International System of Units

The International System of Units (abbreviated SI from the French language name Système International d'Unités) is the modern form of the metric system. It is the world's most widely used system of units, both in everyday commerce and in science.

The older metric system included several groupings of units. The SI was developed in 1960 from the metre-kilogram-second (MKS) system, rather than the centimetre-gram-second (CGS) system, which, in turn, had many variants.

The SI introduced several newly named units. The SI is not static; it is a living set of standards where units are created and definitions are modified with international agreement as measurement technology progresses.

With few exceptions (such as draught beer sales in the United Kingdom), the system is legally being used in every country in the world, and many countries do not maintain official definitions of other units. In the United States, industrial use of SI is increasing, but popular use is still limited. In the United Kingdom, conversion to metric units is official policy but not yet complete. Those countries that still recognize non-SI units (e.g. the US and UK) have redefined their traditional non-SI units in terms of SI units.

History

''See main articles: metre, kilogram, second, ampere, kelvin, candela, and mole.

The metric system was created during the French Revolution. On August 1, 1793 the National Convention adopted the new decimal "metre" with a provisional length as well as the other decimal units with preliminary definitions and terms. On April 7, 1795 (Loi du 18 germinal, an III) the terms gramme and kilogramme replaced the former terms "gravet" and "grave".

A month after the coup of 18 Brumaire, the metric system was definitively adopted in France by the First Consul Bonaparte, (the later Napoleon I) on December 10, 1799. During the history of the metric system a number of variations have evolved and their use spread around the world replacing many traditional measurement systems.

By the end of World War II a number of different systems of measurement were still in use throughout the world. Some of these systems were metric system variations whilst others were based on the Imperial and American systems. It was recognised that additional steps were needed to promote a worldwide measurement system. As a result the 9th General Conference on Weights and Measures (CGPM), in 1948, asked the International Committee for Weights and Measures (CIPM) to conduct an international study of the measurement needs of the scientific, technical, and educational communities.

Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. The six base units recommended were the metre, kilogram, second, ampere, Kelvin degree (later renamed the kelvin), and the candela. In 1960, the 11th CGPM named the system the International System of Units, abbreviated SI from the French name: Le Système International d'Unités. The seventh base unit, the mole, was added in 1970 by the 14th CGPM.

The International System is now either obligatory or permissible throughout the world. It is administered by the standards organisation: the International Bureau of Weights and Measures (BIPM).

Units

Main articles: SI base unit, SI derived unit, SI prefix

The international system of units consists of a set of units together with a set of prefixes. The units of SI can be divided into two subsets. There are the seven base units. Each of these base units are dimensionally independent. From these seven base units several other units are derived. In addition to the SI units there are also a set of non-SI units accepted for use with SI.

SI base units
Name	Symbol	Quantity
kilogram	kg	Mass
second	s	Time
metre	m	Length
ampere	A	Electrical current
kelvin	K	Temperature
mole	mol	Amount of substance
candela	cd	Luminous intensity

A prefix may be added to units to produce a multiple of the original unit. All multiples are integer powers of ten. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth hence there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined: a millionth of a kilogram is a milligram not a microkilogram.

SI writing style

Symbols are written in lower case, except for symbols derived from the name of a person. For example, the unit of pressure is named after Blaise Pascal, so its symbol is written "Pa" whereas the unit itself is written "pascal". The one exception is the litre, whose original symbol "l" is unsuitably similar to the numeral "1". The American National Institute of Standards and Technology recommends that "L" be used instead, a usage which is common in the U.S., Canada and Australia, and has been accepted as an alternative by the CGPM. The cursive "ℓ" is occasionally seen, especially in Japan, but this is not currently recommended by any standards body. For more information, see Litre.
Abbreviated symbols, unlike spelled-out full names of units, should not be pluralized—for example "25 kg", not "25 kgs"—though they sometimes are. For spelled-out unit names in English, all are made plural by adding an 's', except lux, hertz, and siemens, all of which are the same in singular and plural.
Symbols do not have an appended period (.) unless at the end of a sentence.
It is preferable to write symbols in upright Roman type (m for metres, L for litres), so as to differentiate from the italic type used for mathematical variables (m for mass, l for length).
A space should separate the number and the symbol, e.g. "2.21 kg", "7.3×10² m²", "22 °C" ^*. Exceptions are the symbols for plane angular degrees, minutes and seconds (°, ′ and ″), which are placed immediately after the number with no intervening space.
Spaces may be used to group decimal digits in threes, e.g. 1 000 000 or 342 142 (in contrast to the commas or dots used in other systems, e.g. 1,000,000 or 1.000.000). This is presumably to reduce confusion. In print, the space used for this purpose is typically narrower than that between words.
The 10th resolution of CGPM in 2003 declared that "the symbol for the decimal marker shall be either the point on the line or the comma on the line". In practice, the decimal point is used in English, and the comma in most other European languages.
Symbols for derived units formed from multiple units by multiplication are joined with a space or centre dot (·), e.g. N m or N·m.
Symbols formed by division of two units are joined with a solidus (/), or given as a negative exponent. For example, the "metre per second" can be written "m/s", "m s^-1", "m·s^-1" or $\frac{\mathrm{m}}{\mathrm{s}}$ . A solidus should not be used if the result is ambiguous, i.e. "kg·m^-1·s^-2" is preferable to "kg/m·s²".

Spelling variations

Several nations, notably the United States, typically use the spellings 'meter' and 'liter' instead of 'metre' and 'litre' in keeping with standard American English spelling. In addition, the official US spelling for the SI prefix 'deca' is 'deka'.
The unit 'gram' is often spelt 'gramme' outside of the United States of America.

Cultural issues

The swift worldwide adoption of the metric system as a tool of economy and everyday commerce was based mainly on the lack of customary systems in many countries to adequately describe some concepts, or as a result of an attempt to standardize the many regional variations in the customary system. International factors also affected the adoption of the metric system, as many countries increased their trade. Scientifically, it provides ease when dealing with very large and small quantities because it lines up so well with our decimal numeral system.

Differences (as usage of grams instead of kilograms) in the local everyday uses of metric units are often misrepresented as cultural when they are in fact fully economic in base. They are a practical consumer response to using the SI unit, and the decimal number system to achieve the brevity and practicality of everyday use. For example, bread is sold in one-half, one or one and one-half kilogram sizes in many countries, but may be found labelled in grams (500 g, 1000 g, 1500 g) this is simply because the baker sells many similar products in other sizes. The consumer (customer) will opt for the best datum (in this instance perceived as weight) they can get in order to compare value - and the baker (retailer) knows this. So he displays his products in a more easily assessable way - pricewise.

In some countries, the informal cup measurement has become 250 mL, and prices for items are sometimes given per 100 g rather than per kilogram. A profound cultural difference between physicists and engineers, especially radio engineers, existed prior to the adoption of the metre-kilogram-second (MKS) system and hence its descendant, SI. Engineers work with volts, amperes, ohms, farads, and coulombs, which are of great practical utility, rather than the centimetre-gram-second (CGS) units, which, though appropriate for theoretical physics, can be inconvenient for electrical engineering usage and are largely unfamiliar to householders using appliances rated in volts and watts. In the U.S., blood glucose measurements are recorded in milligrams per deciliter (mg/dL); in Europe, the standard is millimole/litre (mmol/L).

The fine-tuning that has happened to the metric base units over the past 200 years, as experts have tried periodically to refine the metric system to fit the best scientific research, does not affect the everyday use of metric units. Since most non-SI units, such as the U.S. customary units, are nowadays defined in terms of SI units, any change in the definition of the SI units results in a change of the definition of the older units as well.

Trade

The European Union has a directive as a result of which non-SI markings will be banned after 31 December 2009 on any goods imported into the European Union. This applies to all markings on products, enclosed directions and papers, packaging, and advertisements.

References

External links

Official

Information

US NIST reference on SI
- chart
SI - Its history and use in science and industry
A Dictionary of Units of Measurement
Cyrillic transcription of SI symbols
Judson, Lewis B., Weights and Measures Standards of the United States: A brief history, NBS Special Publication 447, orig. iss. October 1963, updated March 1976 (46 page PDF file)
Metric system and conversion tables (courtesy French property advice)
metre-info - an encyclopaedia of all metric units
Online metric conversions
USMA International System of Units (SI) reference publications

'' Pro-metric pressure groups

'' Pro-customary measures pressure groups

The British Weights and Measures Association

'' Proposed adjustment to the International system

Dimensional Analysis of the current SI system - Blaze Labs Research

History

Units

SI writing style

Spelling variations

Cultural issues

Trade

See also

References

External links

Further reading

Gourt Home::Gourt :: International System of Units

History

Units

SI writing style

Spelling variations

Cultural issues

Trade

See also

References

External links

Further reading