Find out more about the SI units
Measurement is at the heart of all science and engineering. It is only when we can measure something that scientists can study it and engineers can improve it. And since science and engineering play an important role in our lives, measurement matters for everyone.
Measurement affects our daily lives:
- When we buy a part that ‘just fits’: a nut fits a bolt, or a Lego® brick sticks perfectly to another brick
- When our medical care depends critically on measurements – of concentrations of chemicals in blood, or the intensity of X-rays
- When a satellite navigation system guides us along a road, and it depends on time measured by ultra-precision clocks on satellites
In all these situations, and thousands more, we are enjoying the benefits of a global system of measurement.
The International System of Units
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 NPL's key roles.
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’. They are the kilogram, the metre, the second, the ampere, the kelvin, the mole and the candela.
The seven base units have been chosen so that combinations of these can be used to express all other measurement units, known as ‘derived units’. For example, the unit of force – the newton (N) – is formed from base units with dimensions of mass, length and time: kilogram × metre per second squared (kg·m/s2).
Historically, units of measurement were defined by physical objects or properties of materials. For example, the metre was defined by the length between lines engraved on a metal bar and the kilogram is still defined as the mass of a carefully specified cylinder of platinum-iridium metal – the International Prototype Kilogram (IPK). But physical representations can be unstable – they can change over time or in different environments. So, over the years, the definitions have been improved to be more stable and reproducible, and to meet the needs of today’s research and technological applications.
During the last century, scientists have measured constants of nature, such as the speed of light and the Planck constant, with increasing accuracy and realised that they are more stable than any physical objects. So scientists have utilised these properties to develop new definitions of the units that will meet the demands on the measurement system as science advances.
What are the proposed changes?
The kilogram is the last SI base unit to still be defined as an artefact, but studies of closely similar copies tell us that the mass of the IPK is almost certainly changing … minutely. This implies a tiny but known change in the values of all masses. For mass, and for all units, we need to rule out this type of problem.
In November 2018, at a meeting of the General Conference on Weights and Measures, the global metrology community agreed a revision to the SI. The kilogram is now defined in terms of Planck's constant. This decision will mean that, for the first time, all seven base units are to be defined in terms of natural constants.
The actual changes will impact four of the base units: the kilogram, ampere, kelvin and mole.
- The kilogram – will be defined in terms of the Planck constant
- The ampere – will be defined in terms of the elementary charge
- The kelvin – will be defined in terms of the Boltzmann constant
- The mole – will be defined in terms of the Avogadro constant
Making this revision across the whole SI is a profound change in approach, that will underlie all measurements in science and more widely. But in everyday life it will appear that not much has changed. The redefinition of units is like replacing the weak foundations of a house with new foundations that are exactly the same size, but stronger. The difference isn’t visible on the surface, but substantial changes have been made to underpin the structure for the long term. Similarly, the changes in the SI will ensure that the SI definitions remain robust for the future, ready for advancements in science and technology.
Over the course of history, as our technological knowledge and demands have progressed, the definition of many of the units has changed. So don’t panic – water is still going to boil at 100 ͦ C, and you won’t need to change the time on your watch.
Find out more about the SI units