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
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.
