Metrology is a service discipline - responding to a perceived need for a particular measurement accuracy, either now or in the near future.
Measurement Units
SI Base Units
The SI (Systeme International d'Unites) is a globally agreed system of units based on the metric system.
The SI system consists of seven base units:
- Length - Definition - Beginners Guide - NPL Dimensional area
- Mass - Definition - Beginners Guide - NPL Mass area
- Time - Definition - Beginners Guide - NPL Time area
- Electric current - Definition - Beginners Guide - NPL Electromagnetic area
- Thermodynamic temperature - Definition - Beginners Guide - NPL Thermal area
- Amount of substance - Definition
- Luminous intensity - Definition - Beginners Guide - NPL Optical & Photonics area
- Examples of SI Derived units
- Internationally recognised non-SI units
- SI prefixes
- SI conventions
- Proposed changes to the SI system
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 is provided here <awaiting link> to CODATA Recommended Values (2005). 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
- The recommended values of the fundamental constants are produced by the CODATA Task Group on Fundamental Constants the most recent evaluation was in 1998.
- 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.
- The very term fundamental physical constants invites two questions: are they fundamental and are they constant.
- There are several reasons for maintaining separate national capabilities.
- 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.
- SI units are divided into two classes, base units and derived units. The base units are dimensionally independent.
- The word metrology is derived from the Greek word `metron': to measure.
- The relationships are many and complex. DIUS's quantum metrology programme addresses some of the issues.
- 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.
- 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.
- 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.
- If we did, we would have to change the definition of the metre each time we were able to make a more precise laser.
- The name "kilogram" is a historical quirk.
- 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.