SI measurement system — history, purpose, role in physics
Human history is several thousand years old, and at various stages of its development almost every nation has used some of its conventional reference systems. Now the International System of Units (SI) has become mandatory for all countries.
The system contains seven basic units of measurement: second — time, meter — length, kilogram — mass, ampere — strength of electric current, kelvin — thermodynamic temperature, candela — intensity of light, and mole — amount of substance. There are two additional units: the radian for a flat angle and the steradian for a solid angle.
SI comes from the French Systeme Internationale and stands for the International System of Units.
How the counter is determined
In the 17th century, with the development of science in Europe, calls for the introduction of a universal measure or Catholic meter began to be heard more and more often. It would be a decimal measure based on the natural event and independent of the decision of the person in authority. Such a measure would replace the many different systems of measures then in existence.
The British philosopher John Wilkins proposed to take the length of the pendulum as a unit of length, the half-period of which would be equal to one second. However, depending on the measurement location, the value was not the same. The French astronomer Jean Richet established this fact during a trip to South America (1671 — 1673).
In 1790, Minister Talleyrand proposed to measure the reference longitude by placing the pendulum at a strictly fixed latitude between Bordeaux and Grenoble — 45 ° north latitude. As a result, on May 8, 1790, the French National Assembly decided that the meter is the length of a pendulum with a half-period at 45 ° latitude equal to 1 s. According to today's SI, this meter would be equal to 0.994 m. However, this definition does not sit well with the scientific community.
On March 30, 1791, the French Academy of Sciences accepted the proposal to define a standard of measurement as part of the Paris meridian. The new unit was to be one ten-millionth of the distance from the equator to the North Pole, that is, one ten-millionth of a quarter of the circumference of the earth, measured along the Paris meridian. This became known as the "Meter True and Definitive".
On April 7, 1795, the National Convention passed a law introducing the metric system in France and instructed the commissioners, including Ch. O. Coulomb, J.L. Lagrange, P.-S. Laplace and other scientists experimentally determined the units of length and mass.
In the period from 1792 to 1797, by decision of the revolutionary convention, the French scientists Delambre (1749-1822) and Mechen (1744-1804) measured the same arc of the Paris meridian with a length of 9 ° 40 'from Dunkirk to Barcelona in 6 years. years, laying a chain of 115 triangles across France and part of Spain.
However, it later turned out that due to an incorrect calculation of the Earth's polar compression, the standard turned out to be 0.2 mm shorter. Thus, the meridian length of 40,000 km is only approximate. However, the first prototype of the standard brass meter was made in 1795. It should be noted that the unit of mass (the kilogram, whose definition is based on the mass of one cubic decimeter of water) is also tied to the definition of the meter.
The history of the formation of the SI system
On June 22, 1799, two platinum standards—the standard meter and the standard kilogram—were made in France. This date can rightfully be considered the day of the beginning of the development of the current SI system.
In 1832, Gauss created the so-called Absolute system of units, taking as the basic three units: the unit of time is the second, the unit of length is the millimeter, and the unit of mass is the gram, because using these particular units, the scientist was able to measure the absolute value of the Earth's magnetic field (this system got the name SGS Gauss).
In the 1860s, under the influence of Maxwell and Thomson, the requirement that base and derived units must be compatible with each other was formulated. As a result, the CGS system was introduced in 1874, with prefixes also distributed to denote subsets and multiples of units from micro to mega.
In 1875, representatives of 17 countries, including Russia, the United States, France, Germany, Italy, signed the Metric Convention, according to which the International Bureau of Measures, the International Committee of Measures was established and a regular convention began to function. General Conference on Weights and Measures (GCMW)… At the same time, work began on the development of an international standard for the kilogram and a standard for the measuring instrument.
In 1889 at the first conference of the GKMV, ISS systembased on meter, kilogram and second, like the CGS, however, the ISS units seemed more acceptable due to the convenience of practical use. Optics and electrical units will be introduced later.
In 1948, by order of the French government and the International Union of Theoretical and Applied Physics, the Ninth General Conference on Weights and Measures issued an instruction to the International Committee on Weights and Measures to propose, in order to unify the system of units of measurement, his ideas to create a single system of units of measurement that can be accepted by all countries — parties to the Metric Convention.
As a result, the following six units were proposed and adopted at the tenth GCMW in 1954: meter, kilogram, second, ampere, kelvin, and candela. In 1956, the system was named «Systeme International d'Unities» - the international system of units.
In 1960, a standard was adopted, which for the first time was called the «International System of Units» and was assigned the abbreviation «SI» (SI).
The basic units remained the same six units: meter, kilogram, second, ampere, kelvin and candela, two additional units (radian and steradian) and twenty-seven most important derivatives, without specifying in advance other derivative units that could be added by - late. (The abbreviation in Russian "SI" can be deciphered as "International System").
All these six basic units, both additional units and twenty-seven most important derived units, completely coincided with the corresponding basic, additional and derived units adopted at that time in the USSR state standards for units of measurement for the ISS, MKSA, МКСГ and MSS systems.
In 1963 in the USSR, according to GOST 9867-61 «International system of units», SI is accepted as preferred for the fields of national economy, in science and technology, and for teaching in educational institutions.
In 1968, at the thirteenth GKMV, the unit "degree Kelvin" was replaced by "kelvin", and the designation "K" was also adopted. In addition, a new definition of a second was adopted: a second is a time interval equal to 9,192,631,770 radiation periods corresponding to the transition between two hyperfine levels of the ground quantum state of the cesium-133 atom. In 1997, a clarification would be adopted that this time interval refers to the cesium-133 atom at rest at 0 K.
In 1971, another basic unit «mol» was added to 14 GKMV - a unit for the amount of substance. A mole is the amount of matter in a system containing as many structural elements as there are atoms in carbon-12 weighing 0.012 kg. When a mole is used, the structural elements must be specified and can be atoms, molecules, ions, electrons, and other particles or specified groups of particles.
In 1979, the 16th CGPM adopted a new definition of the candela. The candela is the luminous intensity in a given direction of a source emitting monochromatic radiation with a frequency of 540 × 1012 Hz, whose luminous intensity in that direction is 1/683 W / sr (watts per steradian).
In 1983, a new definition was given to the counter of 17 GKMV.A meter is the length of the path traveled by light in a vacuum in (1/299,792,458) seconds.
In 2009, the Government of the Russian Federation approved the "Regulation on Units of Measurement Permitted for Use in the Russian Federation", and in 2015, amendments were made to it to exclude the "period of validity" of some non-system units.
The main advantages of the SI system are the following:
1. Unification of units of physical quantities for different types of measurement.
The SI system allows any physical quantity found in different fields of technology to have one common unit for them, for example, the joule for all types of work and the amount of heat instead of the currently used different units for this quantity (kilogram - force - meter, erg, calorie, watt-hour, etc.).
2. The universality of the system.
SI units cover all branches of science, technology and the national economy, excluding the need for the use of other units and generally represent a single system common to all areas of measurement.
3. Connectivity (coherence) of the system.
In all physical equations that define the resulting units of measurement, the proportionality factor is always a dimensionless quantity equal to unity.
The SI system makes it possible to significantly simplify the operations of solving equations, performing calculations and drawing up graphs and nomograms, since there is no need to use a significant number of conversion factors.
4. The harmony and coherence of the SI system greatly facilitates the study of physical laws and the pedagogical process in the study of general scientific and special disciplines, as well as the derivation of various formulas.
5.The principles of construction of the SI system provide an opportunity to form new derived units as needed, and therefore the list of units of this system is open to further expansion.
The purpose of the SI system and its role in physics
To date, the international system of physical quantities SI has been accepted throughout the world and is used more than other systems both in science and technology and in people's daily lives - it is a modern version of the metric system.
Most countries use SI units in technology, even if they use traditional units for those territories in everyday life. In the United States, for example, customary units are defined as SI units using fixed coefficients.
The quantity Designation Russian name Russian international Flat angle radian glad rad Solid angle steradian Wed Wed Temperature in Celsius degree in Celsius OS OS Frequency hertz Hz Hz Force Newton Z n Energy joule J J Power watt W W Pressure pascal Pa Pa Luminous flux lumen lm lm Illumination lux OK lx Electric charge pendant CL ° C Potential difference volt V V Resistance ohm Ohm R Electric capacity farad F F Magnetic flux Weber Wb Wb Magnetic induction Tesla T T Inductance Henry Mr. H Electrical conductivity Siemens Cm C Activity of a radioactive source becquerel Bq Bq Absorbed dose of ionizing radiation gray Gr Gy Effective dose of ionizing radiation sievert Sv Sv Activity of the catalyst rolled cat cat
An exhaustively detailed description of the SI system in official form is given in the SI Booklet, published since 1970, and its supplement; these documents are published on the official website of the International Bureau of Weights and Measures. Since 1985these documents are issued in English and French and are always translated into several languages around the world, although the official language of the document is French.
The precise official definition of the SI system is as follows: "The International System of Units (SI) is a system of units based on the International System of Units, together with names and symbols, and a set of prefixes and their names and symbols together with rules for their use adopted by the General Conference on Weights and Measures (CGPM) «.
The SI system is defined by seven basic units of physical quantities and their derivatives, as well as prefixes to them. The standard abbreviations of unit designations and the rules for writing derivatives are regulated. There are seven basic units as before: kilogram, meter, second, ampere, kelvin, mole, candela. Base units are size-independent and cannot be derived from other units.
As for derived units, they can be obtained based on the basic ones, by performing mathematical operations such as division or multiplication. Some of the resulting units, such as "radian", "lumen", "pendant", have their own names.
You can use a prefix before the name of the unit, such as millimeter — one thousandth of a meter and kilometer — one thousand meters. The prefix means that one is to be divided or multiplied by an integer that is a specific power of ten.