von Klitzing constant

On the night of 4 February 1980, at the high-magnetic-field laboratory in Grenoble, Klaus von Klitzing was measuring a silicon MOSFET at 1.5 K in an 18 T field when the Hall resistance stopped varying smoothly and settled onto flat plateaus. The plateau values were h/(ie²) for integer i — reproducible to parts per billion, and utterly indifferent to the sample's size, shape, mobility or impurities. A dirty semiconductor device was producing a fundamental constant on demand. He was awarded the Nobel Prize in 1985, five years later, an unusually short wait.

Because the quantised value depends on nothing but h and e, any laboratory with a Hall bar, a big magnet and a dilution refrigerator can reproduce the same resistance to a part in 10⁹. From 1990 to 2019 the conventional value R_K-90 = 25812.807 Ω served as the world's ohm; the 2019 SI redefinition made h and e exact, so R_K became exact too. Graphene devices now do the job at higher temperature and lower field.