Such magnets may be of very hard steel, containing a small percentage of tungsten or of cobalt and chromium together. The coercive force of cobalt — chromium steel is high, and thus magnets made of this steel are not subject to self-demagnetisation to the same extent as tungsten-steel magnets.
Within the last two decades great advances have been made in the development of magnetic materials having properties suiting them to the construction of permanent magnets. In particular, Alnico (in the united states) or Alcomaax (in England), alloys of iron, nickel and aluminium, having especially good properties with Coercive forces which may be as high as 60.000 AIm.
During manufacture, permanent magnets are artificially aged by being placed in a weak alternating magnetic field, or by heating. This action reduces their strength, somewhat, but ensures permanence of the magnetism remaining.
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