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oersted
oersted
Unit system	Gaussian units
Unit of	magnetic field strength
Symbol	Oe
Named after	Hans Christian Ørsted
Derivation	1 dyn/Mx
Conversions
1 Oe in ...	... is equal to ...
Gaussian base units	1 cm−1/2⋅g1/2⋅s−1
SI units	(4π)−1×103 A/m ≈ 79.57747 A/m

The oersted (/ˈɜːrstɛd/,;^[1] symbol Oe) is the coherent derived unit of the auxiliary magnetic field H in the centimetre–gram–second system of units (CGS).^[2] It is equivalent to 1 dyne per 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. In the SI system, the unit ampere per meter (A/m), which is equivalent to newton per weber, is used for the H-field and the unit of tesla is used for the B-field.^[3]

History

The unit was established by the IEC in the 1930s^[4] 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.^[5]

Definition

The oersted is defined as a dyne per unit pole.^{[clarification needed]}^[6] The oersted is ⁠1000/4π⁠ (≈79.5775) amperes per meter, in terms of SI units.^[7]^[8]^[9]^[10]

The H-field strength inside a long solenoid wound with 79.58 turns per meter of a wire carrying 1 A is approximately 1 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 (G), the CGS unit of magnetic flux density. In vacuum, if the magnetizing field strength is 1 Oe, then the magnetic field density is 1 G, whereas in a medium having permeability $μ r$ (relative to permeability of vacuum), their relation is

B({\text{G}})=\mu _{\text{r}}H({\text{Oe}}).

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:^[11]

1{\text{ Oe}}={\frac {1000}{4\pi }}~{\text{A}}/{\text{m}}.

Stored energy

The stored energy in a magnet, called magnet performance or maximum energy product^[12] (often abbreviated BH_max), is typically measured in units of megagauss-oersteds (MG⋅Oe).

References

^ "Oersted". Random House Webster's Unabridged Dictionary.
^ "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". — 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, "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
^ Kaye, G. W. C, & Laby, T. H.: Table of Physical and Chemical Constants, page 14. Longman, 1973.
^ "IEC history". Archived from the original on 2019-05-21. Retrieved 2006-03-25.
^ "Hans Christian Oersted". Famous Scientists. Retrieved 2020-03-31.
^ Hirst, A. W. Electricity and Magnetism For Engineering Students. Blackie & Son Limited, 1959, p. 411.
^ "Magnetic Conversion Factors". Archived from the original on 2020-09-27. Retrieved 2008-03-11.
^ "EMF Fundamentals". Archived from the original on 2008-04-07.
^ "Oersted". Everything2.
^ "Derived CGS Units with Special Names". Surface Engineering Forum. Gordon England.
^ "Table 9. Non-SI units associated with the CGS and the CGS-Gaussian system of units". SI Brochure: The International System of Units (SI) [8th edition, 2006; updated in 2014]. BIPM. 2006.
^ "What is Maximum Energy Product / BHmax and How Does It Correspond to Magnet Grade?". Dura Magnetics USA. 15 September 2014. Retrieved 2020-01-20.