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Eddy Current Location Misunderstanding


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Occasionally I've read that an eddy current (set up by a metal detector's transmitted signal and which generates a magnetic field to be 'received' by the detector's circuitry) in detected objects are confined to that object's surface.  If true this can be relevant for materials of mixed metals (no, I don't mean alloys) such as some coins and some crown caps, as well as plated objects like cheap jewelry.  There is a simple exposition common to US coin hunting which disproves this contention.

The US 5-cent piece ('nickel'), with the exception of 4 years during WWII, is an alloy of 75% copper (Cu) and 25% nickel (Ni).  Modern US clad dimes and quarters (and some other denominations, all minted after 1964) are a sandwich composed of a pure copper layer in the middle 2/3 of the coin and two surface layers (each 1/6 of the coin's thickness) composed of the same Cu-Ni alloy as the 5-cent piece.

The US nickel's numerical target identification (TID) on every(?) metal detector is considerably lower than a clad dime.  Example 1:  MInelab Equinox -- nickel hits 12-13 and clad dime 25-26, typically.  Example 2:  Fisher F75 -- nickel approximate TID = 30.  Clad dime approximate TID = 70.  Yet a US dime is smaller in both diameter and thickness than a US nickel.  If only the Cu-Ni (outer) layers of the dime contained the eddy currents then the TID of a clad dime should be lower than that of the nickel, not higher.

Conclusion:  at least some of the eddy currents are in the copper core of US clad coinage and thus we can generalize that eddy currents aren't simply confined to the surface of metallic objects.

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You are correct.  It is a misconception to believe that eddy currents generated by the transmitted magnetic field exist only at the surface of a metallic target.  Eddy currents exist as far as the transmitted magnetic field penetrates the target.  The penetration depth is a function of many factors including the frequency of the transmitted field, the thickness of the target and the target’s magnetic and electrical properties.  So the penetration depth varies significantly depending on the type of metal.  

In the case of the clad coin, the magnetic field easily penetrates past the thin clad layer into the copper “core”.  The larger mass and conductivity of the copper core dominates in terms of signal strength over the thin, lower conductive clad and so the detector primarily “sees” a high conductivity target and registers a target ID appropriate to that higher conductive metal.

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As a Geotech1 regular, we've discussed this complicated issue.

The depth that eddy currents penetrate depends significantly on the frequency you're applying, so higher freqs do indeed 'see the skin', and lower freqs penetrate much deeper.
But the other relevant variable is the target metal: specifically the electrical resistivity ( or conductivity ). Good conductors, like copper, silver, don't support much skin penetration.
And lousy conductors, like most cupro-nickel alloys have much deeper skin depths.

For the mathematically inclined: The skin depth varies with the square-root of the resistivity. So for example a metal that has resistivity 4 times greater than pure copper will have a skin depth 2 times greater than copper.
To use real data, coin cupro-nickel has a resistivity about 20 times that of copper, so skin depth is about 4.5 times greater ( square root of 20 ). Currents flow 4.5 times deeper into the metal.

So your typical machine will see 'all' of a 5 cent cupro-nickel coin. But probably only the outer 0.5mm skin of a silver Morgan Dollar, for example.
And the cladding on a clad 25c coin contributes little to the overall response.

And, yes, magnetic characteristics are important, too. Iron/steel/pure nickel are ferromagnetic, and really don't support deep currents at all, and it gets worse as test frequency increases. This is why high freq machines like the 100 kHz Compass models ( Yukon, 66 ? ) can 'see through' small iron. At 100k, they only see the tiniest skin of a nail, which makes the nail very 'small'. - meaning the nail doesn't give much signal itself, plus it doesn't distort the magnetic field much, so there's less masking, and other targets become visible.

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