International System of Units (SI) is a unified system of units adopted and recommended by the International Metrology Conference (CGPM). It is the most widely used standard unit of measurement in the world at present, and adopts the decimal system.
The international system of units is formed according to the principle of consistency in the system of units of measurement, and its multiple and fractional units are formed by decimal system; The unit of multiple and fraction can only be composed of the prefix of SI, and its basic unit and definition can only be decided by the international metrology conference, and the special name and symbol of SI derivative unit can only be selected by the international metrology conference.
1948 according to this resolution, the 9th international metrology congress instructed the international metrology commission (CIPM) to study and formulate a set of rules of measurement units, and strive to establish a scientific and practical system of measurement units. 1954 The 10 International Metrology Conference decided to adopt six quantities: length, mass, time, current, thermodynamic temperature and luminous intensity as the basic quantities of the practical measurement unit system.
Extended data
At the 26th International Metrology Conference, a resolution on revising the international system of units was adopted. Four of the seven basic units of the International System of Units, namely, kilogram, ampere, Kelvin and mole, will be defined by Planck constant, elementary charge constant, Boltzmann constant and Avogadro constant respectively, and the definitions of the other three basic units have also been adjusted to keep consistent with the revised four basic units.
In the new international system of units, two important concepts of measurement, namely, unit definition and measurement (or reproduction) method, are separated. In the old definition, the definition and implementation method of units are completely bound. For example, the reproduction of 1/273. 16K must be carried out in the presence of three phases of water. In order to reproduce 1 kg, it is necessary to contact the original international kilogram device located in France.
After the new definition comes into effect, kilograms can be reproduced by any suitable method, such as Keable balance method and X-ray crystal density method-these two methods are the most accurate reproduction methods in the world.
Refer to Baidu Encyclopedia-International System of Units