Ом: единица измерения электрического сопротивления.

SI derived unit of electrical resistance
the SI derived unit

The ohm (symbol: Ω, the uppercase Greek letter omega) is the unit of electrical resistance in the International System of Units (SI). It is named after German physicist Georg Ohm. Various empirically derived standard units for electrical resistance were developed in connection with early telegraphy practice, and the British Association for the Advancement of Science proposed a unit derived from existing units of mass, length and time, and of a convenient scale for practical work as early as 1861. Following the 2019 redefinition of the SI base units, in which the ampere and the kilogram were redefined in terms of fundamental constants, the ohm is now also defined as an exact value in terms of these constants.

The ohm is defined as an electrical resistance between two points of a conductor when a constant potential difference of one volt (V), applied to these points, produces in the conductor a current of one ampere (A), the conductor not being the seat of any electromotive force. suggesting that standards for electrical units be established and suggesting names for these units derived from eminent philosophers, 'Ohma', 'Farad' and 'Volt'. The BAAS in 1861 appointed a committee including Maxwell and Thomson to report upon standards of electrical resistance. Their objectives were to devise a unit that was of convenient size, part of a complete system for electrical measurements, coherent with the units for energy, stable, reproducible and based on the French metrical system. In the third report of the committee, 1864, the resistance unit is referred to as "B. A. unit, or Ohmad". By 1867 the unit is referred to as simply ohm. The B. A. ohm was intended to be 109 CGS units but owing to an error in calculations the definition was 1.3% too small. The error was significant for preparation of working standards. On 21 September 1881 the International Electrical Congress defined a practical unit of ohm for the resistance, based on CGS units, using a mercury column 1 mm2 in cross section, approximately 104.9 cm in length at 0 °C, similar to the apparatus suggested by Siemens. A legal ohm, a reproducible standard, was defined by the international conference of electricians at Paris in 1884 as the resistance of a mercury column of specified weight and 106 cm long; this was a compromise value between the B. A. unit (equivalent to 104.7 cm), the Siemens unit (100 cm by definition), and the CGS unit. Although called "legal", this standard was not adopted by any national legislation. The "international" ohm was recommended by unanimous resolution at the International Electrical Congress 1893 in Chicago. The mercury column standard was maintained until the 1948 General Conference on Weights and Measures, at which the ohm was redefined in absolute terms instead of as an artifact standard. By the end of the 19th century, units were well understood and consistent. Definitions would change with little effect on commercial uses of the units. Advances in metrology allowed definitions to be formulated with a high degree of precision and repeatability.

Unit Definition Value in B. A. ohms Remarks Absolute foot/second × 107 using imperial units 0.3048 considered obsolete even in 1884 Thomson's unit using imperial units 0.3202 , considered obsolete even in 1884 Jacobi copper unit A specified copper wire 7.62 m long weighing 0.6367 Used in 1850s Weber's absolute unit × 107 Based on the meter and the second 0.9191 Siemens mercury unit 1860. A column of pure mercury 0.9537 100 cm and 1 mm2 cross section at 0 °C British Association (B. A.) "ohm" 1863 1.000 Standard coils deposited at Kew Observatory in 1863 Digney, Breguet, Swiss 9.266–10.420 Iron wire 1 km long and 4 mm2 cross section Matthiessen 13.59 of pure annealed copper wire at 15.5 °C Varley 25.61 One mile of special 1/16 inch diameter copper wire German mile 57.44 A German mile of iron wire diameterAbohm10−9 Electromagnetic absolute unit in centimeter–gram–second unitsStatohm 8.987551787 Electrostatic absolute unit in centimeter–gram–second units

The mercury column method of realizing a physical standard ohm turned out to be difficult to reproduce, owing to the effects of non constant cross section of the glass tubing. Various resistance coils were constructed by the British Association and others, to serve as physical artifact standards for the unit of resistance. The long term stability and reproducibility of these artifacts was an ongoing field of research, as the effects of temperature, air pressure, humidity, and time on the standards were detected and analyzed. Artifact standards are still used, but metrology experiments relating accurately dimensioned inductors and capacitors provided a more fundamental basis for the definition of the ohm. Since 1990 the quantum Hall effect has been used to define the ohm with high precision and repeatability. The quantum Hall experiments are used to check the stability of working standards that have convenient values for comparison. Following the 2019 redefinition of the SI base units, in which the ampere and the kilogram were redefined in terms of fundamental constants, the ohm is now also defined in terms of these constants.

The symbol Ω was suggested, because of the similar sound of ohm and omega, by William Henry Preece in 1867. In documents printed before Second World War the unit symbol often consisted of the raised lowercase omega (ω), such that 56 Ω was written as 56ω. Historically, some document editing software applications have used the Symbol typeface to render the character Ω. Where the font is not supported, the same document may be displayed with a "W" ("10 W" instead of "10 Ω", for instance). As W represents the watt, the SI unit of power, this can lead to confusion, making the use of the correct Unicode code point preferable. Where the character set is limited to ASCII, the IEEE 260.1 standard recommends using the unit name "ohm" as a symbol instead of Ω. In the electronics industry it is common to use the character R instead of the Ω symbol, thus, a 10 Ω resistor may be represented as 10R. This is part of the RKM code. It is used in many instances where the value has a decimal place. For example, 5.6 Ω is listed as 5R6, or 2200 Ω is listed as 2K2. This method avoids overlooking the decimal point, which may not be rendered reliably on components or when duplicating documents. Unicode encodes the symbol as , distinct from Greek omega among letterlike symbols, but it is only included for backward compatibility and the Greek uppercase omega character is preferred. In MS DOS and Microsoft Windows, the alt code ALT 234 may produce the Ω symbol. In Mac OS, does the same.