SI defining constant

SI defining constants

The seven defining constants are the most fundamental feature of the definition of the system of units. Each defining constant consists of an exact numerical value and units. The defining constants are the speed of light in vacuum c, the hyperfine transition frequency of caesiumΔν_Cs, the Planck constanth, the elementary chargee, the Boltzmann constantk, the Avogadro constantN_A, and the luminous efficacyK_cd. The nature of the defining constants ranges from fundamental constants of nature such as c to the purely technical constant K_cd. The values assigned to these constants were fixed to ensure continuity with previous definitions of the base units.