Oersted (unit)

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 have been different definitions. 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). Because a magnetic pole does not exist in nature and must be realized by a long bar magnet, this definition was not practicable and was replaced.

Now 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 0.5 abampere ( = 5 A) for cgs-emu and a current of 0.5 c statampere ( = 5 A) for Gaussian units. Here c is the speed of light (≈ 3⋅10¹⁰ cm/s). This definition follows from the Biot-Savart law, for which (in vacuum B = H in cgs-emu and Gaussian units):

$|\mathbf {H} |={\begin{cases}{\displaystyle {\frac {2i}{4\pi \,r}}}&\quad {\hbox{SI units}}\\\\{\displaystyle {\frac {2i}{r}}}&\quad {\hbox{cgs-emu units}}\\\\{\displaystyle {\frac {2i}{cr}}}&\quad {\hbox{Gaussian units}}\\\end{cases}}$

where r (in cm) is the distance of the field point to the conductor.

The Biot-Savart law in Gaussian units states that the field in the center of a conducting loop of radius r is,

$|\mathbf {H} |={\frac {2\pi i}{cr}},$

where c is speed of light. Hence, one may give a third consistent definition for the Gaussian unit oersted, which is very closely related to the second definition. 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 c/(2π) statA. (A very similar definition holds for the cgs-emu oersted).

One oersted is equivalent to 1000/4π ≈ 79.577 47 A⋅turn/m   (ampere times turn per meter, which is the SI unit for |H|; see solenoid for the explanation of this unit).

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. 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.