The CGPM (General Conference on Weights and Measures) is the ultimate decision-making body for the metric system, and now occurs every four years in Paris to finalize and enact decisions drafted by various consultative committees.
The metric system was brought into international regulation by the Metre Convention of 1875, and the BIPM (Bureau International des Poids et Mesures) was established to run it, based in Sèvres near Paris. The official languages are French and English. The standing committee that runs the BIPM is the CIPM, Comité International des Poids et Mesures, consisting of eighteen appointees from member states. The CIPM, meeting annually, is assisted and advised on technical matters by committees dedicated to specific problems or fields such as the definition of the metre, photometry, or ionizing radiation. (Their names begin with CC, Comité Consultatif.) Agreed decisions on new metric units, redefinitions, refined standards of measurement, and so on, are passed to the CGPM for enactment. The CGPM also approves the budget of the BIPM.
The first CGPM was in 1889 and the twenty-second was in 2003. Many of their resolutions are of the nature of urging national laboratories to continue looking into more reliable future standards for some branch of mensuration, and therefore don't actually do that much. I've extracted the resolutions which had some interesting effect:
First CGPM, 1889
Established the platinum
s of the metre
The Second CGPM, 1895 can be skipped.
Third CGPM, 1901
Resolution 1 distinguishes between weight
. In an unfortunate choice of words we're still stuck with, the ordinary-language weight of an object was named its mass
, and the word weight
was given a technical sense of the gravitational force exerted by that mass. The kilogram
was defined to be a unit of mass.
Resolution 2 defined a litre to be the volume of 1 kg of pure water at maximum density [which is at about 4°C] and at standard pressure.
Then came the Fourth CGPM, 1907, Fifth CGPM, 1913, and Sixth CGPM (I don't know the year, presumably early 1920s after the War -- the BIPM records on their website are incomplete and in a couple of cases mislabelled).
Seventh CGPM, 1927
Resolution 1 was a technical redefinition of the metre
, still using the 1889 prototype bar, but refining the method of taking the measurement.
Then the Eighth CGPM, 1933, then the interruption of the War.
Ninth CGPM, 1948
This was an important one. Resolution 3 defined the thermodynamic triple point
of water, the temperature at which solid, liquid, and gas all coexist. This could be measured more accurately than the melting point
of water. Formerly the degree centigrade
had been defined as having 0°C at the melting point. Now it was redefined so that 0,01°C is the triple point, and melting at 0,00°C is now only an approximate value.
Resolution 7 scheduled a large number of units, and defined their names and the correct ways of writing units, symbols, and numbers.
- The unit of temperature, the degree centigrade, became the degree Celsius, symbol °C
- In the absolute temperature scale [later to be known as the Kelvin scale] the degree absolute, symbol °K, was equal in size to the degree Celsius but with a different starting point.
- The degree, symbol deg, was defined to be the unit of temperature interval, that is the difference between two temperatures (this is a different quantity from temperature, just as distance is different from length). It is equal in size to the degree Celsius and degree absolute.
- Chose the symbols s for second and st for stere [a now obsolete unit dimensionally equal to a metre cubed] (formerly sec and s respectively)
- As well as adopting units that survive into the present day (the volt, watt, coulomb, farad, ohm, henry, hertz, newton, lumen, lux), the 1948 metric system also included ones no longer part of the SI: the tonne, litre, micron, erg (for energy, symbol erg), dyne (for force, symbol dyn), poise (for viscosity, symbol P), stilb (for luminance, symbol sb), calorie (for heat, symbol cal), bar (for pressure, symbol bar), and hour (symbol h).
- Defined principles of writing units and names if named after a person: lower-case name (hertz) but initial capital symbol (Hz)
- The symbols are not abbreviations, and do not take full stops.
- The decimal point can be either a comma or a dot (mid-height or base-line). Thousands can be grouped with blank separators but the comma or dot should never be used.
Tenth CGPM, 1954
Resolution 3 defined the absolute temperature scale
more precisely by declaring that the the triple point
of water was 273,16 degrees Kelvin exactly. (Either I've missed it or they're being sneaky, but the 1948 degree absolute has now quietly changed to the degree Kelvin
In Resolution 4 the term standard atmosphere was defined to mean 101 325 N m-2
Resolution 6 declared the metric system to have six base units, namely the metre, kilogram, second, ampere, degree Kelvin, and candela (or new candle); up from the previous four-base system widely known as MKSA (and which had itself replaced the original MKS).
Eleventh CGPM, 1960
Another very important one. Resolution 6 abolished reference to the physical prototype metre, and defined the metre
to be the length equal to 1 650 763,73 wavelength
s in vacuum of the radiation corresponding to the transition between the levels 2p10
of the krypton
Resolution 7 defined the second as the fraction 1/31 556 925,974 7 of the tropical year for 1900 January 0 at 12 hours ephemeris time.
Resolution 12 adopts the name Système International d'Unités for the present form of the metric system, with the abbreviation SI
In addition to the six base units they adopt two supplementary units, the radian and steradian, for plane and solid angles. ['supplementary' status abolished 1995] They define many derived units, such as watt and square metre, discarding the ones I mentioned under the 1948 CGPM as no longer part of the SI.
They declare that the multiple prefixes to be used are those from deca to tera, and for submultiples deci to pico.
Twelfth CGPM, 1964
Resolution 1 defines the litre
as identically 1 dm3
, abrogating the 1901 definition.
Resolution 7 extends their authority over a non-SI unit, the curie (activity of ionizing radiation, symbol Ci), declaring it to be exactly 3,7 x 1010 s-1.
Resolution 8 adds the submultiples femto and atto.
Thirteenth CGPM, 1967
Resolution 1 redefines the second
again, adopting the present value of the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine
levels of the ground state of the caesium
Resolution 3 abolishes the names degree and degree Kelvin, replacing both with the kelvin. Resolution 7 abolishes micron and new candle, alternative names for the micrometre and candela.
Fourteenth CGPM, 1971
Resolution 3 adopts the mole
, the unit of amount of matter, as a seventh base unit of the SI.
Fifteenth CGPM, 1975
Resolution 2 recommends an exact value for c
, the speed of light
, but it is not in fact adopted until the 1983 redefinition of the metre. Resolution 8 adopts the becquerel
and Resolution 9 the gray
as units for use with ionizing radiation. Resolution 10 adopts the large multiple prefixes peta
Sixteenth CGPM, 1979
Resolution 3 adopts a new definition of the candela
. Resolution 5 adds the radiation unit sievert
because of the health and safety importance of distinguishing it from the dimensionally equal gray
(both J kg-1
Resolution 6 departs from the 1948 symbol capitalization convention by declaring that both l and L are valid symbols for the litre.
Seventeenth CGPM, 1983
Resolution 1 adopts the 1975 proposed value for c
, declaring it to be exactly 299 792 458 m s-1
, thereby redefining the metre
in terms of c
and the second
The Eighteenth CGPM, 1987 didn't do anything so exciting.
Nineteenth CGPM, 1991
Resolution 4 adopts the multiples zetta
and the submultiples zepto
Twentieth CGPM, 1995
Resolution 8 abolishes the class of supplementary units and declares the radian
to be derived units dimensionally equal to 1 (i.e. fully cancelling, such as m m-1
Twenty-first CGPM, 1999
Resolution 12 adopts the katal
, symbol kat, as a derived unit of catalytic activity
, equal to 1 mol s-1
The Twenty-second CGPM, 2003 must have been an awfully dull affair, though they did reaffirm the bits about decimal commas, dots, and thousands blanks from 1948.