The definition of the kilogram, and that of other widely used units of measurement, has changed.
On 20 May 2019, the global metrology community celebrated World Metrology Day and the anniversary of the International System of Units (SI) with new defintions for four of the system's seven base units:
While most won't notice a difference, the change will be important for new technologies that require greater accuracy.
It is the most profound transformation of the SI, which since its establishment in 1960 has been adopted either in full or in parts by most countries in the world, although not the United States.
The system is directed by the General Conference of Weights and Measures (CGPM), which in November 2018 agreed to the system's overhaul that has now all units linked to unchanging fundamental properties of nature.
For the kilogram, the previous definition was based on a single block of platinum-iridium alloy, known as the International Prototype Kilogram or the 'Big K'. It was made in 1889, and has since been kept in a high-security vault in Paris, France.
However, a physical artefact such as the 'Big K' is affected by environmental factors such as temperature. By contrast, the Planck constant that now defines the kilogram is a physical constant relating to the energy of photons.
Australian researchers, including from the CSIRO and ANSTO and the National Measurement Institute (NMI), played a role in the development, according to the NMI's chief metrologist, Dr Bruce Warrington.
“CSIRO fabricated perfect spheres of pure silicon, which can be used as new mass standards. ANSTO measured the mix of silicon isotopes, critical to the precision of these standards. NMI measured the dimensions of the spheres to the nanometre, or billionth of a metre, and also developed leading-edge electrical standards,” Dr Warrington said.