Today’s date — 20 May 2019 — marks a major milestone in measurement history. For the first time, the definitions of the base units that comprise the International System of Units (SI) are entirely derived from constants of nature like the speed of light and Avogadro’s number instead of human-made artifacts. The kilogramme, the SI base unit that held out the longest, will now be defined in terms of the Planck constant rather than the platinum-iridium cylinder known as ‘Le Grand K’*, *or ‘The Big K’.

Physical constants, unlike physical objects, are inherently stable and do not experience minute fluctuations in their properties. As a result, the definitions of all seven SI base units — the second, the meter, the kilogram, the ampere, the kelvin, the mole, and the candela — will remain accurate and unchanging for their countless applications in science, manufacturing, commerce, and other industries where near-perfect calibration is required.

### The End of Big K

So how was Le Grand K, a fixture of scientific measurement for nearly 130 years, finally dethroned? The redefinition of the kilogramme only became possible with the groundbreaking discovery of the quantum Hall effect by German physicist Klaus von Klitzing in 1980. The scientific community immediately recognised the significance of his findings, and he went on to receive the Nobel Prize in Physics a mere five years later.

“At the time of the discovery of the effect, I never believed that this has some influence on the kilogramme,” said von Klitzing during his 2016 Lindau lecture, which focused on how his Nobel Prize-awarded work will contribute to the new and improved SI. The Nobel Laureate will also participate in the upcoming 2019 Lindau Meeting.

The quantum Hall effect is a quantum-mechanical version of the conventional Hall effect, first discovered by American physicist Edwin Hall in 1879. While completing his graduate studies at Johns Hopkins University, he noticed that when a magnetic field was applied perpendicularly to a thin metal sheet through which an electric current is flowing, a small voltage appeared from one side of the sheet to the other. The magnetic field exerts a force on the moving electric charges, and the accumulation of charge on one side of the conductor leaves the other side oppositely charged, leading to the observed potential difference.

### Discovering the Quantum Hall Effect

Von Klitzing wanted to observe the Hall effect in more extreme conditions, at very low temperatures with a much stronger magnetic field. He performed experiments with two-dimensional electron systems, in which electrons are forced to move within an extremely thin layer, and smoothly varied the magnetic field. Surprisingly, he found that the observed Hall resistance — the ratio of the created voltage to the current — changed in discrete steps with exceptionally high accuracy.

In other words, the Hall resistance was exactly quantised. The resistance quantum, h/e^{2}, where e is the electron charge and h is the Planck constant, is now known as the ‘von Klitzing constant’. Because of its extraordinarily high precision, the von Klitzing constant has been used in resistance calibrations worldwide since 1990.

In combination with the Josephson constant (*KJ = 2e/h*), which originates from another electrical phenomenon called the Josephson effect, the von Klitzing constant (*RK = h/e ^{2}*) can be used in experiments to connect mass to the Planck constant. In 1999, scientists Peter Mohr and Barry Taylor at the National Institute of Standards and Technology proposed the redefinition of the kilogramme with such a method, motivated by recent progress in the development of the Kibble balance. Also known as a ‘watt balance’, this device precisely measures mass through the use of electrical measurements

“These two constants were the origin of the change we expect in the future for our SI system,” said von Klitzing in 2016, before the revised definitions were formally accepted. “The Josephson effect and the quantum Hall effect are the driving force for the expected change in the SI system in 2018.”

### The New and Improved SI

And as predicted, in November 2018, representatives from more than 60 countries voted to redefine the kilogramme in terms of the Planck constant during the 26^{th} meeting of the General Conference on Weights and Measures in France. The new SI chosen to come into effect today, on World Metrology Day 2019, whose theme is “The International System of Units – Fundamentally better.” The date itself, 20^{th} May, refers back to the signature of the Metre Convention in 1875 by representatives of 17 nations, which created the International Bureau of Weights and Measures (BIPM).

While most of us won’t notice a difference in everyday life, the new SI improves the precision of measurements for nanotechnology, communications, security, medicine, and emerging technologies such as quantum computing. In other words, the ‘fundamentally better’ SI might not impact you and me directly, but it will provide greater stability and accuracy to countless applications that have a significant effect on society.

*Additional information: During a lecture at the upcoming 69 ^{th} Lindau Nobel Laureate Meeting, Klaus von Klitzing will talk about the “Quantum Hall Effect and the New SI System”. Read an abstract of his lecture here. *