The International System of Units (abbreviated SI from French: Système international d'unités) is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units. The SI was established in 1960, based on the metre-kilogram-second system, rather than the centimetre-gram-second system, which, in turn, had a few variants. The SI is declared as an evolving system, thus prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses, and as the precision of measurements improves.
It is the world's most widely used system of measurement, which is used both in everyday commerce and in science. The system has been nearly globally adopted with the United States being the only industrialized nation that does not mainly use the metric system in its commercial and standards activities. The United Kingdom has officially partially adopted metrication, with no intention of replacing customary measures entirely. Canada has adopted it for all legal purposes but imperial/US units are still in use, particularly in the buildings trade
| Unit name | Unit symbol | Quantity name | Quantity symbol | Dimension symbol |
|---|---|---|---|---|
| metre | m | length | l (a lowercase L), x, r | L |
| kilogram [note 1] | kg | mass | m | M |
| second | s | time | t | T |
| ampere | A | electric current | I (an uppercase i) | I |
| kelvin | K | thermodynamic temperature | T | Θ |
| candela | cd | luminous intensity | Iv (an uppercase i with lowercase non-italicized v subscript) | J |
| mole | mol | amount of substance | n | N |
CGS Units
The centimetre–gram–second system (abbreviated CGS or cgs) is a metric system of physical units based on centimetre as the unit of length, gram as a unit of mass, and second as a unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways of extending the CGS system to cover electromagnetism.
The CGS system has been largely supplanted by the MKS system, based on metre, kilogram, and second. MKS was in turn extended and replaced by the International System of Units (SI). The latter adopts the three base units of MKS, plus the ampere, mole, candela and kelvin. In many fields of science and engineering, SI is the only system of units in use. However, there remain certain subfields where CGS is prevalent.
In measurements of purely mechanical systems (involving units of length, mass, force, energy, pressure, and so on.), the differences between CGS and SI are straightforward and rather trivial; the unit-conversion factors are all powers of 10 arising from the relations 100 cm = 1 m and 1000 g = 1 kg. For example, the CGS derived unit of force is the dyne, equal to 1 g·cm/s2, while the SI derived unit of force is the newton, 1 kg·m/s2. Thus it is straightforward to show that 1 dyne=10−5 newtons.
On the other hand, in measurements of electromagnetic phenomena (involving units of charge, electric and magnetic fields, voltage, and so on), converting between CGS and SI is much more subtle and involved. In fact, formulas for physical laws of electromagnetism (such as Maxwell's equations) need to be adjusted depending on what system of units one uses. This is because there is no one-to-one correspondence between electromagnetic units in SI and those in CGS, as is the case for mechanical units. Furthermore, within CGS, there are several plausible choices of electromagnetic units, leading to different unit "sub-systems", including Gaussian, "ESU", "EMU", and Heaviside–Lorentz. Among these choices, Gaussian units are the most common today, and in fact the phrase "CGS units" is often used to refer specifically to CGS-Gaussian units.
| Quantity | Symbol | CGS unit | CGS unit abbreviation | Definition | Equivalent in SI units |
|---|---|---|---|---|---|
| length, position | L, x | centimetre | cm | 1/100 of metre | = 10−2 m |
| mass | m | gram | g | 1/1000 of kilogram | = 10−3 kg |
| time | t | second | s | 1 second | = 1 s |
| velocity | v | centimetre per second | cm/s | cm/s | = 10−2 m/s |
| acceleration | a | gal | Gal | cm / s2 | = 10−2 m/s2 |
| force | F | dyne | dyn | g cm / s2 | = 10−5 N |
| energy | E | erg | erg | g cm2 / s2 | = 10−7 J |
| power | P | erg per second | erg/s | g cm2 / s3 | = 10−7 W |
| pressure | p | barye | Ba | g / (cm s2) | = 10−1 Pa |
| dynamic viscosity | μ | poise | P | g / (cm s) | = 10−1 Pa·s |
| wavenumber | k | kayser | cm−1 | cm−1 | = 100 m−1 |
MKS Units
The MKS system of units is a physical system of units that expresses any given measurement using fundamental units of the metre, kilogram, and/or second (MKS).
Historically the MKS system of units succeeded the cgs system of units and laid the blueprint for the International System of Units, which now serves as the international standard. Therefore the exact composition of the MKS system is a historical issue. As a matter of historical record the MKS system incorporated fundamental units other than the metre, kilogram, and second in addition to derived units. An incomplete list of the fundamental and derived units appears below. Since the MKS system of units never had a governing body to rule on a standard definition the list of units depended on different conventions at different times.
- cycle (This dimensionless quantity became synonymous with the term "cycle per second" a an abbreviation. This circumstance confused the exact definition of the term cycle. Therefore the term "cycle per metre" became ill-defined. The cycle did not become an SI unit.)
- cycle per second
- cycle per metre (This measure of wavenumber became ill-defined due to the abbreviation of "cycle per second" as "cycle".)
FPS Units
The foot-pound-second system or FPS system is a system of units built on the three fundamental units foot for length, (avoirdupois) pound for either mass or force (see below) and second for time.
