Base Units
Just as the original conception of the metric system had grown out of the problems scientists encountered in dealing with the medieval system, so a new system grew out of the problems a vastly enlarged scientific community faced in the proliferation of subsystems improvised to serve particular disciplines. At the same time, it had long been known that the original 18th-century standards were not accurate to the degree demanded by 20th-century scientific operations; new definitions were required. After lengthy discussion the 11th General Conference on Weights and Measures (11th CGPM), meeting in Paris in October 1960, formulated a new International System of Units (abbreviated SI). The SI was amended by subsequent convocations of the CGPM. The following base units have been adopted and defined:
Length: metre
Since 1983 the metre has been defined as the distance traveled by light in a vacuum in 1/299,792,458 second.
Mass: kilogram
The standard for the unit of mass, the kilogram, is a cylinder of platinum-iridium alloy kept by the International Bureau of Weights and Measures, located in Sèvres, near Paris. A duplicate in the custody of the National Institute of Standards and Technology serves as the mass standard for the United States.
The kilogram is the only base unit still defined by an artifact. However, in 1989 it was discovered that the prototype kept at Sèvres was 50 micrograms lighter than other copies of the standard kilogram. To avoid the problem of having the kilogram defined by an object with a changing mass, the CGPM in 2018 agreed that effective on May 20, 2019, the kilogram would be defined not by a physical artifact but by a fundamental physical constant. The constant chosen was Planck’s constant, which was defined to be equal to 6.62607015 × 10−34 joule second. One joule is equal to one kilogram times metre squared per second squared. Since the second and the metre were already defined in terms of the frequency of a spectral line of cesium and the speed of light, respectively, the kilogram would then be determined by accurate measurements of Planck’s constant.
Time: second
The second is defined as the duration of 9,192,631,770 cycles of the radiation associated with a specified transition, or change in energy level, of the cesium-133 atom.
Electric current: ampere
The ampere was defined as the magnitude of the current that, when flowing through each of two long parallel wires separated by one metre in free space, results in a force between the two wires (due to their magnetic fields) of 2 × 10−7 newton (the newton is a unit of force equal to about 0.2 pound) for each metre of length. However, in 2018 the CGPM agreed that effective on May 20, 2019, the ampere would be redefined such that the elementary charge was equal to 1.602176634 × 10−19 coulomb.
Thermodynamic temperature: kelvin
The thermodynamic, or Kelvin, scale of temperature used in SI has its origin or zero point at absolute zero and has a fixed point at the triple point of water (the temperature and pressure at which ice, liquid water, and water vapour are in equilibrium), defined as 273.16 kelvins. The Celsius temperature scale is derived from the Kelvin scale. The triple point is defined as 0.01 degree on the Celsius scale, which is approximately 32.02 degrees on the Fahrenheit temperature scale. However, in 2018 the CGPM agreed that effective on May 20, 2019, the kelvin would be redefined such that Boltzmann’s constant was equal to 1.380649 × 10−23 joule per kelvin.
Amount of substance: mole
The mole is defined as the amount of substance containing the same number of chemical units (atoms, molecules, ions, electrons, or other specified entities or groups of entities) as exactly 12 grams of carbon-12. However, in 2018 the CGPM agreed that effective on May 20, 2019, the mole would be redefined such that the Avogadro constant was equal to 6.02214076 × 1023 per mole.
Light (luminous) intensity: candela
The candela is defined as the luminous intensity in a given direction of a source that emits monochromatic radiation at a frequency of 540 × 1012 hertz and that has a radiant intensity in the same direction of 1/683 watt per steradian (unit solid angle).