Magnetic Materials: Units and Terminology
In the study of magnetism there are two systems of units currently in use:
• the mks (metres-kilograms-seconds) system, which has been adopted as the S.I. units
• the cgs (centimetres-grams-seconds) system, also known as the Gaussian system.
The cgs system is used by many magnets experts due to the numerical equivalence of the magnetic
induction (B) and the applied field (H).
When a field is applied to a material it responds by producing a magnetic field, the magnetisation
(M). This magnetisation is a measure of the magnetic moment per unit volume of material, but can
also be expressed per unit mass, the specific magnetisation (σ). The field that is applied to the
material is called the applied field (H) and is the total field that would be present if the field were
applied to a vacuum. Another important parameter is the magnetic induction (B), which is the total
flux of magnetic field lines through a unit cross sectional area of the material, considering both lines
of force from the applied field and from the magnetisation of the material. B, H and M are related by
equation 1a in S.I. units and by equation 1b in cgs units.
B = µo (H + M)
Equ.1a
B = H + 4 π M
Equ.1b
In equation 1a, the constant µo is the permeability of free space (4π x 10-7 Hm-1), which is the ratio
of B/H measured in a vacuum. In cgs units the permeability of free space is unity and so does not
appear in equation 1b. The units of B, H and M for both S.I. and cgs systems are given in table 2.
Note that in the cgs system 4πM is usually quoted as it has units of Gauss and is numerically
equivalent to B and H.
Another equation to consider at this stage is that concerning the magnetic susceptibility (x),
equation 2, this is the same for S.I. and cgs units. The magnetic susceptibility is a parameter that
demonstrates the type of magnetic material and the strength of that type of magnetic effect.
Equ.2
Sometimes the mass susceptibility (χ
m
) is quoted and this has the units of m
3
kg
-1
and can be
calculated by dividing the susceptibility of the material by the density.
Another parameter that demonstrates the type of magnetic material and the strength of that type of
magnetic effect is the permeability (µ) of a material, this is defined in equation 3 (the same for S.I.
and cgs units).
Equ.3
In the S.I. system of units, the permeability is related to the susceptibility, as shown in equation 4
and can also be broken down into µ
0
and the relative permeability (µ
r
), as shown in equation 5.
