Oersted

Historical background

The oersted (Oe) is a measure of magnetic field strength. The unit oersted is named after the Danish physicist and chemist Hans Christian Oersted. Hans Christian Oersted discovered the magnetic effect of electric current in 1820 and thus made a significant contribution to research into magnetism. He was not the first physicist to recognise a connection between magnetism and electricity. However, earlier discoveries were repeatedly forgotten. Many physicists simply confused electrical and magnetic forces. It was not until Hans Christian Oersted recognised the significance of his discovery for modern electrotechnical applications.

The relationship between electricity and magnetism was fully described by Maxwell in 1864 with the help of Maxwell's equations. Maxwell's formulations gave traditional electrodynamics its formulation, which is still accepted today.

The oersted unit

The oersted unit for magnetic field strength is defined in the so-called cgs system. In the cgs system, a quantity is formed from the physical base units centimetre (cm), gram (g) and second (s). There are also the base units for current (ampere), temperature (kelvin), amount of substance (mol) and luminous intensity (candela).

Today, the international system of units or SI system is generally accepted. According to this system, the kilogram (kg) is used instead of the gram and the metre (m) instead of the centimetre. In the SI system, magnetic field strength is measured in amperes per metre (A/m).

In contrast to most other cases, there is no straight conversion for the oersted in the SI system. The oersted is defined in such a way that a magnetic field of 1 Oe in a vacuum corresponds to a magnetic flux density of 0.1 millitesla (mT).

However, unlike the oersted, the tesla is an SI unit and the relationship between magnetic field H and magnetic flux density B is determined by the permeability constant of the vacuum (μ₀):

B=μ₀H.

In the SI system, the permeability constant μ₀ has the value of \(\mu_0=4\pi\cdot10^{-7}\frac{Vs}{Am}\). This results in the non-straight relationship 1 oersted = 79,577 A/m = \(\frac{1000}{4\pi} A/m\).

The unit tesla is often used in literature as a measure of the magnetic field strength. However, this is not entirely correct. As mentioned, tesla and gauss are the units of magnetic flux density. The magnetic field is given in the unit oersted (1 Oe = 79,577 A/m), i.e. in amperes per metre (A/m).