page_title: Metrology
section_title: Realisation of units
hierarchical_section_title: Metrology / Concepts / Realisation of units
caption_attribution_description: A computer-generated image of the International Prototype kilogram (IPK), which is made from an alloy of 90% platinum and 10% iridium (by weight) and machined into a right-circular cylinder (height = diameter) of 39.17&#160;mm. The IPK is kept at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures) in Sèvres on the outskirts of Paris. The geometry of this computer model was based on the actual specifications being used for experiments in new manufacturing techniques to produce new kilogram mass standards. Solid model and ray-traced image was created using Cobalt CAD software. This image was provided by the contributor because of the dearth of uncopyrighted photographs of the actual IPK. This is expected to be a chronic limitation given that… the IPK is stored in a vault nearly all the time, there is no general public access to the BIPM (and certainly none to the vault), and working copies of the IPK are used at the BIPM for routine calibrations for years on-end.
context_page_description: Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in France, when a length standard taken from a natural source was proposed. This led to the creation of the decimal-based metric system in 1795, establishing a set of standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875; to ensure conformity between the countries, the Bureau International des Poids et Mesures was established by the Metre Convention. This has evolved into the International System of Units as a result of a resolution at the 11th Conference Generale des Poids et Mesures in 1960.
  Metrology is divided into three basic overlapping activities.
  The first being the definition of units of measurement, second the realisation of these units of measurement in practice, and last traceability, which is linking measurements made in practice to the reference standards. These overlapping activities are used in varying degrees by the three basic sub-fields of Metrology.
context_section_description: The realisation of a unit of measure is its conversion into reality. Three possible methods of realisation are defined by the international vocabulary of metrology (VIM): a physical realisation of the unit from its definition, a highly-reproducible measurement as a reproduction of the definition (such as the quantum Hall effect for the ohm), and the use of a material object as the measurement standard.
caption: Computer-generated image realising the international prototype of the kilogram (IPK), made from an alloy of 90-percent platinum and 10-percent iridium by weight
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