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| ===Time=== | ===Time=== | ||
| The realisation of time has gone through three phases. During both the first and second phases, man used ]—during the first phase, realisation of time was by observing the ] using such devices as the ] or ]. During the second phase actual ] such as ]es or ]s were used. If the user needed to know ] rather than elapsed time, clocks were ] with ]. The third phase made use of clocks that were sufficiently accurate that they could measure variations in the |
The realisation of time has gone through three phases. During both the first and second phases, man used ]—during the first phase, realisation of time was by observing the ] using such devices as the ] or ]. During the second phase actual ] such as ]es or ]s were used. If the user needed to know ] rather than elapsed time, clocks were ] with ]. 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. <!--Early man measured two aspects of time - seasons and time of day. The seasons were measured by keeping a record on how many days had passed since an astronomical phenomena as is believed to have happened at ]. Time of day was measured using . One of the earliest time-measuring devices was the introduced into Europe by Luitprand, a monk at the cathedral in ], France in the 8th century AD.--> | ||
| ====Direct measurement of solar time==== | ====Direct measurement of solar time==== | ||
| *]s and ]s | *]s and ]s | ||
| ====Timekeepers==== | ====Timekeepers==== | ||
| * |
*Accuracy of clocks | ||
| ====Time generators==== | ====Time generators==== | ||
| * |
*Radiation frequency and SI | ||
| ===Length=== | ===Length=== | ||
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| ===Volume=== | ===Volume=== | ||
| * |
*Jugs etc. in ancient times | ||
| * |
*Not a base unit in SI | ||
| ===Mass=== | ===Mass=== | ||
| *Grains | |||
| *grains | |||
| * |
*Artefacts held by governments (e.g. the ]) | ||
| *] |
*] and Avogadro experiment | ||
| ===Electric charge=== | ===Electric charge=== | ||
| *] deposits | *] deposits | ||
| *] | *] | ||
| *] | *] | ||
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| ===Photometry=== | ===Photometry=== | ||
| * |
*Sensitivity of the eye | ||
| ===Amount of substance=== | ===Amount of substance=== | ||
Revision as of 13:10, 29 January 2024
Conversion of the definition of a unit of measure into reality
In metrology, the realisation of a unit of measure is the conversion of its definition into reality. The International vocabulary of metrology 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.
The International Bureau of Weights and Measures maintains the techniques for realisation of the base units in the International System of Units (SI).
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 International vocabulary of metrology 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—during the first phase, realisation of time was by observing the Earth's rotation using such devices as the sundial or astrolabe. During the second phase actual timing devices such as hourglasses or clocks were used. If the user needed to know time-of-day rather than elapsed time, clocks were synchronized 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
- Sundials and astrolabes
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 - barleycorn, feet
- regal units - measures held by ruler
- using speed of light
An example of a modern realisation is the realisation of the meter 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 IPK)
- Kibble balance and Avogadro experiment
Electric charge
Temperature
- freezing & boiling water
- non-linearity etc.
- Boltzmann's constant
Photometry
- Sensitivity of the eye
Amount of substance
References
- ^ "Realise". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
- International Bureau of Weights and Measures (2012). "Practical realization of the definitions of some important units". p. 46. Retrieved 23 April 2013.
- International vocabulary of metrology—Basic and general concepts and associated terms (VIM) (PDF) (3rd ed.). International Bureau of Weights and Measures on behalf of the Joint Committee for Guides in Metrology. 2012. Retrieved 26 April 2013.
- Quinn, T. J. (2003). "Practical realisation of the definition of the metre, including recommended radiations of other optical frequency standards (2001)" (PDF). Metrologia. 40: 103–133. Bibcode:2003Metro..40..103Q. doi:10.1088/0026-1394/40/2/316. Retrieved 6 December 2013.
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