This is an audio version of the Wikipedia Article:
00:02:49 1 Units
00:02:57 1.1 Base units
00:04:06 1.2 Derived units with special names
00:05:57 1.3 Auxiliary and accessory units
00:10:03 2 Realisation of units
00:12:31 3 Properties as a system
00:13:08 3.1 Units based on the natural world
00:14:02 3.2 Base and derived unit structure
00:15:35 3.3 Decimal ratios
00:16:27 3.4 Prefixes for multiples and submultiples
00:18:26 3.5 Coherence
00:19:05 3.6 Rationalisation
00:19:30 4 International System of Units
00:19:39 4.1 Historical variants
00:20:05 4.1.1 Gaussian second and the first mechanical system of units
00:22:36 4.1.2 The EMU, ESU and Gaussian systems of electrical units
00:22:58 4.1.3 Centimetre–gram–second systems
00:23:21 4.1.4 International system of electrical units
00:23:41 4.1.5 MKS and MKSA systems
00:24:37 4.1.6 Metre–tonne–second systems
00:25:05 4.1.7 Gravitational systems
00:26:19 5 Conversion, calculation and symbol confusion incidents
00:26:47 6 Conversion table
00:27:25 7 See also
00:29:19 8 Notes
00:30:13 9 References
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"The only true wisdom is in knowing you know nothing."
The metric system is an internationally recognised decimalised system of measurement. It is in widespread use, and where it is adopted, it is the only or most common system of weights and measures (see metrication). It is now known as the International System of Units (SI). It is used to measure everyday things such as the mass of a sack of flour, the height of a person, the speed of a car, and the volume of fuel in its tank. It is also used in science, industry and trade.
In its modern form, it consists of a set of base units including metre for length, kilogram for mass, second for time and ampere for electrical current, and a few others, which together with their derived units, can measure any physical quantity. Metric system may also refer to other systems of related base and derived units defined before the middle of the 20th century, some of which are still in limited use today.
The metric system was designed to have properties that make it easy to use and widely applicable, including units based on the natural world, decimal ratios, prefixes for multiples and sub-multiples, and a structure of base and derived units. It is also a coherent system, which means that its units do not introduce conversion factors not already present in equations relating quantities. It has a property called rationalisation that eliminates certain constants of proportionality in equations of physics.
The units of the metric system, originally taken from observable features of nature, are now defined by phenomena such as the microwave frequency of a caesium atomic clock which accurately measures seconds. One unit, the kilogram, remains defined in terms of a man-made artefact, but scientists recently voted to change the definition to one based on Planck's constant via a Kibble balance. The new definition is expected to be formally propagated in 2019.
While there are numerous named derived units of the metric system, such as watt and lumen, other common quantities such as velocity and acceleration do not have their own unit, but are defined in terms of existing base and derived units such as metres per second for velocity.
Though other currently or formerly widespread systems of weights and measures continue to exist, such as the British imperial system and the US customary system of weights and measures, in those systems most or all of the units are now defined in terms of the metric system, such as the US foot which is now a defined decimal fraction of a metre.
The metric system is also extensible, and new base and derived units are defined as needed in fields such as radiology and chemistry. The most recent derived unit, the katal, for catalytic activity, was added in 1999. Recent changes are directed toward defining base units in terms of invariant constants of physics to provide more precise realisations of units for advances in science and industry.