned his doctorate in 1879 with a thesis on new proofs of the Earth's rotation. As professor of physics at Leiden he rebuilt the laboratory around one goal: reaching very low temperatures so that he could test the theories of his countrymen Van der Waals and Lorentz.
His inaugural lecture gave him a lifelong motto: "Door meten tot weten", knowledge through measurement. In 1901 he founded a school for instrument-makers and glass-blowers in the laboratory, because the apparatus he needed did not exist. He died in Leiden on 21 February 1926.
How did he liquefy helium?
Helium was the last gas to resist liquefaction. In his 1913 Nobel lecture Kamerlingh Onnes described the day it worked, which the Nobel facts page dates to 1908. The experiment began at half past five in the morning and ended at half past nine at night. Liquid helium was so clear that it was not noticed flowing in; only when the vessel was full could its surface be seen, standing "sharply against the vessel like the edge of a knife". He measured its boiling point as 4.25 K and estimated its critical temperature as 5 K.
He gave special thanks to his mechanic, G. J. Flim. The Nobel biography adds that by pumping on the helium he later reached 0.9 K, the closest approach to absolute zero at the time.
What did he find in 1911?
With liquid helium in hand, he studied the resistance of metals. Theory at the time was uncertain. He reasoned that impurities made the resistance of gold level off rather than reach zero, so he chose mercury, which could be purified much better.
In the Nobel lecture he described the result. The mercury was purified, frozen inside U-shaped capillary tubes only 1/20 mm across, and its resistance measured as it cooled. The resistance did disappear, but "the disappearance did not take place gradually but abruptly". At 4.2 K it fell from 1/500 of its value at 0 °C to a millionth part, and at 1.5 K it was less than a thousand-millionth of its value at normal temperature. He wrote that the mercury had entered "a new state, which, owing to its particular electrical properties, can be called the state of superconductivity."
He soon found that tin (about 3.8 K) and lead also become superconducting. He also found limits: above a threshold current, or above a threshold magnetic field (for lead at the boiling point of helium, 600 gauss), normal resistance returned. He thanked his colleague Holst for help with the electrical work.
Worked example: using his numbers
Suppose a mercury wire has a resistance of 100 Ω at 0 °C.
Just above the transition. At 1/500 of the 0 °C value, R = 100/500 = 0.2 Ω.
Just below 4.2 K. At a millionth of the 0 °C value, R = 100 × 10⁻⁶ = 1 × 10⁻⁴ Ω.
At 1.5 K. Less than a thousand-millionth: R < 100 × 10⁻⁹ = 1 × 10⁻⁷ Ω.
Now pass a current of 0.5 A and find the heating, P = I²R.
At 0.2 Ω: P = 0.25 × 0.2 = 0.05 W.
At 1 × 10⁻⁴ Ω: P = 0.25 × 10⁻⁴ = 2.5 × 10⁻⁵ W, two thousand times less.
This is why superconductors interest engineers: in principle, current can flow without wasting energy as heat. Our post on the heating effect of current has more practice with P = I²R.
Common mistakes:
Converting kelvin wrongly. 4.2 K is about −269 °C. Subtract 273.15, do not add.
Saying "very low resistance". Below the transition, resistance is too small to measure, not merely small. That sudden change is what defines superconductivity.
Forgetting the limits. Too much current or too strong a magnetic field destroys the state, as Onnes found.
What can a student take from his way of working?
Measure carefully. His motto, knowledge through measurement, is what made a surprising result believable.
Build the infrastructure. He trained instrument-makers and glass-blowers because good apparatus came first.
Choose your sample to test an idea. He picked mercury because it could be made purer than gold.
Credit your team. His Nobel lecture names the mechanic and colleagues who made the work possible.
Kamerlingh Onnes received the 1913 Nobel Prize in Physics "for his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium". The story continues with Pyotr Kapitsa and superfluid helium, and began with James Dewar's vacuum flask. For the basics of resistivity and how it changes with temperature, the Fermi Concept Library covers every concept in physics with its own lesson.
Next step: sketch resistance against temperature for an ordinary metal and for mercury on the same axes, and mark where the two curves differ.
