Analysis:
unit of length
In ancient times, a part of the human body was often used as a unit of length. For example, the book "Confucius' Family Language" compiled by Wang Su during the Three Kingdoms period (the beginning of the third century A.D.) records: "The cloth refers to the inch, the hand knows the ruler, and the elbow knows the search. "The arm stretched out eight feet long, this is a search. It is also recorded that: "Ten feet is one foot, and people are eight feet long, so they are called husbands. "Visible, in ancient times, inch and finger, ruler and hand, search and body is one-to-one correspondence.
The so-called "wrist ruler", about 52 ~ 53 cm, is commonly used in ancient western countries and is closely related to the length from the middle fingertip to the elbow.
There are also physical objects as the basis of length units. For example, the inch in the English system comes from the length of three round dried barley grains arranged one after another.
Over the years, various countries in the world have been using various units of length, and even a country or region has adopted different units of length at different times, which is chaotic and extremely inconsistent, bringing a lot of trouble to commodity circulation. Therefore, with the progress of science and technology, the units of length gradually tend to be unified, and this process began hundreds of years ago.
1790, the French National Assembly passed a resolution instructing the French Academy of Sciences to study how to establish the benchmarks of basic physical quantities such as length and mass, so as to lay a solid foundation for unifying measurement units. The following year, it was decided to take one-tenth of a quarter of the earth meridian passing through Paris as the unit of length, and the word "metron" in ancient Greek was chosen as the name of this unit, which later evolved into "meter" and translated into "meter" or "meter" in Chinese. From 1792, French astronomers spent seven years measuring the meridian of the earth passing through Paris, and made a prototype of platinum meter based on the measurement results, which has been kept in the Paris archives.
After the French initiated the metric system, because it is scientific and easy to use, countries in continental Europe have adopted it.
Later, it was measured that one meter of hominid was not exactly equal to a quarter of the meridian of the earth, but was 0.2 mm larger. I believe that the measurement technology will continue to improve in the future and the deviation will be found again. Instead of modifying the length of the rice field, it is better to unify all length measurements based on this platinum rice field.
1875 On May 20th, France * * * came forward, held a meeting of representatives of 20 countries, formally signed the metric convention, and recognized the metric system as an international unit of measurement. At the same time, it was decided to establish the International Metrology Committee and the International Bureau of Metrology. As of June 1985 and June 10, the metric convention has 47 members. Participation of China 1977.
After several years of research, the International Bureau of Metrology has carefully designed and manufactured 30 original rice devices with X-shaped cross sections, of which 90% are platinum and 10% are iridium. This shape is the strongest and the most material-saving. Platinum-iridium alloy is characterized by very small expansion coefficient. These 30 original devices were compared with platinum original devices, and after selection, one of them was rated as an international original device. 1889, the International Metrology Committee approved this work, and announced that the length of 1 meter is equal to the distance between the marking marks at both ends of the X-shaped platinum-iridium alloy ruler at the melting point temperature.
Some other rice prototypes were compared with international rice prototypes, and most of them were later distributed to member countries and became national benchmarks for each country. In the future, regular verification should be carried out every few decades to ensure the consistency of the length benchmark.
But in fact, the length given by the original rice is not necessarily exactly1m. Because of dicing technology and measurement methods, there is always an error in the reproduction of numerical values, which is not less than 0. 1 micron, that is to say, the relative error can reach 1× 10e(-7). After a long time, it is difficult to ensure that the original device itself will not change, and the original device is in danger of being destroyed at any time. Therefore, with the development of science and technology, people increasingly hope to build the length on a more scientific, convenient and reliable basis, rather than the size of an object. Spectroscopic studies show that the wavelengths of visible light are some very accurate and stable lengths, which can be used as the benchmark of length. At the end of 19, the red spectral line of natural cadmium (Cd) was found in the experiment, which has very good clarity and reproducibility. In dry air at 15℃, its wavelength is equal to y = 6438.4696×10e (-10) meters.
According to the international agreement 1927, it is decided to use this spectral line as the length standard of spectroscopy, and it is determined that 1 m = 155 164438+03 YCD, and people have found the intangible standard that can be used to define the meter for the first time.
Scientists continued their research, and later found that the orange spectral line of krypton (86 Kr) was better than the red spectral line of cadmium. 1960, at the 1 1 International Metrology Conference, it was decided to replace the red line of cadmium with the orange line of krypton (86Kr), and the definition of meter was changed to: "The length of meter is equal to 1650763.73 times of radiation wavelength, which is equivalent to krypton (86Kr) atom in vacuum.
The accuracy of this benchmark is quite high, and the relative error is less than 4× 10e(-9)? , equivalent to 1 km length measurement is not less than 4 mm
However, the wavelength of atomic spectrum is too short, which will inevitably be affected by factors such as current and temperature, so the accuracy of reproduction is still limited. After the 1960s, due to the appearance of laser, people discovered a superior light source. Using laser instead of krypton spectrum can make the length measurement more accurate. As long as a certain time interval is determined, the unit of length can be defined from the product of the speed of light and this time interval. In 1980s, the speed of light C in vacuum was measured by laser, and C = * * * * * * * m/s was obtained.
1983 10 the 17th international metrology congress adopted a new definition of meter: "meter is the length of the distance that light travels in a vacuum of1* * * * * * * seconds". The new definition of rice is of great scientific significance. Since then, the speed of light c has become an accurate value. By unifying the length unit to time, we can use highly accurate time measurement and greatly improve the accuracy of length measurement.