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# Measurement Units

## SI Base Units

#### The SI (Système International d'Unités) is a globally agreed system of units, with seven base units.

Formally agreed by the 11th General Conference on Weights and Measures (CGPM) in 1960, the SI is at the centre of all modern science and technology. The definition and realisation of the base and derived units is an active research topic for metrologists with more precise methods being introduced as they become available.

There are two classes of units in the SI: base units and derived units. The base units provide the reference used to define all the measurement units of the system, whilst the derived units are products of base units and are used as measures of derived quantities:

There are recommendations as to how to use SI units. SI prefixes are used to form decimal multiples and submultiples of the units:

Some non-SI units are still widely used:

The definitions of the SI units have a continuing history of change:

## Fundamental Constants and Units

#### The fundamental physical constants, such as the speed of light, the Planck constant and the mass of the electron provide a system of natural units.

However, these must be related to the SI units by experiment. This experimental work is a global effort mostly undertaken in national standards laboratories to which NPL contributes. The constants provide the link between the SI units and theory and also between one part of physics and the SI and another.

For more information, a review article describing the background to the change to units based on fundamental constants is available.

NPL has activity in the Planck constant (watt balance) Rydberg constant (Hydrogen spectroscopy) Stefan-Boltzman constant (ARD).

## Recommended Values of the Constants

#### A list of values and uncertainties of the most frequently used constants to CODATA Recommended Values (2005) is available.

These values are taken from the recommended values of the constants which are produced by the CODATA Task Group on Fundamental Constants, based on a review of all the available data. The latest review is available at the CODATA fundamental constants page at NIST. This should be consulted for values of the less frequently used constants or for covariances between the constants.

## SI, Units & Constants FAQs

• ### Haven't all the problems of the units long since been solved? Why is there any need for ever more precise measurements? (FAQ - Quantum)

Metrology is a service discipline - responding to a perceived need for a particular measurement accuracy, either now or in the near future.
• ### How often are the fundamental constants of physics updated and by whom? (FAQ - Quantum)

The recommended values of the fundamental constants are produced by the CODATA Task Group on Fundamental Constants the most recent evaluation was in 1998.
• ### How would we know if the kilogram was changing with time? (FAQ - Quantum)

We already can set limits on the drift over a long period by looking at the values obtained for fundamental constants that depend critically on mass.
• ### How would we know if the speed of light was varying with time? (FAQ - Quantum)

The very term fundamental physical constants invites two questions: are they fundamental and are they constant.
• ### If everything has already been internationally agreed upon, why isn't it enough to have a single state institute for the whole world? (FAQ - Quantum)

There are several reasons for maintaining separate national capabilities.
• ### If the constants are constant why do they need updating? (FAQ - Quantum)

What is evolving is our knowledge of the constants not as far as we know their values, which for the purposes of evaluation are considered constant.
• ### What is a base unit? (FAQ - Quantum)

SI units are divided into two classes, base units and derived units. The base units are dimensionally independent.
• ### What is metrology? (FAQ - Quantum)

The word metrology is derived from the Greek word `metron': to measure.
• ### What is the connection between SI base units and the fundamental constants? (FAQ - Quantum)

The relationships are many and complex.
• ### What is the minimum number of base units that is required? (FAQ - Quantum)

A difficult question perhaps impossible. In what way the minimum? Some would say seven have been introduced into the SI because seven are needed. It has also been argued that with the use of fundamental constants only one unit is needed.
• ### What is the most accurate measurement known? (FAQ - Quantum)

No dimensioned measurement can be made more accurately than its corresponding SI unit is known. Thus the measurements with the smallest uncertainty are those of frequencies as the second is the most precisely realised unit.
• ### Why are there seven base units in the SI system? (FAQ - Quantum)

The international system is a set of seven base units chosen to fulfil the requirements of science and technology. The selection of seven base units is a matter of choice.
• ### Why do we need both thermodynamic and practical temperature scales? (FAQ - Engineering Measurements)

Temperature is an intensive property and we can only measure thermodynamic temperature via measurable quantities which change with temperature. Because this is re-measured from time to time and the values revised, this scale may differ from the true thermodynamic temperature scale.
• ### Why do you not use a stabilised laser to define the metre? (FAQ - Quantum)

If we did, we would have to change the definition of the metre each time we were able to make a more precise laser.
• ### Why is the kilogram (and not the gram) the base unit of mass? (FAQ - Quantum)

The name 'kilogram' is a historical quirk.
• ### Wouldn't it be preferable to organise metrology by co-operation of national metrology institutes, each of them being responsible for special fields, such as mechanics, electricity, optics, or ionizing radiation? (FAQ - Quantum)

Discussions in Europe under the MERA project are pointing in the direction of this alternative to a single world or European institute, but some duplication and collaboration will probably always be required.