New definitions for the kilogram, mole, ampere, and kelvin

J.L. Lee/NIST

METROLOGISTS from over 60 countries met at the General Conference on Weights and Measures in Versailles, France, earlier this month and unanimously voted for new definitions of the kilogram, mole, ampere, and kelvin.

The new definitions are based on fundamental physical constants, meaning that all seven base units in the International System of Units (SI) – which includes the second, candela, and metre – are now defined in a manner that ensures future stability. The metre used to be defined as the distance between two scratches on a bar of platinum-iridium metal, but was redefined in 1983 based on the speed of light.

Since 1879, the kilogram has been defined based on a cylinder of platinum and iridium, known as the International Prototype Kilogram (IPK), which sits in a vault at the International Bureau of Weights and Measures (BIPM) in France. Copies of the reference weight were distributed worldwide for others to calibrate their kilogram, but if the original got scratched or lost some mass, the value of the kilogram would change. In 1990, it was discovered that the IPK had become lighter than its six official copies by around 50 micrograms, however it took decades of work to develop the technology needed to accurately and precisely measure the kilogram.

The kilogram will now be defined using the Planck constant, which is measured with a piece of equipment called the Kibble balance. Currently only two labs have produced a Kibble balance, but it is hoped that eventually anyone will be able to accurately weigh a kilogram. The uncertainty in the kilogram also impacted the mole, which has been defined in relation to the kilogram since 1971. The mole will now be defined with respect to Avogadro’s number. The ampere will be defined with respect to the electric charge carried by a single proton, and the kelvin with respect to the Boltzmann constant. The size of the units will not change; they will just be more stable as they are no longer reliant on physical objects.

Martin Milton, director at BIPM, said: "The SI redefinition is a landmark moment in scientific progress. Using the fundamental constants we observe in nature as a foundation for important concepts, such as mass and time, means that we have a stable foundation from which to advance our scientific understanding, develop new technologies and address some of society's greatest challenges. We will now no longer be bound by the limitations of objects in our measurement of the world, but have universally accessible units that can pave the way to even greater accuracy, and even accelerate scientific advancement."

The change will come into effect on 20 May 2019.

This article has been updated to give the correct acronym for the International Bureau of Weights and Measures (BIPM). The previous version incorrectly gave it as IBWM.