Due to the serious deformation of archival rice, 1872 abandoned the definition of archival rice and used rice made of original platinum alloy (90% platinum, 10% iridium) as the length unit. The original rice cooker was made according to the length of the "file rice". At that time, * * * made 3 1 piece, the cross section was approximately X-shaped, and the length of the archive meter was engraved on the groove (neutral surface) of the ruler with two 6-8 micron wide scribes. 1889 At the First International Metrology Conference, the prototype of No.6 rice identified by the International Bureau of Metrology (one of the prototype of 3 1 meter which is closest to the length of the archive meter at 0℃) was selected as the international rice prototype, which was kept as the most authoritative length benchmark in the world in the basement of the International Bureau of Metrology in Paris, and the remaining rulers were distributed to the participating countries as auxiliary rulers. It is stipulated that when the ambient air temperature is 0℃, the distance between the two ends of the rice cooker is 1m. 1927. The definition of rice was strictly stipulated in the Seventh International Metrology Conference. In addition to the temperature requirement, it is also proposed that the raw rice container should be kept at the standard atmospheric pressure 1, and its placement method is specified in detail.
However, there are many shortcomings in taking rice as the objective standard of rice, such as material deformation; The measurement accuracy is not high (only 0. 1 micron). It is difficult to meet the needs of precise measurement such as metrology. In addition, if the original rice cooker is damaged, there will be no basis for copying, especially it is difficult to ensure that the replica is completely consistent with the original rice cooker, which brings difficulties to the use of various countries. Therefore, the idea of using natural values as unit benchmarks has always been desired by people. In 1950s, with the development of isotope spectrum light source. The narrow Krypton -86 isotope spectrum was discovered. With the success of interference technology, people finally found a natural standard that is not easy to be destroyed, that is, the natural standard with the wavelength of light wave as the length unit.
1960 The definition of meter was changed to: "The length of meter is equal to 1650763.73 times of the radiation wavelength passing between 2P 10 and 5d 1 levels of krypton -86 atoms in vacuum. This natural benchmark has stable performance, no deformation problem, easy replication and high replication accuracy. China also established the krypton -86 isotope length standard in 1963. After the definition of rice is changed, the original international rice instruments are still kept in the International Bureau of Metrology according to the original regulations.
With the progress of science and technology, since the 1970s, the measurement of time and the speed of light has reached a high precision. Therefore, 1983, 10 adopted a new definition of the meter at the 17th international metrology conference held in Paris in June: "the meter is the length of light propagating in a vacuum with a time interval of 1/299792458 seconds". In this way, the definition of meter based on spectral line wavelength is replaced by the new definition of meter.
(I add that the definition of rice has been set by people, not since ancient times, so people have been looking for a relative standard to define rice, and it is this truth to define rice with light. )