SI base unit referenced to the values of the fundamental constants, namely the Planck constant ("h"), the electron charge ("e"), the Boltzmann constant ("k"), and the Avogadro constant ("N"). This approach was approved in 2018, only after measurements of these constants were achieved with sufficient accuracy. SI base unit The International System of Units (SI) defines seven units of measure as a basic set from which all other SI units can be derived. The SI base units and their physical quantities are the meter for measurement of length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin
Current density is the charge density (SI unit: coulombs per cubic metre), in which "n"(r, "t") is the number of particles per unit volume ("number density") (SI unit: m), "q" is the charge of the individual particles with density "n" (SI unit: coulombs). A common approximation to the current density assumes the current simply is proportional to the electric field, as expressed by: where E is the electric field and "σ" is the electrical conductivity. Conductivity "σ" is the reciprocal (inverse) of electrical resistivity and has the SI units of siemens per metre (S⋅m), and E has the SI units of newtons