What Does a Reading of W Refer to When Working in Ohms

SI derived unit of measurement of electrical resistance

This article is about the SI derived unit of measurement. For other uses, see Ohm (disambiguation).

Ohm
A laboratory one-ohm standard resistor, circa 1917.
Full general information
Unit system	SI derived unit
Unit of	Electric resistance
Symbol	Ω
Named after	Georg Ohm
Derivation	Ω = V/A
Conversions
1 Ω in ...	... is equal to ...
SI base of operations units	kg⋅m2⋅due south−iii⋅A−ii

The ohm (symbol: Ω) is the SI derived unit of electrical resistance, named after German physicist Georg Ohm. Various empirically derived standard units for electric resistance were developed in connexion 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 every bit early as 1861. Every bit of 2020, the definition of the ohm is expressed in terms of the quantum Hall result.

Definition [edit]

Ane of the functions of many types of multimeters is the measurement of resistance in ohms.

The ohm is divers as an electrical resistance between ii points of a conductor when a constant potential difference of one volt, applied to these points, produces in the conductor a current of one ampere, the usher not being the seat of any electromotive force.^[1]

${\displaystyle \Omega ={\dfrac {\text{5}}{\text{A}}}={\dfrac {i}{\text{S}}}={\dfrac {\text{W}}{{\text{A}}^{2}}}={\dfrac {{\text{V}}^{2}}{\text{Westward}}}={\dfrac {\text{s}}{\text{F}}}={\dfrac {\text{H}}{\text{s}}}={\dfrac {{\text{J}}{\cdot }{\text{s}}}{{\text{C}}^{ii}}}={\dfrac {{\text{kg}}{\cdot }{\text{1000}}^{ii}}{{\text{s}}{\cdot }{\text{C}}^{2}}}={\dfrac {\text{J}}{{\text{south}}{\cdot }{\text{A}}^{2}}}={\dfrac {{\text{kg}}{\cdot }{\text{m}}^{two}}{{\text{due south}}^{3}{\cdot }{\text{A}}^{2}}}}$

in which the following units appear: volt (5), ampere (A), siemens (Due south), watt (West), second (s), farad (F), henry (H), joule (J), coulomb (C), kilogram (kg), and metre (m).

Following the 2019 redefinition of the SI base units, in which the ampere and the kilogram were redefined in terms of key constants, the ohm is affected past a very small scaling in measurement.

In many cases the resistance of a conductor is approximately abiding within a sure range of voltages, temperatures, and other parameters. These are called linear resistors. In other cases resistance varies, such every bit in the case of the thermistor, which exhibits a strong dependence of its resistance with temperature.

A vowel of the prefixed units kiloohm and megaohm is unremarkably omitted, producing kilohm and megohm.^{[two] [3] [4] [5]}

In alternate current circuits, electrical impedance is also measured in ohms.

Conversions [edit]

The siemens (symbol: S) is the SI derived unit of electric conductance and comprisal, as well known as the mho (ohm spelled backwards, symbol is ℧); information technology is the reciprocal of resistance in ohms (Ω).

Power as a function of resistance [edit]

The power dissipated by a resistor may exist calculated from its resistance, and the voltage or current involved. The formula is a combination of Ohm's law and Joule's police force:

${\displaystyle P=V\cdot I={\frac {V^{two}}{R}}=I^{ii}\cdot R}$

where:

P is the power

R is the resistance

V is the voltage across the resistor

I is the current through the resistor

A linear resistor has a constant resistance value over all applied voltages or currents; many practical resistors are linear over a useful range of currents. Not-linear resistors have a value that may vary depending on the applied voltage (or current). Where alternating current is applied to the circuit (or where the resistance value is a function of fourth dimension), the relation above is truthful at whatsoever instant but calculation of average power over an interval of time requires integration of "instantaneous" power over that interval.

Since the ohm belongs to a coherent system of units, when each of these quantities has its respective SI unit (watt for P, ohm for R, volt for Five and ampere for I, which are related as in § Definition, this formula remains valid numerically when these units are used (and idea of as existence cancelled or omitted).

History [edit]

The rapid rise of electrotechnology in the last half of the 19th century created a demand for a rational, coherent, consistent, and international organization of units for electrical quantities. Telegraphers and other early users of electricity in the 19th century needed a practical standard unit for resistance. Resistance was ofttimes expressed every bit a multiple of the resistance of a standard length of telegraph wires; different agencies used unlike bases for a standard, so units were not readily interchangeable. Electric units and then defined were not a coherent system with the units for free energy, mass, length, and fourth dimension, requiring conversion factors to exist used in calculations relating energy or power to resistance.^[6]

Two different methods of establishing a system of electrical units tin can be chosen. Diverse artifacts, such as a length of wire or a standard electrochemical jail cell, could exist specified as producing defined quantities for resistance, voltage, and then on. Alternatively, the electrical units tin be related to the mechanical units by defining, for instance, a unit of current that gives a specified forcefulness between ii wires, or a unit of accuse that gives a unit of forcefulness between 2 unit charges. This latter method ensures coherence with the units of energy. Defining a unit for resistance that is coherent with units of energy and fourth dimension in effect likewise requires defining units for potential and electric current. It is desirable that one unit of electrical potential will force one unit of electrical current through 1 unit of electrical resistance, doing ane unit of work in one unit of time, otherwise, all electrical calculations will require conversion factors.

Since then-called "absolute" units of charge and electric current are expressed every bit combinations of units of mass, length, and fourth dimension, dimensional analysis of the relations between potential, electric current, and resistance show that resistance is expressed in units of length per time – a velocity. Some early definitions of a unit of measurement of resistance, for example, defined a unit resistance as i quadrant of the Earth per second.

The accented-units system related magnetic and electrostatic quantities to metric base units of mass, time, and length. These units had the great advantage of simplifying the equations used in the solution of electromagnetic problems, and eliminated conversion factors in calculations about electrical quantities. Nonetheless, the centimeter-gram-second, CGS, units turned out to have impractical sizes for practical measurements.

Various artifact standards were proposed equally the definition of the unit of measurement of resistance. In 1860 Werner Siemens (1816–1892) published a suggestion for a reproducible resistance standard in Poggendorffs Annalen der Physik und Chemie.^[vii] He proposed a column of pure mercury, of one square millimeter cross section, one metre long: Siemens mercury unit. All the same, this unit was non coherent with other units. One proposal was to devise a unit of measurement based on a mercury cavalcade that would be coherent – in effect, adjusting the length to make the resistance i ohm. Not all users of units had the resources to acquit out metrology experiments to the required precision, and then working standards notionally based on the physical definition were required.

In 1861, Latimer Clark (1822–1898) and Sir Charles Bright (1832–1888) presented a paper at the British Clan for the Advancement of Science meeting ^[8] 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 written report upon standards of electrical resistance.^[9] Their objectives were to devise a unit of measurement that was of user-friendly size, role of a complete system for electrical measurements, coherent with the units for energy, stable, reproducible and based on the French metrical system.^[10] In the tertiary study of the committee, 1864, the resistance unit is referred to as "B.A. unit, or Ohmad".^[11] By 1867 the unit is referred to as only ohm.^[12]

The B.A. ohm was intended to be 10^ix CGS units merely attributable to an fault in calculations the definition was 1.3% too small. The fault was meaning for preparation of working standards.

On 21 September 1881 the Congrès internationale des électriciens (international briefing of electricians) defined a practical unit of ohm for the resistance, based on CGS units, using a mercury column 1 sq. mm. in cross-section, approximately 104.9 cm in length at 0 °C,^[13] similar to the appliance suggested by Siemens.

A legal ohm, a reproducible standard, was defined by the international conference of electricians at Paris in 1884^{[ commendation needed ]} as the resistance of a mercury column of specified weight and 106 cm long; this was a compromise value between the B. A. unit of measurement (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.^[14] The unit was based upon the ohm equal to ten^ix units of resistance of the C.G.S. system of electromagnetic units. The international ohm is represented past the resistance offered to an unvarying electric current in a mercury cavalcade of abiding cross-exclusive area 106.3 cm long of mass xiv.4521 grams and 0 °C. This definition became the basis for the legal definition of the ohm in several countries. In 1908, this definition was adopted by scientific representatives from several countries at the International Conference on Electric Units and Standards in London.^[14] The mercury column standard was maintained until the 1948 General Briefing on Weights and Measures, at which the ohm was redefined in accented terms instead of as an artifact standard.

By the terminate of the 19th century, units were well understood and consequent. Definitions would change with little event on commercial uses of the units. Advances in metrology allowed definitions to exist formulated with a high caste of precision and repeatability.

Historical units of resistance [edit]

Unit of measurement[15]	Definition	Value in B.A. ohms	Remarks
Absolute human foot/second × 10seven	using imperial units	0.3048	considered obsolete even in 1884
Thomson's unit	using imperial units	0.3202	100 million ft/s (30,480 km/southward), considered obsolete fifty-fifty in 1884
Jacobi copper unit	A specified copper wire 25 ft (7.620 m) long weighing 345 gr (22.36 g)	0.6367	Used in 1850s
Weber'south accented unit of measurement × 107	Based on the metre and the 2d	0.9191
Siemens mercury unit	1860. A column of pure mercury	0.9537	100 cm and ane mm2 cantankerous department at 0 °C
British Association (B.A.) "ohm"	1863	1.000	Standard coils deposited at Kew Observatory in 1863[16]
Digney, Breguet, Swiss		9.266–10.420	Iron wire i km long and 4 mmtwo cantankerous section
Matthiessen		13.59	1 mi (i.609 km) of one⁄xvi -inch-diameter (1.588 mm) pure annealed copper wire at xv.5 °C
Varley		25.61	One mile of special 1⁄16 -inch-diameter copper wire
German mile		57.44	A German mile (viii,238 yd or 7,533 m) of atomic number 26 wire 1⁄half dozen  in (four.233 mm) diameter
Abohm		x−9	Electromagnetic absolute unit of measurement in centimeter–gram–2nd units
Statohm		8.987551 787 ×ten11	Electrostatic absolute unit in centimeter–gram–2d units

Realization of standards [edit]

The mercury column method of realizing a concrete 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 synthetic past the British Association and others, to serve as physical artifact standards for the unit of measurement of resistance. The long-term stability and reproducibility of these artifacts was an ongoing field of research, every bit the effects of temperature, air pressure, humidity, and time on the standards were detected and analyzed.

Artifact standards are still used, merely metrology experiments relating accurately-dimensioned inductors and capacitors provided a more cardinal basis for the definition of the ohm. Since 1990 the quantum Hall upshot has been used to define the ohm with high precision and repeatability. The breakthrough Hall experiments are used to check the stability of working standards that have user-friendly values for comparison.^[17]

Following the 2019 redefinition of the SI base units, in which the ampere and the kilogram were redefined in terms of key constants, the ohm is at present also defined in terms of these constants.

Symbol [edit]

The symbol Ω was suggested, considering of the like sound of ohm and omega, by William Henry Preece in 1867.^[18] In documents printed earlier WWII the unit symbol often consisted of the raised lowercase omega (ω), such that 56 Ω was written as 56^ω.

Historically, some document editing software applications have used the Symbol typeface to render the character Ω.^[nineteen] Where the font is not supported, a W is displayed instead ("10 W" instead of "10 Ω", for case). As W represents the watt, the SI unit of power, this can lead to defoliation, making the utilise of the right Unicode code point preferable.

Where the character set is limited to ASCII, the IEEE 260.1 standard recommends substituting the symbol ohm for Ω.

In the electronics industry it is common to use the grapheme R instead of the Ω symbol, thus, a ten Ω resistor may be represented every bit 10R. This is the British standard BS 1852 code. It is used in many instances where the value has a decimal place. For example, five.6 Ω is listed as 5R6. This method avoids overlooking the decimal betoken, which may non be rendered reliably on components or when duplicating documents.

Unicode encodes the symbol every bit U+2126 Ω OHM SIGN, distinct from Greek omega amid letterlike symbols, but it is merely included for backward compatibility and the Greek uppercase omega character U+03A9 Ω GREEK Capital letter OMEGA (HTMLΩ· Ω, Ω) is preferred.^[20] In MS-DOS and Microsoft Windows, the alt code ALT 234 may produce the Ω symbol. In Mac OS, ⌥ Opt+Z does the same.

See besides [edit]

• Electronic color code
• History of measurement
• International Committee for Weights and Measures
• Orders of magnitude (resistance)
• Resistivity

Notes and references [edit]

1. ^ BIPM SI Brochure: Appendix i, p. 144
2. ^ SASB/SCC14 - SCC14 - Quantities, Units, and Alphabetic character Symbols (2002-12-30). IEEE/ASTM SI x-2002: IEEE/ASTM Standard for Employ of the International Organisation of Units (SI): The Modern Metric System. {{cite book}}: CS1 maint: multiple names: authors list (link)
3. ^ Thompson, Ambler; Taylor, Barry N. (November 2008) [March 2008]. "Chapter ix.3 Spelling unit names with prefixes". Guide for the Employ of the International System of Units (SI) (PDF) (second corrected press, 2008 ed.). Gaithersburg, Maryland, USA: National Institute of Standards and Engineering, U.S. Department of Commerce. CODEN NSPUE3. NIST Special Publication 811. Archived (PDF) from the original on 2021-01-31. Retrieved 2021-01-31 . p. 31: Reference [6] points out that there are three cases in which the concluding vowel of an SI prefix is commonly omitted: megohm (not megaohm), kilohm (not kiloohm), and hectare (not hectoare). In all other cases in which the unit name begins with a vowel, both the concluding vowel of the prefix and the vowel of the unit of measurement proper noun are retained and both are pronounced. (85 pages)
4. ^ "NIST Guide to the SI". Gaithersburg, Maryland, USA: National Plant of Standards and Technology (NIST), Concrete Measurement Laboratory. 2016-08-25 [2016-01-28]. Affiliate 9: Rules and Style Conventions for Spelling Unit Names, ix.3: Spelling unit names with prefixes. Special Publication 811. Archived from the original on 2021-01-31. Retrieved 2021-01-31 . [1]
5. ^ Aubrecht II, Gordon J.; French, Anthony P.; Iona, Mario (2012-01-twenty). "About the International Arrangement of Units (SI) Part IV. Writing, Spelling, and Mathematics". The Physics Instructor. 50 (ii): 77–79. Bibcode:2012PhTea..fifty...77A. doi:10.1119/1.3677278.
6. ^ Hunt, Bruce J. (1994). "The Ohm Is Where the Art Is: British Telegraph Engineers and the Development of Electrical Standards" (PDF). Osiris. two. ix: 48–63. doi:10.1086/368729. S2CID 145557228. Archived from the original on 2014-03-08. Retrieved 2014-02-27 .
7. ^ Siemens, Werner (1860). "Vorschlag eines reproducirbaren Widerstandsmaaßes". Annalen der Physik und Chemie (in German). 186 (5): 1–20. Bibcode:1860AnP...186....1S. doi:10.1002/andp.18601860502.
8. ^ Clark, Latimer; Brilliant, Sir Charles (1861-11-09). "Measurement of Electrical Quantities and Resistance". The Electrician. one (1): 3–4. Retrieved 2014-02-27 .
9. ^ Report of the Xxx-First Coming together of the British Association for the Advancement of Scientific discipline; held at Manchester in September 1861. September 1861. pp. xxxix–40.
10. ^ Williamson, A.; Wheatstone, C.; Thomson, W.; Miller, W. H.; Matthiessen, A.; Jenkin, Fleeming (September 1862). Conditional Report of the Committee appointed past the British Association on Standards of Electrical Resistance. Xxx-2d Meeting of the British Association for the Advancement of Scientific discipline. London: John Murray. pp. 125–163. Retrieved 2014-02-27 .
11. ^ Williamson, A.; Wheatstone, C.; Thomson, West.; Miller, W. H.; Matthiessen, A.; Jenkin, Fleeming; Bright, Charles; Maxwell, James Clerk; Siemens, Carl Wilhelm; Stewart, Balfour; Joule, James Prescott; Varley, C. F. (September 1864). Report of the Committee on Standards of Electrical Resistance. Xxx-fourth Meeting of the British Association for the Advancement of Science. London: John Murray. p. Foldout facing page 349. Retrieved 2014-02-27 .
12. ^ Williamson, A.; Wheatstone, C.; Thomson, W.; Miller, West. H.; Matthiessen, A.; Jenkin, Fleeming; Vivid, Charles; Maxwell, James Clerk; Siemens, Carl Wilhelm; Stewart, Balfour; Varley, C. F.; Foster, Yard. C.; Clark, Latimer; Forbes, D.; Hockin, Charles; Joule, James Prescott (September 1867). Report of the Committee on Standards of Electric Resistance. 30-seventh Meeting of the British Association for the Advancement of Science. London: John Murray. p. 488. Retrieved 2014-02-27 .
13. ^ "System of measurement units". Engineering and Engineering History Wiki . Retrieved 2018-04-13 .
14. ^ ^{a b} Fleming, John Ambrose (1911). "Units, Physical". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 27 (11th ed.). Cambridge Academy Press. pp. 738–745, see page 742. An Electric Congress was held in Chicago, U.S.A. in August 1893, to consider......and at the terminal one held in London in Oct 1908 were finally adopted
15. ^ Gordon Wigan (trans. and ed.), Electrician'due south Pocket Book, Cassel and Company, London, 1884
16. ^ Historical Studies in International Corporate Concern. Teich p34
17. ^ R. Dzuiba and others, Stability of Double-Walled Maganin Resistors in NIST Special Publication Proceedings of SPIE, The Establish, 1988 pp. 63–64
18. ^ Preece, William Henry (1867), "The B.A. unit for electrical measurements", Philosophical Magazine, vol. 33, p. 397, retrieved 2017-02-26
19. ^ E.yard. recommended in HTML 4.01: "HTML 4.01 Specification". W3C. 1998. Section 24.1 "Introduction to character entity references". Retrieved 2018-11-22 .
20. ^ Excerpts from The Unicode Standard, Version 4.0, accessed eleven October 2006

External links [edit]

• Scanned books of Georg Simon Ohm at the library of the Academy of Practical Sciences Nuernberg
• Official SI brochure
• NIST Special Publication 811
• History of the ohm at sizes.com
• History of the electrical units.

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Source: https://en.wikipedia.org/wiki/Ohm

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