# Oersted

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Oersted (abbreviated as Oe) is the unit of the auxiliary magnetic field H in the CGS system of units.

## Difference between CGS and SI systems

In CGS the unit of H-field is the oersted and the unit of B‑field is the gauss. In the SI system, ampere per meter (A/m) is used for H‑field and tesla is used for B‑field.1

## History

The unit was established by the IEC in 1930 2 in honour of the Danish physicist Hans Christian Ørsted. Ørsted discovered the relationship between magnetism and electric current when a magnetic field produced in a current-carrying coil deflected an ammeter (an instrument used to measure current) when it was switched on and off.

## Definition

The oersted is defined as a dyne per unit pole.3 The oersted is 1000/4π (≈79.5774715) amperes per meter, in terms of SI units.4567

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, the CGS unit of magnetic flux density. In a vacuum, if the magnetizing field strength is 1 Oe, then the magnetic field density is 1 G, whereas, in a medium having permeability $\mu$r (relative to permeability of vacuum), their relation is:

$B(\mbox{G})= \mu_r H(\mbox{Oe})$

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:

$H(\mbox{Oe})= \frac{4 \pi}{1000} \frac{I(\mbox{A})}{l(\mbox{m})}$8

The stored energy in a magnet, called magnet performance or maximum energy product (often abbreviated BHmax), is typically measured in units of megagauss-oersteds (MGOe). One MGOe is approximately equal to 7957.74715 J/m3.9

## References

1. ^ Kaye, G. W. C, & Laby, T. H.: Table of Physical and Chemical Constants, page 14. Longman, 1973.
2. ^ IEC history
3. ^ Hirst, A. W. Electricity and Magnetism For Engineering Students. Blackie & Son Limited, 1959, p.411
4. ^ Magnetic Conversion Factors
5. ^ EMF Fundamentals
6. ^ Everything2.com • Oersted
7. ^ Derived CGS Units with Special Names
8. ^ Griffiths, David (1999). Introduction to Electrodynamics. Prentice Hall. p. 559. ISBN 0-13-805326-X.
9. ^ eFunda: Glossary: Units: Energy Density Units: Megagauss-Oersted (MGOe)

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