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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 scientist Hans Christian Oersted.

Before 1930 there was much confusion about the difference between the gauss (the Gaussian 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. The gauss is defined through a time-dependent change in magnetic flux density and the oersted is defined through the field created by an electric current.

For Gaussian units, the oersted is defined 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 that carries a current of ½⋅c statA (statampere). Here c is the speed of light (≈ 3⋅1010 cm/s).

This definition follows from the Biot-Savart law, where r is the distance of the field point to the conductor. In the SI definition r is in meter, whereas in Gaussian units r is in centimeter.

The relation to the corresponding SI unit (ampere times turn per meter) is

1 Oe = 1000/4π A⋅turn/m.

See solenoid for the explanation of the SI unit, which is one of the few electromagnetic units that carries no name.

In order to explain the factor 1000/4π, we consider an infinite wire carrying a current of ½⋅3⋅1010 statA and measure |H| at one cm distance from the wire, by definition the field has the strength 1 Oe. Note that the current is equal to

½⋅3⋅1010 statA = ½⋅3⋅1010 A/(3⋅109) = ½⋅10 A,

where for clarity we wrote out the speed of light in the respective units (m and cm). Applying the Biot-Savart equation valid in SI units (current i in A, distance r in m), we find that the field in A⋅turn/m is The historically oldest definition of the oersted 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). Because a magnetic pole does not exist in nature and must be realized by a long bar magnet, this definition was not practicable and is now obsolete.