Why some objects are difficult to find.

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Filed under: Articles, Machines — Norfolk Wolf

This was put up on a forum that I’m on as a technical advisor, I thought it would be useful for others to see it on here, (well I wrote the answer!).

One of the most common medieval items found in my neck of the woods by archaeologists is earrings. They are made from silver wire, usually about 2mm thick, and turned back a little at the end to make an unclosed loop, which just slips onto the earlobe. As you can imagine they must have got lost pretty often. So why is it that I have never found any?

Well I found the answer this week. When detecting a medieval church site, I saw an earring on the surface. I ran the coil over it before I picked it up and got nothing! I had to run the coil over it at less than two inches before it gave a broken signal. I thought maybe the machine had malfunctioned so I ran a tiny buckle under the coil and the machine blasted out at 6 inch and more.

So today I did a test. I made unclosed loops of silver, aluminium, and copper, then ran the Goldmaxx Mk I and Mk II v4 over them in various settings. I could only pick them up at an inch. Obviously there is something about the dynamics of an unclosed loop that causes problems for detectors. Unfortunately, I am unable to test with other detectors.

Nigel wrote “John will know the reason I’m sure but it’s probably to do with magnetic fields of flux being easier to generate when a circular object is picked up, break the circuit and it dramatically reduces in range.”
Here was my reply.

Sorry to pull you up Nigel, it is the detector that generates the lines of flux in the magnetic field, the object induces eddy currents.

A few years back I was playing around with some washers that had fooled me out in the field, I was trying trying everything to find a way of sussing them out and eventually ended up by hacksawing through one side of of one of them. Bingo, no more being fooled. Unfortunately you don’t normally find washers with splits in them! However, this led me on to more thinking. Why would a flat circular object give a good signal and yet one with a break in it give a lousy one? This is all down to the laws of physics, (the chapter that covers it in my book “Advanced Detecting” is the hardest to understand and was a blinkin’ site harder to write down!)

The Conductivity of the object induces the flow of eddy currents, (low conductivity, low eddy currents). Then you have to take into account: -

The Resistivity of the object, (resists the eddy currents) this is the reciprocal of conductivity and then you also have to take into account the Impedance (impedes the eddy current) of the object.
Another factor is Inductance Permeability (how easily the object can be magnetised by the eddy currents).
Still with me? Now we go on to the depth of penetration of the object by the eddy currents.:-
It Decreases with an Increase in Frequency. It Decreases with an increase in Conductivity. It Decreases with an increase of Permeability. Don’t it just blow yer socks off just readin’ it? An’ I ain’t finished yet!

Because the electro-magnetic field produced by the detector is A C (alternating current) it generates something called Skin Effect. This was discovered by Lord Kelvin back in 1887 and I quote, “This is the tendency of an alternating current to distribute itself within a conductor so that the current density near the surface of the conductor is greater than at its core. It causes the effective resistance of the conductor to increase with the frequency of the current”.

Okay, now let’s put that into layman’s terms regarding the earring. They are made of silver, = high conductivity= decrease in depth of penetration. (Eddy currents) On the plus side it has a lot lower resistivity than most other metals and is also non magnetic.
The problem being is that they are very thin and irregular in shape; once again you just can’t get enough depth of penetration. If as Nigel quite rightly points out, it was a complete circle, the signal given off would be manyfold better. (Eddy currents love this shape.)

So would a different frequency help? A lower frequency (say below 7.5 KHz) wouldn’t be able to do the biz, as these perform better on thicker sectioned objects. A higher frequency than 18 KHz would also give diminished returns “It Decreases with an Increase in Frequency.”

You just have to accept the fact that some objects because of their shape and composition will be very hard to detect. Notice I haven’t mentioned anything about the mineralisation or ground conditions where these were found.

These conditions don’t go away just because you have used the ground reject or discrimination to quieten them down, ultimately these will have the greatest effect on whether your detector gives you enough of a recognisable signal to induce you to dig as you have found out in doing those air tests. It’s difficult to explain some of this stuff in layman’s terms, but I hope it has been understandable, John