Metric system
Though the rules governing the metric system have changed over time, the modern definition, the International System of Units (SI), defines the metric prefixes and seven base units: metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd).
Ancient non-metric but SI-accepted multiples of time, minute and hour, are base 60 (sexagesimal).
Similarly, the angular measure degree and submultiples, arcminute, and arcsecond, are also sexagesimal and SI-accepted.
Although used in some contexts, the US has resisted full adoption; continuing to use "a conglomeration of basically incoherent measurement systems".
The only exceptions are for the SI-accepted units of time (minute and hour) and angle (degree, arcminute, arcsecond) which, based on ancient convention, use base-60 multipliers.
These relations can be written symbolically as:[4] The decimalised system is based on the metre, which had been introduced in France in the 1790s.
The definition of base units has increasingly been realised in terms of fundamental natural phenomena, in preference to copies of physical artefacts.
[5]: 15–18 The metric system is extensible since the governing body reviews, modifies and extends it needs arise.
For example, the katal, a derived unit for catalytic activity equivalent to one mole per second (1 mol/s), was added in 1999.
[7] Where possible, definitions of the base units were developed so that any laboratory equipped with proper instruments would be able to realise a standard without reliance on an artefact held by another country.
[8] In 1791 the commission originally defined the metre based on the size of the earth, equal to one ten-millionth of the distance from the equator to the North Pole.
In the SI, the standard metre is now defined as exactly 1⁄299792458 of the distance that light travels in a second.
Replicas made in 1879 at the time of the artefact's fabrication and distributed to signatories of the Metre Convention serve as de facto standards of mass in those countries.
For example, 12 inches is one foot, but the larger unit in the same system, the mile is not a power of 12 feet.
[16]: 222–223 During the 19th century the prefix myria-, derived from the Greek word μύριοι (mýrioi), was used as a multiplier for 10000.
[4] For the most part, the metric prefixes are used uniformly for SI base, derived and accepted units.
A notable exception is that for a large measure of seconds, the non-SI units of minute, hour and day are customary instead.
Coherence was a design aim of SI, which resulted in only one unit of energy being defined – the joule.
relating to steradians, representative of the fact that electric charges and magnetic fields may be considered to emanate from a point and propagate equally in all directions, i.e. spherically.
This factor made equations more awkward than necessary, and so Oliver Heaviside suggested adjusting the system of units to remove it.
[21] The basic units of the metric system have always represented commonplace quantities or relationships in nature; even with modern refinements of definition and methodology.
In cases where laboratory precision may not be required or available, or where approximations are good enough, the commonplace notions may suffice.
The French Revolution (1789–99) enabled France to reform its many outdated systems of various local weights and measures.
In 1790, Charles Maurice de Talleyrand-Périgord proposed a new system based on natural units to the French National Assembly, aiming for global adoption.
These were chosen so that the corresponding electrical units of potential difference, current and resistance had a convenient magnitude.
[20] It also includes numerous coherent derived units for common quantities like power (watt) and irradience (lumen).
[43] The system was promulgated by the General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM) in 1960.
The kilogram was defined by a cylinder of platinum-iridium alloy until a new definition in terms of natural physical constants was adopted in 2019.
