Basic units of the International System

The seven core units of the SI and the interdependence of its definitions

The International System of Units (SI) defines seven basic units or fundamental physical units, which are described by an operational definition and are independent from the point of view dimensional. These base SI units and their physical quantities are the meter for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the candela for light intensity, and the mole for the amount of substance.

All other units used to express physical quantities can be derived from these base units and are known as derived units. The derivation is carried out by means of dimensional analysis.

Magnitudes

Length

A meter (m) is defined as the distance light travels in a vacuum in 1/299,792.458 s.

Mass

A kilogram (kg) is defined taking the fixed numerical value of the Planck constant, h, equal to 6.626 070 15 x 10^{- 34} when expressed in J·sunit equal to kg·m2·s^-1, being the meter and the second defined according to c and Δ Δ v{displaystyle Delta v,}_Cs.

Time

One second (s) is the time required by 9,192,631,770 cycles of radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

Temperature

One kelvin (K) is defined as 1/273.16 of the thermodynamic temperature of the triple point of water.

Electrical current intensity

An ampere (A) is defined as the intensity of a constant electric current that, maintained in two parallel, rectilinear conductors, of infinite length, of negligible circular section and located at a distance of one meter from each other in a vacuum, would produce a force equal to 2×10^-7 newtons per meter of length.

Amount of substance

A mole (mole) is the amount of substance in a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon 12, approximately 6.022 141 29 (30) × 10²³.

When using the mole, the elementary entities must be specified and can be atoms, molecules, ions, electrons, other particles, or specific groups of such particles.

The amount of substance is defined as a fundamental unit that is proportional to the number of elemental entities present.

Luminous intensity

A candela (cd) is the luminous intensity, in a given direction, of a source emitting monochromatic radiation with a frequency of 540 × 10¹² Hz so that the intensity of radiation emitted, in the indicated direction, is 1/683 W per steradian.

Possible future redefinition of units

A commission of the International Committee of Weights and Measures (CIPM) has proposed to review the formal definitions of the base units of the SI, proposals that are being examined by it, and that could be considered by the 26th CGPM in 2018 The proposed changes are:

"There will still be the same seven basic units (meter, second, kilogram, amper, kelvin, mol and candela). Of these, the kilogram, the amper, the kelvin and the mol will be redefined according to the calculation of the exact numerical values of the Planck constant, of the elemental electric charge, of the Boltzmann constant and of the Avogadro constant, respectively. The second, the metro and the candela are already defined by physical constants and it is only necessary to re-edit their current definitions. The new definitions will improve the SI without changing the size of the units, thus ensuring continuity with current measurements."

More details are in the draft chapter of the Ninth SI Units Pamphlet.

There have been numerous criticisms of the revised definitions since their initial proposal, and it has been argued that the SI reform proposal requires a frank and open discussion before decisions are made on it.