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I am not sure if it is increasing or decreasing as some sources say it is increasing whereas some say it is not. What actually happens when a metallic object is put under a coil that is meant to be used as a detection coil for metal detector?

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What actually happens when a metallic object is put under a coil that is meant to be used as a detection coil for metal detector?

If the metal is (say) iron then there are two opposing effects. Iron has a magnetic permeability much greater than 1 and this will tend to increase the inductance of the search coil but, iron is also a conductor and, it will act as a shorted turn and reduce the inductance of the search coil. Somewhere between the two is the answer and this depends on: -

  • Operating frequency
  • Relative dimensions of the piece of iron (i.e. shape)
  • Size

Incidentally some grades of stainless steel are very difficult to find using a metal detector because the two effects cancel out. I discovered this when designing metal detectors for food and pharmaceutical lines.

For other (non-ferromagnetic) metals/conductors their presence reduces the inductance of the search coil.

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It depends on the magnetic properties of the object. Some metals are ferromagnetic, some are paramagnetic, and some are only diamagnetic.

Generally speaking, if the magnetic permeability is greater than one (more specifically, if it is greater than that of air), inductance increases; if it is less, inductance decreases.

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    \$\begingroup\$ you are implicitly assuming that the object is not conductor. \$\endgroup\$ – andre314 Dec 20 '19 at 13:25

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