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, the French government came forward, held a meeting of representatives of 20 governments, and formally signed the Metric Convention, recognizing 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 the 1980s, the speed of light C in vacuum was measured by laser, and C = 299,792,458 m/s was obtained. ..
1983 10 the 17th international metrology congress adopted a new definition of meter: "meter is the length of the distance traveled by light in a vacuum 1/299792458 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.
mass unit
The situation of ancient mass units and length units is similar, and there are various forms. For example, Persia uses Garasa as the unit of mass, which is about 0.834kg, and Egypt uses Gade, which is about 9.33g g. According to the weights and measures system of Qin Dynasty in China, 1 stone = 4 jun, 1 jun = 30 kg, 1 kg = 16 liang. Compared with the modern international system of units, 1 kg is about 0? 256 kilograms. British units are pounds, ounces, drams and grains: 1 pound = 16 ounces = 265 drams = 7000 grains.
The British Empire made a weighing instrument out of pure platinum, which is a pure platinum cylinder with a height of about 1.35 inches and a diameter of 1. 1.5 inches.
The original unit of mass in kilograms was the length unit of meters adopted by France at the end of 18. The mass of 1 cubic decimeter of pure water at the maximum density (the temperature is about 4 degrees Celsius) is determined as 1 kg.
During the period of 1799, France made a prototype of platinum rice, and at the same time made a benchmark of platinum kilogram, which was kept in the Paris Archives.
Later, it was found that this benchmark is not completely equal to the mass of pure water with the maximum density of 1 cubic decimeter, but equal to 1? 000028 cubic decimeter. Therefore, after the Metric Congress of 1875, the alloy containing 90% platinum and 10% iridium was also used as the original kilogram, and one * * * was made into three. Compared with the original kilogram of platinum preserved by the Paris Archives Bureau, one of them was selected as the international original kilogram. The original international kilogram device was carefully kept in a special place by experts from the International Bureau of Metrology. It was covered with three layers of glass, and the outermost glass was evacuated to prevent air and impurities from entering. Subsequently, 40 platinum-iridium alloy cylinders were copied, compared with the original international kilogram device, and distributed to member States as national benchmarks. Like the rice prototype, the kilogram prototype also needs to be verified regularly to ensure the stability and reliability of the quality benchmark.
unit of time
Among the natural phenomena observed by human beings, what happens in the sky is the most obvious and regular, so it is natural to take the period of the earth's rotation as the measurement of time. This period is the solar day. 1 sec = 1/86400 flat solar day. Since the earth's rotation is uneven and unstable, the international metrology conference of 1960 confirmed that the time standard should be changed to be based on the earth's revolution around the sun, that is, the second should be defined as 1/365438+ 1900, which is the time required for the earth to make one revolution around the sun in its orbit.50000.000000000005
But according to this definition, it is difficult to directly compare the second itself. During this period, the measurement technology of time and frequency has developed greatly.
1967 The 13th International Metrology Conference redefined the time unit: "One second is the duration of 9 19263 1770 radiation cycles corresponding to two ultra-fine energy level transitions in the ground state of cesium-133 atom."
thermometric scale
In the current international system of units, the temperature is expressed in Kelvin (K), and this degree is also the basic unit. Strictly speaking, the choice of degree-day is actually a question of temperature scale. In the history of thermal development, there are Fahrenheit scale, cylindrical scale, Langer scale, centigrade scale scale, gas scale and thermodynamic scale. The thermodynamic temperature scale was first proposed by Kelvin in 1848, and the thermodynamic temperature defined by the thermodynamic temperature scale has the strictest scientific significance. The others belong to the empirical temperature scale, and their common feature is that a specific thermometer and several temperature fixed points are artificially selected to define the temperature scale, so there is no objective standard. These empirical temperature scales have become history, but they still have some origins with modern temperature scales.
Fahrenheit scale was put forward by German D.G.Fahrenheit around 17 10, which stipulated that the freezing point of water was 32 degrees and the boiling point of water was 2 12 degrees. Fahrenheit is still popular in Britain, America and other countries.
Liech's temperature scale was put forward by R.A.F.Reaumur in 1730, which stipulated that the freezing point of water was zero and the boiling point of water was 80 degrees. Liesl scale was once popular in Germany.
Langmuir scale was put forward by the Englishman Rankin, and its definition is
tR=tF+459.67
In fact, Langer's temperature is the Fahrenheit temperature starting from absolute zero, which is expressed as 0R. Now it is rarely used in the field of science and technology.
Centigrade scale was proposed by Swedish astronomer A. Celsius 1742. His original plan was that the boiling point of water was zero and the freezing point of water was 100 degrees. The next year, the Frenchman Christian reversed the two scales and became the common scale now.
The gas temperature scale formed by calibrating the temperature with a gas thermometer is closest to the thermodynamic temperature scale. Due to the poor reproducibility of gas thermometers, the international practical temperature scale has been agreed to unify the international temperature values. After several changes, the temperature set for this purpose is as close as possible to the thermodynamic temperature.
As early as 1887, the International Metrology Committee decided to adopt the constant volume hydrogen thermometer as the basis of the international practical temperature standard.
1927 The Seventh International Metrology Conference decided to adopt platinum resistance thermometer as the interpolation instrument of temperature standard, and stipulated that a series of repeatable temperatures or fixed points should be determined between the freezing point of oxygen (-182.97 degrees Celsius) and the freezing point of gold (1063 degrees Celsius).
1948 The 1 1 International Metrology Conference made some important revisions to the international practical temperature scale. For example, replace the freezing point of gold with a financial point; Replace Wayne's law with Planck's law of blackbody radiation; Reference more accurate constant values; The calculation formula is more accurate; And the measuring range of the optical pyrometer is expanded.
An important modification has been added to 1960, that is, the triple point of water is taken as the only definition point, and its absolute temperature is specified as 273. 16 (accurate), instead of the original freezing point of water as 0.00 degrees Celsius (accurate). According to the actual measurement, the freezing point of water should be 273.438+0.500 0.038+0K. Using the triple point of water as the only definition point is a great progress in temperature measurement, because it can avoid the difference of temperature measurement caused by the change of freezing point around the world.
1968, the international practical temperature scale was revised again with the code IPTS-68. It is characterized by adopting the latest achievements of thermodynamics, which makes the international practical temperature scale closer to the thermodynamic temperature scale. This time, it is also stipulated that the symbol K is used to represent the absolute temperature, and the original symbol (K) is cancelled. It is stipulated that the temperature of Celsius is exactly equal to the absolute degree-day of the thermodynamic temperature scale, and the temperature of Celsius is T = absolute temperature T-273. 15 (accurate).
IPTS-68 was revised and supplemented in 1975 and 1976 respectively, and the lower limit of the temperature range was extended from 13.8K to 0.5K, but there were still some shortcomings, mainly because it was found that IPTS-68 was quite different from the thermodynamic temperature defined by the international system of units in some temperature regions.
1988 recommended by the International Committee of Weights and Measures, the18th International Metrology Congress and the 77th International Metrology Committee made resolutions to adopt the revised international temperature scale all over the world from 10/9901,and this time it was named1. Compared with IPTS-68, the temperature at 100℃ is 0.026℃ lower, that is, the boiling point of water in standard state is 99.974℃ instead of 100℃.
Obviously, the implementation of ITS-90 will bring benefits to precision temperature measurement, which is another sign of scientific and technological development.