metrology Metrology is the scientific study of measurement. It establishes a common understanding of Unit of measurement, units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to stan ...

, the realisation of a

unit of measure A unit of measurement, or unit of measure, is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can ...

is the conversion of its definition into reality. An example of the concept of realisation, is the unit of length, the

meter The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...

. The metre was originally defined in 1791 as one ten-millionth of the distance from the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

to the

North Pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True North Pole to distingu ...

along a

great circle In mathematics, a great circle or orthodrome is the circular intersection of a sphere and a plane passing through the sphere's center point. Discussion Any arc of a great circle is a geodesic of the sphere, so that great circles in spher ...

. To actually measure a length, this definition must be converted into a physical tool, which can be used to complete the measurement. The meter stick is the realisation of the meter. The

International Bureau of Weights and Measures The International Bureau of Weights and Measures (, BIPM) is an List of intergovernmental organizations, intergovernmental organisation, through which its 64 member-states act on measurement standards in areas including chemistry, ionising radi ...

maintains the techniques for realisation of the base units in the

International System of Units The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official s ...

(SI), all seven of which are defined in terms of natural physical constants, rather than human artefacts such as the standard kilogram. Following the

2019 revision of the SI In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artefacts such as the standard kilogram. Effective 20 May 2019, the 144th ...

all fundamental units of metrology are now defined in terms of natural physical constants, rather than human artefacts. The realization of these units is also defined by a published "Practice for the Realization of the Unit", for each unit. This is a detailed set of technical instructions for the construction of a device that will produce a practical realization of each unit. Any competent person, who follows these instructions can realize any unit. For example, the metre is defined as 1/ of the distance light travels in one second. The Practice for the Realization of the Metre describes how to build an apparatus to determine this distance. Using this apparatus it is possible to construct a metre stick which is the realization of the metre. The

International vocabulary of metrology The Joint Committee for Guides in Metrology (JCGM) is an organization in Sèvres that prepared the ''Guide to the Expression of Uncertainty in Measurement'' (GUM) and the ''International Vocabulary of Metrology'' (VIM). The JCGM assumed responsibil ...

identifies three distinct methods of realisation: # Realisation of a measurement unit from its definition. # Reproduction of measurement standards. # Adopting a particular artefact as a standard.

Overview

The Oxford English Dictionary defines the word "realise" (also spelt "realize") as "to convert (something imagined, planned, etc.) into real existence or fact". The

identifies three distinct ways in which this is done – the first being the realisation of a measurement unit from its definition, the second the reproduction of measurement standards and the third the process of actually adopting a particular artefact as a standard.

Techniques

Time

The realisation of time has gone through three phases. During both the first and second phases, man used

solar time Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based ...

– during the first phase, realisation of time was by observing the

Earth's rotation Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in progra ...

using such devices as the

sundial A sundial is a horology, horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the position of the Sun, apparent position of the Sun in the sky. In the narrowest sense of the ...

astrolabe An astrolabe (; ; ) is an astronomy, astronomical list of astronomical instruments, instrument dating to ancient times. It serves as a star chart and Model#Physical model, physical model of the visible celestial sphere, half-dome of the sky. It ...

. During the second phase actual timing devices such as hourglasses or

clock A clock or chronometer is a device that measures and displays time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month, a ...

s were used. If the user needed to know time-of-day rather than elapsed time, clocks were synchronised with astronomical time. The third phase made use of clocks that were sufficiently accurate that they could measure variations in the Earth's rotation – such clocks taking over from the rotation of the earth as the prime measure of time.

Direct measurement of solar time

Sundial A sundial is a horology, horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the position of the Sun, apparent position of the Sun in the sky. In the narrowest sense of the ...

s and

Timekeepers

* Accuracy of clocks

Time generators

* Radiation frequency and SI

Length

Units of length, along with mass (or weight) and time, are one of the earliest quantities that was measured by man. Historically two distinct approaches were used – one was to use a naturally occurring phenomenon such as a particular seed or part of the human body, the other was to use a standard length that was held by a community leader. *

Natural units In physics, natural unit systems are measurement systems for which selected physical constants have been set to 1 through nondimensionalization of physical units. For example, the speed of light may be set to 1, and it may then be omitted, equa ...

– barleycorn, feet * Regal units – measures held by ruler * Using

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...

An example of a modern realisation is the realisation of the

metre The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...

in terms of optical frequency standards.

Volume

* Jugs etc. in ancient times * Not a base unit in SI

Mass

* Grains * Artefacts held by governments (e.g. the

International Prototype of the Kilogram The International Prototype of the Kilogram (referred to by metrology, metrologists as the IPK or Le Grand K; sometimes called the ''wiktionary:ur-#Prefix, ur-kilogram'', or ''urkilogram'', particularly by German-language authors writing in Engli ...

) *

Kibble balance A Kibble balance (also formerly known as a watt balance) is an electromechanical measuring instrument that measures the weight of a test object very precisely by the electric current and voltage needed to produce a compensating force. It is a Me ...

and Avogadro experiment

Electric charge

* Silver nitrate deposits * force between conductors * charge on the electron

Temperature

freezing Freezing is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point. For most substances, the melting and freezing points are the same temperature; however, certain substances possess dif ...

boiling Boiling or ebullition is the rapid phase transition from liquid to gas or vapor, vapour; the reverse of boiling is condensation. Boiling occurs when a liquid is heated to its boiling point, so that the vapour pressure of the liquid is equal to ...

water * Non-linearity etc. *

Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative thermal energy of particles in a ideal gas, gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin (K) and the ...

Photometry

* Sensitivity of the eye

Amount of substance

* Development of the mole

References

{{DEFAULTSORT:Realisation Systems of units