Historical changes of unit system
As early as 17 ~ 18 century, people began to feel the impact of the confusion of measurement units and measurement systems on the development of science and technology and production. 1795, French scientists established the metric system with the metric system as the basic unit. 1875, 17 countries formally signed the metric convention, which laid an international foundation for the spread and development of the metric system, and the metric system became the earliest recognized system in the world. With the development of science and technology, many different units have been derived from metric system, such as centimeter, gram and second system, meter, kilogram force and second system, absolute electrostatic unit system and absolute electromagnetic unit system. The coexistence of various units not only hinders international trade, but also is not conducive to scientific and cultural exchanges between countries. Therefore, the unified unit system has become a common requirement of all countries in the world. 1956, the international metrology Committee named the draft system of measurement agreed by 2/kloc-0 countries as the international system of units, which was represented by the international common symbol SI. 1960 the international system of units was formally adopted at the 1 1 international metrology conference. Because the international system of units is advanced, practical, simple and scientific, and it is suitable for all fields of culture and education, science and technology and economic construction, it is widely adopted by countries and international organizations all over the world. 1977, China clearly stipulated that the international system of units should be gradually adopted. 1984, the legal units of measurement in People's Republic of China (PRC) promulgated by China was based on the international system of units. 1948 The 9th International Metrology Conference made a decision to require the International Metrology Committee to create a simple and scientific practical unit system, which can be used by all member countries of the Metrology Convention. 1954 the 10th international metrology conference decided to adopt the meter (m), kilogram (kg), second (s), ampere (a), kelvin (k) and Candeira (cd) as the basic units. 1960 the 11th international metrology congress decided to name the practical measurement system based on these SIx units as "international system of units" and specify its symbol as "si". Later 1974, the 14th international metrology congress decided to increase the unit mole (mol) of substances as the basic unit. Therefore, the current international system of units has seven basic units. The international system of units has two auxiliary units, namely radian and sphericity. SI derived units are derived from SI basic units by definition, and there are many. There are three types here: some SI derived units expressed by SI basic units; SI export unit with special name; Part of the derived unit of the international system of units represented by the auxiliary unit of the international system of units. There are 19 SI derivative units with special names. There are 17 named after outstanding scientists, such as Newton, Pascal and Joule, to commemorate their contributions in this field. At the same time, for the convenience of representation, these derived units can also be combined with other units to represent other more complex derived units. The international system of units is the basis and core of metrological research. In particular, the replication, preservation and value transfer of the seven basic units are the most fundamental research topics in metrology.