![]() ![]() So, over the years, the definitions have evolved to depend instead on constants of nature that are more stable and reproducible, meeting the demanding needs of today’s research and technological applications.ĭuring the last century, scientists measured constants of nature, such as the speed of light and the Planck constant, with increasing accuracy. However, such physical representations can change over time and are susceptible to damage or loss. ![]() In these two examples, the definition was also the realisation – the physical form – of the unit. For example, the metre was defined by the length between lines engraved on a metal bar and the kilogram was defined as the mass of a single cylinder of platinum-iridium metal – the International Prototype of the Kilogram (IPK). Historically, units of measurement were defined by physical objects or properties of materials. In November 2022, measurement scientists and government representatives from around the world voted to expand the range of prefixes used within the International System of Units, meaning that four new prefixes, (which were proposed by NPL's Head of Metrology, Richard Brown) will now be used to express measurements worldwide.įind out more about the expansion of the SI prefixes here Equally, having reliable information on climate change, pollution and medical diagnostics is important to society and builds trust, allowing effective decisions to be made.įind out more about the redefinition of the SI units For instance, mass is measured every day, and having agreement on the definition of the kilogram means that consumers can trust that the shop is really providing the mass they say they are. Standardising such measurements not only helps to keep them consistent and accurate, but also helps society have confidence in data. This International System of Units is necessary to ensure that our everyday measurements remain comparable and consistent worldwide. The SI covers units for every type of measurement, but at the heart of the SI is a set of seven units known as the ‘base units’. The globally-agreed system of measurement units was formally named the 'International System of Units' (SI) in 1960. ![]()
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