The time has come for another historic retirement. The ‘original’ kilo’, a cylinder made of an alloy of platinum and iridium that for the last 130 years has been the International Prototype for measuring weight and mass may soon be put out to grass.
On November 16, 2018, metrologists from more than 60 countries, including Greece, will come together to hold a formal vote on the redefinition of several standards in the International System of Units (SI), changing the world’s definition of the kilogram, the ampere, the kelvin and the mole.
If agreed, these changes will come into force on May 20 in 2019. The vote will take place at the General Conference on Weights and Measures (CGPM) at the Palais des Congrès in Versailles, which is organised by the Bureau International de Poids et Mesures (BIPM). The decision will mean that all SI units will be defined in terms of constants that describe the natural world. This will assure the future stability of the SI and allow the use of new technologies, including quantum technologies, to implement the definitions.
The decision will end the use of physical objects to define measurement units. The kilogram, for example, is currently defined as being the mass of the International Prototype of the Kilogram, a cylinder made out of a platinum-iridium alloy stored at the BIPM. This object has served science and technology well for nearly 130 years, but its stability during this period could only be confirmed by comparisons with identical copies, which is a difficult process and potentially inaccurate.
The proposed change will see the kilogram defined in terms of the Planck constant – the fundamental constant of quantum physics. It is ready for use everywhere and always.
The expected new definitions impact four of the seven base units of the SI: the kilogram, the ampere, the kelvin and the mole, and all units derived from them, such as the volt, ohm and joule.
The kilogram will be defined by the Planck constant (h)
The ampere will be defined by the elementary electrical charge (e)
The kelvin will be defined by the Boltzmann constant (k)
The mole will be defined by the Avogadro constant (NA)
Although the size of these units will not change (a kilogram will still be a kilogram), the four redefined units will join the second, the metre and the candela to ensure that the set of SI base units will continue to be both stable and useful. The revised SI will maintain its relevance by facilitating technical innovations. Just as the redefinition of the second in 1967 provided the basis for technology that has transformed how we communicate across the globe, through GPS and the internet, the new changes will have wide-reaching impact in science, technology, trade, health and the environment, among many other sectors.
Until today, the original kilogram or the “Grand K” used since 1889 was guarded in a laboratory in the building of the BIPM in Chevres, France. The iconic cylinder, which is the last physical construction still used to define a measure of the SI system will not be thrown away. BIPM will keep it and will calibrate it from time to time as secondary protype for measuring mass.