2 Units SI base units SI : Système International d'unités (International System of Units) Unite name (symbol) Definition established mass kilogram (kg) The mass of the International Prototype of the Kilogram (IPK). IPK is a rightcircular cylinder of 39.17 mm made of Pt 90 Ir 10 wt%. 1889 length meter (m) The length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. 1983 time second (s) 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 cesium 133 atom. electric current ampere (A) The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2x10-7 newton per meter of length 1948 thermodynamic temperature kelvin (K) The fraction 1/273.16 of the thermodynamic temperature of the triple point of water. amount of substance mole (mol) The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. luminous intensity candela (cd) The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x10 12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. 1979
SI base units (old definition) Unite name (symbol) Old definition mass kilogram (kg) The mass of a cubic decimeter of water. 18th century length meter (m) The length of a pendulum having a half-period of one second. One ten-millionth of the length of the earth's meridian along a quadrant (one fourth the circumference of the earth). The distance between two lines inscribed on a highly stable platinumiridium (Pt 90 Ir 10 wt%) bar. That is to be measured at the melting point of ice. 18th century 1889 time second (s) The fraction 1/86 400 of the mean solar day. electric current ampere (A) The current that would deposit 0.001118000 grams of silver per second from a silver nitrate solution (:International Ampere = 0.99985 A). 1893 thermodynamic temperature kelvin (K) The fraction 1/273.15 of the thermodynamic temperature of the freezing point of water. ( freezing point depends on atmospheric pressure) amount of substance mole (mol) The amount of substance of a system which contains as many elementary entities as there are atoms in 0.016 kilogram of oxygen. ( isotope problem: 16 O, 17 O, 18 O) luminous intensity candela (cd) Each country had its own, and rather poorly reproducible, unit of luminous intensity. US, UK and France adopted the International Candle represented by carbon filament lamps.
SI derived units Derived quantity Unit name Symbol Expression in terms of other SI units Expression in terms of SI base units plane angle radian rad m m -1 = 1 solid angle steradian sr m 2 m-2 = 1 frequency hertz Hz s -1 force newton N m kg s -2 pressure pascal Pa N/m 2 m-1 kg s-2 energy, work, heat joule J N m m2 kg s-2 power, radian flux watt W J/s m2 kg s-3 electric charge, quantity of electricity coulomb C s A electric potential difference, electromotive force volt V W/A m2 kg s-3 A-1 capacitance farad F C/V m-2 kg-1 s4 A2 electric resistance ohm Ω V/A m2 kg s-3 A-2 electric conductance siemens S A/V m-2 kg-1 s3 A2 magnetic flux weber Wb V s m2 kg s-2 A-1 magnetic flux density tesla T Wb/m 2 kg s-2 A-1 inductance henry H Wb/A m2 kg s-2 A-2 Celsius temperature degree Celsius C K luminous flux lumen lm cd sr m 2 m-2 cd = cd illuminance lux lx lm/m 2 m 2 m-4 cd = m -2 cd activity (of a radionuclide) becquerel Bq s -1 absorbed dose, specific energy (imparted), kerma gray Gy J/kg m2 s-2 dose equivalent sievert Sv J/kg m2 s-2 catalytic activity katal kat s -1 mol
SI prefix (Metric prefix) SI A unit prefix to indicate a decadic multiple or fraction of the basic unit of measure in the International System of Units (SI) 10 n prefix symbol 10 24 yotta () Y 10 21 zetta () Z 10 18 exa () E 10 15 peta () P 10 12 tera () T 10 9 giga () G 10 6 mega () M 10 3 kilo () k 10 2 hecto () h 10 1 deca () da 10 n prefix symbol 10-1 deci () d 10-2 centi () c 10-3 milli () m 10-6 micro ( ) µ 10-9 nano () n 10-12 pico () p 10-15 femto ( ) f 10-18 atto () a 10-21 zepto () z 10-24 yocto () y
International prototype kilogram (IPK) Established in 1889 at the 1st CGPM meeting (Conférence générale des poids et mesures ) Preserved at the International Bureau of Weights and Measures (BIPM: Bureau international des poids et mesures ) mass drift over time: Δm 1 μg/year G. Girard, Metrologia 31, 317 (1994) mass gain through surface adsorption of atmospheric contamination mass reduction by surface cleanings IPK from HP of BIPM right-circular cylinder (height = diameter) of 39.17 mm made of Pt 90 Ir 10 (wt%) Change in mass Δm of the six official copies and of prototype No. 25 with respect to the mass of the IPK. mass drift of IPK over the last 125 years: Δm -50 μg or more providing uncertainties of 5x10-8 on SI units