Electric current electrons into the conduction band. The current-carrying electrons in the conduction band are known as "free electrons", though they are often simply called "electrons" if that is clear in context. Current density is a measure of the density of an electric current. It is defined as a vector whose magnitude is the electric current per cross-sectional area. In SI units, the current density is measured in amperes per square metre. where "formula_6" is current in the conductor, formula_7 is the current density, and formula_8 is the differential cross-sectional area vector. The current density (current per unit area) "formula_7" in materials
History of the metric system the Treaty of the Metre was extended to cover electrical units with the CGPM merging its work with that of the IEC. The twentieth century history of measurement is marked by five periods: the 1901 definition of the coherent MKS system; the intervening 50 years of coexistence of the MKS, cgs and common systems of measures; the 1948 "Practical system of units" prototype of the SI; the introduction of the SI in 1960; and the evolution of the SI in the latter half century. The need for an orthogonal electromagnetic unit to resolve the difficulties of defining such units in