1. A brief history

‘A brief history’ shows that measurement is at least as old as civilization. Different systems and different units of measurement were developed in different places, with the physical size of natural biological objects frequently being used as a basic unit. The key drivers for a uniform measurement system were trade, the industrial revolution, and scientific advance. In 1960 the Système International d’Units (SI units) was introduced, consisting of seven basic units: length (metre), mass (kilogram), time (second), electric current (ampere), temperature (degree kelvin), quantity of substance (mole), and luminous intensity (candela). Another twenty-two named units were defined as powers and combinations of these basic seven.

On the five base quantities of nature and SI (The International System of Units)

It is shown here that five base quantities (and the corresponding five base units) of nature are sufficient to define all derived quantities (and their units) and to describe all natural phenomena. The base quantities (and their base units) are: length (m), mass (kg), time (s), temperature (K) and electric charge (C). The amount of substance (mole) is not taken as a base quantity of nature and the Avogadro constant is not considered as a fundamental constant of nature, as they are both based on an arbitrary definition (due to the arbitrary value of 0.012 kg for the mass of 1 mole of C-12 isotope). Therefore, the amount of substance (mole) is moved from the list of base quantities to the category of the supplementary units (to be re-created after its abrogation in 1995). Based on its definition, the luminous intensity (cd) is not a base quantity (unit), therefore it is moved to the list of derived quantities (units). The ampere and coulomb are exchanged by places in the list of base and derived units, as ampere is a speed of coulombs (but SI defines meter, not its speed as a base unit). The five base quantities are re-defined in this paper by connecting them to five fundamental constants of nature (the most accurately known frequency of the hydrogen atom, the speed of light, the Planck constant, the Boltzmann constant and the elementary charge) with their numerical values fixed in accordance with their CODATA 2006 values (to be improved by further experiments).