Flux Density/Coercivity (B/H) of Round Rod magnets in Air
Under some conditions, a magnet which has been magnetized in a fixture or coil, which is then placed in a magnetic circuit, is found to produce a weaker field than was calculated, based on its fully magnetized B/H curve. Another form of the same problem occurs when a magnet which has been magnetized in place, in a magnetic circuit, is found to produce its intended field initially but would become weaker if removed from the circuit and then returned to it. Magnets under these conditions are said to have undergone "shape demagnetization". The problem is much more severe with older Alnico materials which have a high flux-density B but relatively little coercivity H, and also a B/H curve which is more of a continuous arc than a straight line with a fairly sharp knee. Designers who have worked only with the newer magnet materials may not be aware that the danger of partial demagnetization by shape exists in these materials too.
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