National Physical Laboratory

A redefinition of the kilogram

Mass is the only base SI unit that is defined in terms of a physical artefact – a 1 kilogram cylinder of platinum-iridium.

The main problem with a physical object is that its mass could change due to loss of material or contamination from the surrounding environment. It is also possible, however unlikely, that it could be lost or damaged.

The search is on in a number of scientific laboratories to try to find a way of defining the kilogram in terms of a fundamental constant. Today, two key approaches are being pursued internationally:

NPL can provide mass traceability for Kibble balances via a vacuum facility capable of weighing both Kibble balance working masses and the silicon spheres used in measurements of the Avogadro constant. Research into storage of masses in vacuum and inert gas at NPL is also crucial to the goal of redefining the kilogram.

Watt Balance

Kibble balance
Avagadro Sphere

Avogadro sphere
Last Updated: 22 Jul 2016
Created: 10 Sep 2007


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