1.1 SI base units 

Definitions of SI base units: 

– Unit of time 

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency Δν_Cs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s^–1.

– Unit of length 

The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m/s, where the second is defined in terms of Δν_Cs.

– Unit of mass 

The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10^–34 when expressed in the unit J s, which is equal to kg m² s^–1, where the metre and the second are defined in terms of c and Δν_Cs.

– Unit of electric current 

The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10^–19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of Δν_Cs.

– Unit of thermodynamic temperature 

The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10^–23 when expressed in the unit J K^–1, which is equal to kg m² s^–2 K^–1, where the kilogram, metre and second are defined in terms of h, c and Δν_Cs.

– Unit of amount of substance 

The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 × 10²³ elementary entities. This number is the fixed numerical value of the Avogadro constant, N_A, when expressed in the unit mol^–1 and is called the Avogadro number.

The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.

– Unit of luminous intensity 

The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10¹² Hz, K_cd, to be 683 when expressed in the unit lm W^–1, which is equal to cd sr W^–1, or cd sr kg^–1 m^–2 s³, where the kilogram, metre and second are defined in terms of h, c and Δν_Cs.