But how do we know that ‘one metre’ in the UK is the same as ‘one metre’ in Japan? And how do we ensure that ‘one metre’ today is the same as it was 20 years ago?
We do this by using the globally agreed International System of Units, known as the SI (from the French, Système International d’Unités). The SI sets out what the agreed units of measurement are, how they are defined and how they are realised in practice.
The widespread adoption of the SI allows science, industry and trade to measure physical objects and phenomena using the same units, so that the results can be compared meaningfully, worldwide.
The SI was formally agreed in 1960 and is directed from the International Bureau of Weights and Measures (BIPM), a laboratory situated in diplomatically-protected territory in Sèvres, just outside Paris. At BIPM, scientists from all over the world meet to agree upon definitions of precisely what we mean by each of the SI base units. Typically, these scientists work at national measurement institutes which have responsibility for making the benefits of the SI available within individual countries. In the UK, this is one of the key roles of the National Physical Laboratory.