Oersted

In the Gaussian system, the unit of the H-field is the oersted and the unit of the B-field is the gauss. In the SI, the unit ampere per metre (A/m), which is equivalent to newton per weber, is used for the H-field and the unit tesla is used for the B-field.^[3]

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]

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

The H-field strength inside a long solenoid wound with 79.58 turns per metre 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, and has exactly ⁠1000/4π⁠ turns per metre.

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

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

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]

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

• Centimetre–gram–second system of units
• Ampere's model of magnetization