Retentivity / Remanence
Reluctance is the reciprocal of permeability: i.e.
When a magnetising force is removed from a ferromagnetic material the amount of magnetism remaining will vary between materials and depends upon the permeability of the material. The remaining magnetism is termed residual magnetism and the material is said to have retentivity or retained magnetism.
Residual field as per BS EN 1330:7 – magnetic field remaining after magnetisation. As per ASNT handbook – when the magnetic field intensity is reduced to zero from the saturation point the flux density slowly decreases it lags the field intensity and does not reach zero. The amount of flux density remaining in the material is called residual magnetism of remanence.
If a material has high permeability it is easily magnetised, but it will just as easily give up the magnetic force, therefore it has a low retentivity. If a material has low permeability it is very difficult to magnetise, that is to say it has high magnetic reluctance, but once magnetisation has been achieved then it does not give up the magnetic force easily, therefore it has high retentivity.
High retentive materials can result in a problem known as magnetic writing. For example, if a material with high retentivity has been magnetised for testing then comes into contact with another ferromagnetic material. The new material is magnetised, and when this is tested the area could show as a false indication.
