Editing Oersted

{{Short description|Unit of the auxiliary magnetic field H in the CGS system of units}}
{{Other uses}}
{{Infobox unit
| name       = oersted
| image      =
| caption    =
| standard   = [[Gaussian units]]
| quantity   = [[magnetic field strength]]
| symbol     = Oe
| namedafter = [[Hans Christian Ørsted]]
| extralabel  = Derivation
| extradata   = 1 dyn/Mx
| units1     = Gaussian base units
| inunits1   = 1&nbsp;cm<sup>−1/2</sup>⋅g<sup>1/2</sup>⋅s<sup>−1</sup>
| units2     = [[SI units]]
| inunits2   = (4π)<sup>−1</sup>{{e|3}}&nbsp;A/m ≈ {{convert|1e0|Oe|A/m|sigfig=7|disp=out}}
}}

The '''oersted''' ({{IPAc-en|ˈ|ɜːr|s|t|ɛ|d}},;<ref>[http://dictionary.reference.com/browse/oersted "Oersted"]. ''[[Random House Webster's Unabridged Dictionary]]''.</ref> symbol '''Oe''') is the [[coherent derived unit]] of the [[Magnetic field#The H-field|auxiliary magnetic field '''H''']] in the [[centimetre–gram–second system of units]] (CGS).<ref>"as late as 1936 a subcommittee of the IEC [[International Electrotechnical Commission]] proposed the names 'maxwell', 'gauss' and 'oersted' for the cgs electromagnetic units of flux, induction and magnetic field strength, respectively". &mdash; John James Roche, ''The Mathematics of Measurement: A Critical History'', The Athlone Press, London, 1998, {{ISBN|0-485-11473-9}}, page 184 and John James Roche, [http://materias.df.uba.ar/ft1a2016c2/files/2016/09/B-vs-H.pdf "''B'' and ''H'', the intensity vectors of magnetism: A new approach to resolving a century-old controversy"], ''American Journal of Physics'', vol. 68, no. 5, 2000, doi: 10.1119/1.19459, p. 438; in both cases giving the reference as Claudio Egidi, editor, ''Giovanni Giorgi and his Contribution to Electrical Metrology: Proceedings of the meeting held in Turin (Italy) on 21 and 22, September 1988'', Politecnico di Torino, Turin (IT), 1990, {{ISBN|978-8885259003}}, pp. 53–56</ref> It is equivalent to 1 [[dyne]] per [[maxwell (unit)|maxwell]].

==Difference between CGS and SI systems==
In the CGS system, the unit of the [[H-field]] is the oersted and the unit of the [[B-field]] is the [[gauss (unit)|gauss]]. In the SI system, the unit ampere per meter (A/m), which is equivalent to [[newton (unit)|newton]] per [[weber (unit)|weber]], is used for the H-field and the unit of [[tesla (unit)|tesla]] is used for the B-field.<ref>Kaye, G. W. C, & Laby, T. H.: ''Table of Physical and Chemical Constants'', page 14. Longman, 1973.</ref>

==History==
The unit was established by the [[International Electrotechnical Commission|IEC]] in the 1930s<ref>{{Cite web |url=http://www.iec.ch/about/history/ |title=IEC history |access-date=2006-03-25 |archive-date=2019-05-21 |archive-url=https://web.archive.org/web/20190521213548/http://www.iec.ch/about/history/ |url-status=dead }}</ref> in honour of Danish physicist [[Hans Christian Ørsted]]. Ørsted discovered the connection between magnetism and electric current when a magnetic field produced by a current-carrying copper bar deflected a magnetised needle during a lecture demonstration.<ref>{{Cite web |url=https://www.famousscientists.org/hans-christian-oersted/ |title=Hans Christian Oersted |website=Famous Scientists |language=en-US |access-date=2020-03-31}}</ref>

==Definition==
[[File:TDK AM Pro cassette - wrapper detail with magneric specs.png|thumb|[[Cassette tape]] label with [[coercivity]] (a measure of the external [[magnetic flux]] required to magnetize the tape) measured in oersteds]]
The oersted is defined as a [[dyne]] per [[Magnetic moment#Magnetic pole model|unit pole]].{{Clarify|date=February 2024|reason=What unit is used to measure the magnetic pole strength here? Presumably the SI and CGS systmes use different units.}}<ref>Hirst, A. W. ''Electricity and Magnetism For Engineering Students''. Blackie & Son Limited, 1959, p.&nbsp;411.</ref> The oersted is {{sfrac|1000|4π}} (≈{{convert|1.000000|Oe|A/m|disp=number}}) amperes per meter, in terms of [[SI units]].<ref>{{Cite web |url=http://www-d0.fnal.gov/hardware/cal/lvps_info/engineering/mag_conv.htm |title=Magnetic Conversion Factors<!-- Bot generated title --> |access-date=2008-03-11 |archive-date=2020-09-27 |archive-url=https://web.archive.org/web/20200927170425/https://www-d0.fnal.gov/hardware/cal/lvps_info/engineering/mag_conv.htm |url-status=dead }}</ref><ref>{{cite web |url=http://www.vitatech.net/emf_fundamentals.php4 |title=EMF Fundamentals |url-status=dead |archive-url=https://web.archive.org/web/20080407011207/http://www.vitatech.net/emf_fundamentals.php4 |archive-date=2008-04-07}}</ref><ref>{{cite web |url=http://everything2.com/index.pl?node_id=1145558 |website=Everything2 |title=Oersted}}</ref><ref>{{cite web |url=http://www.gordonengland.co.uk/conversion/sinon3.htm |title=Derived CGS Units with Special Names |website=Surface Engineering Forum |publisher=Gordon England}}</ref>

The H-field strength inside a long [[solenoid]] wound with 79.58 turns per meter of a wire carrying 1&nbsp;[[ampere|A]] is approximately 1&nbsp;oersted. The preceding statement is exactly correct if the solenoid considered is infinite in length with the current evenly distributed over its surface.

The oersted is closely related to the [[Gauss (unit)|gauss]] (G), the CGS unit of magnetic flux density. In vacuum, if the magnetizing field strength is 1&nbsp;Oe, then the magnetic field density is 1&nbsp;G, whereas in a medium having [[magnetic permeability|permeability]] {{math|''&mu;''<sub>r</sub>}} (relative to permeability of vacuum), their relation is
: <math>B(\text{G}) = \mu_\text{r} H(\text{Oe}).</math>

Because oersteds are used to measure magnetizing field strength, they are also related to the [[magnetomotive force]] (mmf) of current in a single-winding wire-loop:<ref>{{cite book |publisher=[[BIPM]] |title=SI Brochure: The International System of Units (SI) [8th edition, 2006; updated in 2014] |year=2006 |chapter=Table 9. Non-SI units associated with the CGS and the CGS-Gaussian system of units |chapter-url=http://www.bipm.org/en/publications/si-brochure/table9.html}}</ref>
: <math>1\text{ Oe} = \frac{1000}{4 \pi}~\text{A}/\text{m}.</math>

===Stored energy===
{{main|maximum energy product}}
The stored energy in a magnet, called ''magnet performance'' or ''maximum energy product''<ref>{{Cite web |url=https://www.duramag.com/techtalk/tech-briefs/what-is-maximum-energy-product-bhmax-and-how-does-it-correspond-to-magnet-grade/ |title=What is Maximum Energy Product / BHmax and How Does It Correspond to Magnet Grade? |website=Dura Magnetics USA |date=15 September 2014 |language=en-US |access-date=2020-01-20}}</ref> (often abbreviated BH<sub>max</sub>), is typically measured in units of megagauss-oersteds (MG⋅Oe).

==See also==
* [[Centimetre–gram–second system of units]]
* [[Ampere model of magnetization|Ampere's model of magnetization]]

{{portal|Physics|Science}}

==References==
{{Reflist}}

{{CGS units}}

[[Category:Centimetre–gram–second system of units]]
[[Category:Units of magnetic induction]]