| 摘要 |
<p>A method of detecting a magnetic feature on an article. The method comprises:
i) causing relative movement between the feature (1) and a single bias magnetic field having, along the direction of relative movement, a first region (Y) with a relatively strong field strength of a first polarity and a second region (X) with a relatively weak field strength and a second polarity different from the first polarity, whereby the feature is moved successively through the first and second regions, the magnetic feature being magnetized by the biasing field;
ii) removing the magnetic feature (1) from the influence of the bias magnetic field and detecting the polarity of the magnetized feature to determine if the material has a sufficiently low coercivity that its polarity is influenced by the bias magnetic field in the second region,
wherein the magnetic material of the magnetic feature is sufficiently hard to retain its magnetic polarity between steps i) and ii).
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