**UNITS**, *units of physical quantities*. E. and. arose at early stages of development of material culture and originally covered insignificant number of physical quantities (length, weight, the area, volume), various in the different countries and geographical districts. A large number various by the sizes and names of units was formed. Expansion of commercial relations between the people, development of science and technology was resulted in need of unification E. and. and creations of system of units. In 1795 in France the metric system of measures was for the first time developed and approved by the special government decree, in a cut the meter representing ten-million part 1/4 of length of the Parisian geographical meridian was taken for a unit of length. This decision was caused by desire to be such unit the basis for system of units, to-ruyu it is possible to coordinate to almost invariable object of the nature. The sizes and names of other units in this system were chosen taking into account a possibility of their subsequent use and in other countries. In 1875 17 countries, including and Russia, signed the Metric convention for ensuring the international unity of measurements and improvement of metric system of measures. In Russia this system of units was allowed to use (in an optional order) in 1899 and entered as obligatory by the decree of SNK RSFSR of September 14, 1918, and for the USSR — the resolution SNK USSR of July 21, 1925. By 1972. The metric convention was signed by 41 states. The International bureau of measures and scales is created, the International committee of measures and scales is organized, general conferences on measures and scales are regularly convened. The metric system of measures

was evolved by the private, covering separate sections of physics or the equipment of system of units, and also stand-alone units. At the same time system E. and. are subdivided into the main units (e.g., meter, second, kilogram) chosen randomly and the derivative units (e.g., meter per second, kilogram per cubic meter etc.) formed on the equations of communication between sizes. Stand-alone E. and. are historically formed out of communication with creation of systems of units. These units divide on independent (defined without the aid of other units, napr, degree Celsius, equal 0,01 intervals between temperatures of thawing of ice and boiling of water) and randomly chosen, but defined through other units (e.g., the horsepower equal of 735,5 W; bar equal to 10 N/m, etc.); a nek-eye the name in honor of any outstanding scientist is appropriated to units (e.g., dalton — in honor of the English chemist and the physicist J. Dalton; one dalton is in number equal to the mass of one hydrogen atom).

For practical convenience of expression of sizes, considerably different from the main units, use multiple and submultiple units (e.g., kilogram and milligram — one thousand grams or a thousand share of gram respectively). In metric systems E. and. multiple and submultiple units (except for units of time and a corner) are formed by multiplication of system unit on 10^{ n } , where n — a positive or negative number (e.g., 1 kg = 10^{ 3 } 1 g = 10^{ 3 } kg). To each of such numbers (see below tab. 9) there corresponds one of the accepted decimal prefixes (a kilogram - mega-etc.).

Six main systems of units (MKGSS, MKSA, MKSG, MCC, MKS and SGS) on the basis of which arose entered practice of various fields of science and technology, and since 1960 the International System of Units — Sistem International — SI (SI) began to be applied preferential in the increasing number of the countries.

In system of units of MKGSS the main units are the meter (unit of length), a kilogram force (unit of force), second (unit of time); the system is not coordinated (not coherently) with units of electrical and magnetic quantities. With adoption of the International System of Units this system gradually goes out of use. If necessary the MKGSS system is applied in addition to the International System of Units or to other units allowed to use.

System of units of MKSA — system of units of electrical and magnetic quantities. Main units: meter (unit of length), kilogram (mass unit), second (unit of time) and ampere (unit of force of electric current). The system of units of MKSA was included a component into the International System of Units.

System of units of MKSG — system of units of thermal sizes. Main units: meter (unit of length), kilogram (mass unit), second (unit of time), kelvin (unit of thermodynamic temperature). This system of units was also included into the International System of Units.

System of units MCC — system of units for light sizes. The main units in this system are the meter (unit of length), second (unit of time) and a candle (unit of luminous intensity). System of units MCC — a part of the International System of Units.

Systems of units of the ISS — system of units for mechanical and acoustic sizes. Main units: meter (unit of length), kilogram (mass unit), second (unit of time). Systems of units of the ISS were included as components into the International System of Units.

Systems of units of SGS — system of units of mechanical, acoustic, electrical and magnetic quantities. Main units: centimeter (unit of length), gram (mass unit) and second (unit of time). Within the SGS systems some units received own name: dyne (unit of force), erg (unit of work and energy), puaz (unit dynamic or just viscosity), Stokes (unit of kinematic viscosity), Maxwell (unit of a magnetic flux), gauss (unit of magnetic induction), Gilbert (unit of magnetomotive power), oersted (unit of a magnetic field strength). Put seven types of the SGS systems for electrical and magnetic quantities into practice: electrostatic — SGSE (dielectric permeability of vacuum is accepted to equal dimensionless unit); electromagnetic — SGSM (magnetic permeability of vacuum is taken for dimensionless unit); symmetric SGS, or Gauss's system (electric units match electric units of the SGSE system, and magnetic — magnetic units of SGSM); SGSE0 (magnetic permeability of vacuum — the fourth main unit); SGSF (the fourth main unit — unit of electric charge — franklin); SGSB (the fourth main unit — unit of force of electric current — bio).

In the physicist and the technician preferential apply the SGS symmetric system.

In 1960 the XI General conference on measures and scales accepted the International System of Units. Since January 1, 1963 in the USSR the International System of Units was recommended for preferable use in all fields of science, the equipment and the national economy (GOST 9867 — 61 «International System of Units») for the purpose of unification of units. Seven main units (lengths, masses, time, force of electric current, thermodynamic temperature, amount of substance and luminous intensity), and also two additional units are the basis for the International System of Units (for a flat corner and a space angle). All other units are their derivatives and are formed on the equations of communication between physical. the sizes corresponding to the simplest shape of bodies or the phenomena. Acceptance for all countries of the uniform International System of Units physical. sizes it is designed to eliminate the difficulties connected with transfer of numerical values physical. sizes, and also constants from any operating system of units (SGS, MKS, etc.) in another.

The international organization for education, science and culture at the UN (UNESCO) suggested all member countries of this organization — to accept the International System of Units.

Ground rules of designation of units of the International system and use of them.

1. Designation of units which name is given by name the scientist, provides writing them from a capital letter, e.g.: ampere — And, volt — In, watt — W, X-ray — P etc. All other designations write from a lowercase letter.

2. Use of the reduced designations instead of full names of units, and also the room of designations of units at line with the formulas expressing dependence between sizes is not allowed. E.g., it is necessary to write «force is expressed in newtons», «force makes 1 N», but it is impossible to write: «force is expressed in N».

3. Name E. and. at figure do not decline. E.g., 10 mol, 10 Ohms, but not 10 moths and not 10 ohms.

4. Designation of unit is placed in a line with numerical value of size without transfer the next line; between the last figure and alphabetic reference of unit leave the admission.

5. Designation of units entering the work is divided by points on the centerline, e.g. Nanometer (newton-meter). In designation of units formed by division apply the slanting line, napr, kg/m^{ 3 } (kilogram per cubic meter). At the same time the work of units in a denominator bracket, e.g. W (m^{ 2 } • J) — watt on square meter - kelvin.

(Tab. 1 — 8) gives the main, additional below, and also derivatives and some taken most roots units (outdated, stand-alone, etc.). During the use of tables it must be kept in mind the following:

a) units of measurements of the International system are allocated with a bold-face type, units of measurements which did not enter it are given by a usual font, and the units of measurements applied earlier, but which are subject to withdrawal from the practical use are given with an asterisk;

b) as before adoption of the International System of Units alphabetic references of units in many domestic editions, and in particular in the BME editions, were given in the italics, designation of the corresponding units is given in the beginning according to the International System of Units, i.e. a direct font (without italics), and nearby in brackets the designation applied earlier, italics, napr with (sec.), W (W), P (p) etc.;

c) the concept of dimension (i.e. a symbol of sizes) presented in one of columns of tables 1 — 8, reflects communication this physical. sizes with the main sizes of system of units (tab. 1) is also the work of the main sizes built in the corresponding degrees. E.g., dimension of force in the International System of Units represents expression:

LMT^{ - 2 } or m • kg/s^{ 2 }

where L, M and T — dimensions of length, weight and time (meter, kilogram and second respectively). All members of equation, describing any physical. process, shall have identical dimension;

d) all accepted international reductions of units are given according to the International System of Units.

The main, additional, major derivative International System Units of units (SI), and also some stand-alone units which were not a part of the system the SI are listed in tab. 1 — 9.

Additional instructions to use

of tables 1. The bold-face type designated International System Units of units (SI).

2. The asterisk designated the units which did not enter the International System of Units and subjects to withdrawal.

3. The units which did not enter the International System of Units, but allowed to use are given by a usual direct font.

4. Designations of the corresponding units are given according to the International System of Units by a direct font without italics in the beginning, and nearby the designations applied earlier, e.g. are given in brackets: with (sec.), W (W), m (m) etc.

**Table 1. MAIN AND ADDITIONAL UNITS of MEASUREMENTS of the INTERNATIONAL SYSTEM OF UNITS (SI). (Explanations to the table — see the text of article)**

**Table 2. The most important units of mechanical sizes, space and time**

**Table 3. The most important units of electrical and magnetic quantities**

**Table 4. The most important units of thermal sizes**

**Table 5. The most important units of acoustic sizes**

**Table 6. The most important units of radiant and light sizes of optical radiation**

**Table 7 the most important derivative units of sizes of physical chemistry and molecular physics**

**Table 8. The most important units of ionizing radiation**

**Table 9. Prefixes and multipliers for formation of decimal multiple and submultiple units and their names**

**Bibliography:** Burdun G. D. Units of physical quantities, M., 1967; it, Reference book on the International System of Units, M., 1971; With e of N and L. A. Units of physical quantities and their dimension, M., 1969.

*Century. I am Maximov, Yu. M. Petrusevich.*