What is "remanence"

Remanence \Rem"a*nence\ (r?m"?*nens), Remanency \Rem"a*nen*cy\ (-nen*s?), n. [Cf. OF. remanence, LL. remanentia, fr. L. remanens. See Remanent, a.] The state of being remanent; continuance; permanence. [R.]
--Jer. Taylor.

The remanence of the will in the fallen spirit.
--Coleridge.

1660s, "that which remains," from remanent + -ence. Meaning "continuance, permanence" is from 1810.

n. 1 (context physics English) The magnetization left behind in a medium after an external magnetic field is removed. 2 (context archaic English) The state of being remanent; continuance; permanence.

Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence. The remanence of magnetic materials provides the magnetic memory in magnetic storage devices, and is used as a source of information on the past Earth's magnetic field in paleomagnetism.

The equivalent term residual magnetization is generally used in engineering applications. In transformers, electric motors and generators a large residual magnetization is not desirable (see also electrical steel) as it is an unwanted contamination, for example a magnetization remaining in an electromagnet after the current in the coil is turned off. Where it is unwanted, it can be removed by degaussing.

Sometimes the term retentivity is used for remanence measured in units of magnetic flux density.