# Encyclopedia Magnetica

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# Magnetic field strength

 Stan Zurek, Magnetic field strength, Encyclopedia Magnetica, E-Magnetica.pl

Magnetic field strength H - a physical quantity used as one of the basic measures of the intensity of magnetic field.1)2) The unit of magnetic field strength3) is ampere per metre or A/m.

Electric current $I$ generates magnetic field strength $H$, whose magnitude is independent on the type of the uniform isotropic surrounding medium (magnetic or non-magnetic)

From the engineering viewpoint, magnetic field strength $H$ can be thought of as excitation and the magnetic flux density $B$ as the response of the medium.4)5) This naming convention is defined in the SI system of units.

From theoretical physics viewpoint, the field $H$ is defined as the vectorial difference between flux density $B$ and magnetisation $M$. The H field is sometimes referred to as “auxiliary” or simply “field H”.6)7)8)

These two approaches are identical in the sense of the physical quantities in question (with the same physical units of A/m), but are referred to by different names, and different emphasis put on their meaning and use in derivation of some equations.

Magnetic field is a vector field in space, and is a kind of energy whose full quantification requires the knowledge of the vector fields of both magnetic field strength $H$ and flux density $B$ (or other values correlated with them, like magnetisation M or polarisation J). In vacuum, at each point the $H$ and $B$ vectors are oriented along the same direction and are directly proportional through permeability of free space, but in other media they can be misaligned (especially in non-uniform or anisotropic materials).

The requirement of two quantities is analogous for example to electricity. Both electric voltage $V$ and electric current $I$ are required to fully quantify the effects of electricity, e.g. the amount of transferred energy.9)

The name magnetic field strength and the symbol $H$ are defined by International Bureau of Weights and Measures (BIPM) as a one of the coherent derived physical units.10) Therefore, strictly speaking, other names like magnetic field intensity or magnetic field (or even just field) which can be encountered in everyday technical jargon11) are incorrect if used when referring to a specific value of H in A/m.

There are many other names which are used in the literature: magnetic field intensity H12), magnetic field H 13)14)15), field H 16), field H' 17), H-field strength 18), H-field 19), magnetization field strength H 20), magnetizing field strength H 21), magnetising force H 22), magnetic force H 23), intensity of magnetic force H 24), auxiliary field H 25), and probably several others.