Although the Faraday shield will be discussed at greater length at a later stage, it may be noted here that the shield is always an incomplete 'turn' of a conductor - usually aluminium foil - and that if the turn is ever completed, the oscillation ceases. This is simply explained in terms of the Eddy current that is then induced into the short circuit shield - since the field of that current then opposes the original oscillation, the circuit simply is damped so severely that it will not work.
Eddy Currents and their effects on practical metal detection
From the formulae on the first page, you will see that:
m = B / H
and so, B, the total induction = m H
Now where H, the intensity of the magnetising field, remains constant - as in the case of the metal locator circuit - the total inductance now depends on the permeability. So high m materials (ferromagnetic) may be expected to increase B.
It can be shown that B is directly proportional to L - the inductance.
Thus we have a direct correlation between materials, and their expected effect on a tuned circuit since the reactance of L,
XL = 2 p f L, where f = frequency
For the BFO, or single search oscillator system, where the basis of operation is the change in frequency of an RF oscillator, the frequency is set by
fo = 1 / 2 p Ö (L C)
(The basic resonant circuit formula, where the capacitive and inductive reactances are shown to be equal).
Thus any alteration to L is reflected in the change of frequency, and increased L, brought about by a high m material present in the field, thus causes a lowering of the frequency. i.e. L is inversely proportional to fo.
However, a metal Detector will not always behave in such a predictable manner, and anyone who has actually used one will discover that a lump of iron, may give a similar reaction to a lump of brass. Why ?
The operative word here is lump, for as we have seen with the motor armature, the non-laminar solid armature suffered from induced eddy currents that tended to oppose the applied forces. In other words, the conductor exhibited a field that opposed the original field applied. So with lumps of iron in the range of the metal locator
Furthermore, since eddy currents are directly proportional to the frequency of the field change that produces them, the high frequency fields of circa 100kHz that are present with most metal locators are strongly opposed by the induced eddy currents, thus in fact lowering the total induction - hence lowering L, and according to the formula for resonance, this increases fo, the frequency of the search oscillator system.
So whilst iron may be high m , this is over ridden by the effect of the eddy current, and the net result is an appearance that the material is in fact non-ferrous. Materials used in high frequency coil construction are ferrites, which are iron oxides, where the particles are individually isolated (electrically) from each other, thus making the m effect prevail over the eddy current effect, since no circuit exists for the eddy current to assume significant proportions.
At higher frequencies, the effects of RF eddy currents can be quite surprising. An apparently 'earthed' metal chassis on a powerful HF transmitter can give a substantial RF burn - and when the realms of microwaves are reached, the term 'earth' gets to be very meaningless since a tuned circuit may only be a few millimetres.
The problem of eddy current considerations is the nebulous nature of the effect. i.e. It cannot be readily quantified, that is, expressed mathematically and thus made predictable within given parameters. Further reading will be found in the pages of Physics text books, especially those which concentrate on the properties of conductive materials.
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