The current international form of the metric system, and indeed that normally referred to by the latter phrase within a contemporary context. The name was adopted by the CIPM in 1956, for a system founded on six base units adopted by the 10th CGPM in 1954, these being the metre, kilogram, second, ampere, kelvin, and candela. (Hence it is an m.k.s. system rather than a c.g.s. system as long applied routinely, though not exclusively, to the metric system. Thus, for example, the derived unit for force is the newton, = m·kg·s-2, rather than the dyne, = cm·g·s-2. Because of being based on the ampere, rather than the volt or some other electrical unit, the SI system is sometimes referred to as an m.k.s.A. system.) The name and the international standard label SI were ratified by the 11th CGPM in 1960, along with a repertoire of basic, supplementary, and derived units.
[Le Système International d'Unités (Sèvres, France: Bureau International de Poids et Mesures, 1985)] The current repertoire, with their purposes and, for all derived units, their expression in base-unit terms, is thus as shown in Table 49 (the derived units being methodically indented to indicate their relative dependence). Reference should be made to the individual unit names for relevant definitions, plus details of adjustments in values.
| Base units: |
| length: m = metre |
| mass: kg = kilogram |
| time: s = second |
| electric current strength: A = ampere |
| thermodynamic temperature: K = kelvin |
| luminous intensity: cd = candela |
| amount of substance: mol = mole |
| Derived units (the first two being ratios): |
| rad = radian = m·m-1 for plane angle: |
| sr = steradian = m2·m-2 for solid angle: |
| degree C = degree Celsius = K for temperature |
| C = coulomb = s·A for electric charge, quantity of electricity |
| N = newton = m·kg·s-2 for force |
| Pa = pascal = N·m-2 for pressure, stress |
| J = joule = N·m for energy, work, quantity of heat |
| W = watt = J·s-1 for power, radiant flux |
| V = volt = W·A-1 for voltage, e.m.f., potential difference |
| F = farad = C·V-1 for electric capacitance |
| Ω = ohm = V·A-1 for electric resistance |
| S = siemens = Ω-1 for electric conductance |
| Wb = weber = V·s for magnetic flux |
| T = tesla = Wb·m-2 for magnetic flux density |
| H = henry = Wb·A-1 for electric inductance |
| Gy = gray = J·kg-1 for absorbed radiation |
| Sv = sievert = J·kg-1 for dose equivalent |
| lm = lumen = cd·sr for luminous flux |
| lx = lux = lm·m-2 for illuminance |
| Hz = hertz = (cycles)·s-1 for frequency |
| Bq = becquerel = (disintegrations)·s-1 for activity of a radionuclide |
| kat = katal = mol·s for catalytic activity. |
| Other established units acceptable temporarily where already used: |
| length: nautical mile = 1 852 m |
| speed: knot = 1 nautical mile per hour = 1852/3600 = 0.514 444 4~ m·s-1 |
| length: Å = angstrom = 0.1 nm (10-10 m) |
| area: a = are = 100 m2, hence ha = hectare = 10 000 m2 |
The familiar range of prefixes was expanded and the upper/lower case distinction of D- for deca- and d- for deci- (both largely shunned outside Europe) replaced by da-versus d-. Hecto- (of similar limited use) and kilo-are now routinely and officially lower case in their symbol form; myria has been discarded. The larger multipliers than kilo- continue the tradition of being capitalized as symbols (and of being derived from Greek, but their opposites all wander from the Latin tradition). The full range of SI prefixes, with etymological derivation and symbol (old in brackets) is as shown in Table 50. The inner terms, all in use by 1951, were redefined in 1960 by the 11th CGPM, the outer as shown in the table.
| Symbol | Prefix | Value | Etymological source | Approved |
|---|---|---|---|---|
| y | yocto- | 10-24 | oct-‘eight’, for 1 000-8 | 1990 CIPM |
| z | zepto- | 10-21 | sept-‘seven’, for 1 000-7 | 1990 CIPM |
| a | atto- | 10-18 | ‘eighteen’ in Danish | 1964 12th CGPM |
| f | femto- | 10-15 | ‘fifteen’ in Danish | 1964 12th CGPM |
| p | pico- | 10-12 | ‘small’ in Spanish | |
| n | nano- | 10-9 | ‘dwarf’ in Greek | |
| μ | micro- | 10-6 | ‘small’ in Latin | |
| m | milli- | 10-3 | ‘thousand’ in Latin | |
| c | centi- | 10-2 | ‘hundred’ in Latin | |
| d | deci- | 10-1 | ‘ten’ in Latin | |
| da (D) | deca- | 101 | ‘ten’ in Greek | |
| h (H) | hecto- | 102 | ‘hundred’ in Greek | |
| k (K) | kilo- | 103 | ‘thousand’ in Greek | |
| M | mega- | 106 | ‘great’ in Greek | |
| G | giga- | 109 | ‘giant’ in Greek | |
| T | tera- | 1012 | ‘monster’ in Greek | |
| P | peta- | 1015 | pent- ‘five’, for 1 0005 | 1975 15th CGPM |
| E | exa- | 1018 | hex- ‘six’, for 1 0006 | 1975 15th CGPM |
| Z | zetta- | 1021 | sept- ‘seven’, for 1 0007 | 1990 CIPM |
| Y | yotta- | 1024 | oct- ‘eight’, for 1 0008 | 1990 CIPM |
See metric system for further historical background.
