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Permeability

Electromagnetism and formation of magnetic fields

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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.

MediumPermeability
- 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 0) 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.

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