Permeability
Electromagnetism and formation of magnetic fields.
Permeability is the ability to support formation of magnetic fields in a material.
Permeability is measured in H/m (henries/m) or newtons/ampere2 (N/A2).
The Permeability of Free Space
The permeability of free space µ0 (the permeability constant or the magnetic constant) is
µ0 = 4π 10-7 (H/m)
≈ 1.257 10-6 (H/m, N/A2)
The Relative Permeability
The relative permeability is the ratio of the permeability of a specific medium to the permeability of free space µ0
µr = µ / µ0 (1)
where
µr = the relative permeability
µ = permeability of the medium (H/m)
The lowest relative magnetic permeability of a paramagnetic material is 1.0 - and the magnetic response of the material is the same as 'free space' or complete vacuum.
| Medium | Permeability - µ - (H/m) | Relative permeability - μ / μ0 - |
|---|---|---|
| Air | 1.25663753 10-6 | 1.00000037 |
| Aluminum | 1.256665 10-6 | 1.000022 |
| Austenitic stainless steel1) | 1.260 10-6 - 8.8 10-6 | 1.003 – 7 |
| Bismuth | 1.25643 10-6 | 0.999834 |
| Carbon Steel | 1.26 10-4 | 100 |
| Cobalt-Iron (high permeability strip material) | 2.3 10-2 | 18000 |
| Copper | 1.256629 10-6 | 0.999994 |
| Ferrite (nickel zinc) | 2.0 10-5 – 8.0 10-4 | 16 – 640 |
| Ferritic stainless steel (annealed) | 1.26 10-3 - 2.26 10-3 | 1000 – 1800 |
| Hydrogen | 1.2566371 10-6 | 1 |
| Iron (99.8% pure) | 6.3 10-3 | 5000 |
| Iron (99.95% pure Fe annealed in H) | 2.5 10-1 | 200000 |
| Martensitic stainless steel (annealed) | 9.42 10-4 - 1.19 10-3 | 750 – 950 |
| Martensitic stainless steel (hardened) | 5.0 10-5 - 1.2 10-4 | 40 – 95 |
| Nanoperm | 1.0 10-1 | 80000 |
| Neodymium magnet | 1.32 10-6 | 1.05 |
| Nickel | 1.26 10-4 - 7.54 10-4 | 100 – 600 |
| Permalloy | 1.0 10-2 | 8000 |
| Platinum | 1.256970 10-6 | 1.000265 |
| Sapphire | 1.2566368 10-6 | 0.99999976 |
| Superconductors | 0 | 0 |
| Teflon | 1.2567 10-6 | 1 |
| Vacuum (µ0) | 4π 10-7 | 1 |
| Water | 1.256627 10-6 | 0.999992 |
| Wood | 1.25663760 10-6 | 1.00000043 |
1) Permeability of austenitic stainless steels is not like ferritic, martensitic and duplex stainless steel. Austenitic steel can be classed as paramagnetic with relative permeability approaching 1.0 in the fully austenitic condition. The low permeability enables austenitic steel to be used where a non-magnetic material is required.