Realisation (metrology)

For a topical guide to this subject, see Outline of the metric system.

The realisation of a unit of measure is the conversion of its definition into reality.[1] 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.

In the case of the International System of Units, realisation techniques for the base units base are maintained by the International Bureau of Weights and Measures [2]

Overview

The Oxford English Dictionary defines the word "realise" (also spelt "realize") as "to convert (something imagined, planned, etc.) into real existence or fact."[1] 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.[3]


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

Timekeepers

Time generators

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.

An example of a modern realisation is the realisation of the meter in terms of optical frequency standards.[4]

Volume

Mass

Electric charge

Temperature

Photometry

Amount of substance

References

  1. 1 2 "Realise". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005. (Subscription or UK public library membership required.)
  2. ↑ "Practical realization of the definitions of some important units". International Bureau of Weights and Measures. 2012. p. 46. Retrieved 23 April 2013.
  3. ↑ International vocabulary of metrology—Basic and general concepts and associated terms (VIM) (PDF) (3rd ed.). International Bureau of Weights and Measures on behlaf of the Joint Committee for Guides in Metrology. 2012. Retrieved 26 April 2013.
  4. ↑ 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. doi:10.1088/0026-1394/40/2/316. Retrieved 6 December 2013.
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