If the magnetic field exists in a material other
than a vacuum, the permeability in the equation is that of the material and not a vacuum. The
permeability of a material is written as μ but it is usual to speak of the relative permeability of
the material. This is the ratio of the permeability of the material to that of a vacuum, and is
written as μr (note that μr is
a pure number with no units or dimensions).
Therefore: μ = μrμo
Some
values for relative permeabilities are given below. The value for iron is many thousands of times
that of air; this is why the introduction of an iron core into a solenoid produces a very large
increase in the field within it.
| Material | Realative permeability |
| Mild steel | 2000 |
| Silicon-iron (4.25% Si) | 9000 |
| Supermalloy (Fe 16%,Ni 79%,Mo 5%) | 1 000 000 |