The oersted (abbreviated as Oe) is the unit of magnetic field strength |H| in the cgs-emu (centimeter-gram-second electromagnetic unit) and Gaussian systems of units. The field strength 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).
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 alternatively define the Gaussian unit oersted as 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 A/m (ampere per meter, which is the SI unit for |H|).
The oersted is named after the Danish physicist Hans Christian Oersted. Before 1932 the oersted was known as gauss, a name sometimes still applied, though now more properly used for the cgs-emu and Gaussian unit of strength of magnetic induction |B|.