In physics, the oersted (symbol Oe) is the unit of magnetic field strength |H| in the cgs-emu (centimeter-gram-second electromagnetic unit) and Gaussian systems of units. The oersted is named for the Danish physicist Hans Christian Oersted.
There are different (consistent) definitions in the literature. The oldest definition is: The magnetic field strength |H| in a point in vacuum is 1 Oe, if a unit magnetic pole in the point experiences a force of 1 dyne ( = 1⋅10−5 newton).
Later the oersted was redefined as the strength of the magnetic field at a distance of 1 centimeter from a straight conductor of infinite length and negligible circular cross section which carries a current of 0.5 abampere ( = 5 A).
The Biot-Savart law in Gaussian units, for which in vacuum B = H, states that the field in the center of a conducting loop of radius r is,
where c is speed of light. Hence, one may give a third consistent definition for the Gaussian unit oersted. It is the magnetic field strength |H| in the center of a conducting loop with radius of 1 cm, carrying an electric current i of 1/(2π) ampere. Note here that ampere/c (the statampere) is the unit of current and the centimeter is the unit of length in the Gaussian system of units.
One oersted equals 1000/4π ≈ 79.577 47 A/m (ampere per meter, which is the SI unit for |H|).
Before 1930 there was much confusion about the difference between the gauss (the cgs unit of magnetic flux density B) and the oersted. At its meeting in Stockholm in 1930 the Advisory Committee on Nomenclature of the International Electrotechnical Commission eliminated all ambiguity by adopting the gauss for the unit of magnetic flux density and the oersted for the unit of magnetic field strength.