<P> Seen in some magnetic materials, saturation is the state reached when an increase in applied external magnetic field H cannot increase the magnetization of the material further, so the total magnetic flux density B more or less levels off . (It continues to increase very slowly due to the vacuum permeability .) Saturation is a characteristic of ferromagnetic and ferrimagnetic materials, such as iron, nickel, cobalt and their alloys . </P> <P> Saturation is most clearly seen in the magnetization curve (also called BH curve or hysteresis curve) of a substance, as a bending to the right of the curve (see graph at right). As the H field increases, the B field approaches a maximum value asymptotically, the saturation level for the substance . Technically, above saturation, the B field continues increasing, but at the paramagnetic rate, which is several orders of magnitude smaller than the ferromagnetic rate seen below saturation . </P> <P> The relation between the magnetizing field H and the magnetic field B can also be expressed as the magnetic permeability: μ = B / H (\ displaystyle \ mu = B / H) or the relative permeability μ r = μ / μ 0 (\ displaystyle \ mu _ (r) = \ mu / \ mu _ (0)), where μ 0 (\ displaystyle \ mu _ (0)) is the vacuum permeability . The permeability of ferromagnetic materials is not constant, but depends on H. In saturable materials the relative permeability increases with H to a maximum, then as it approaches saturation inverts and decreases toward one . </P>

When does saturation occur in an air mass