Target geometry and response

[Anonymous]

I have read accounts on the Australian "Finders" site of both a gold specimen (quartz & gold), and a small piece of twisted wire, that could not be detected with SD/GP detectors. Apparently, the fellow with the specimen regularly uses it to win beer from other SD/GP users (who after seeing it are certain they will be able to detect it).
Apparently, the specimen has a small, flat surface (I think it was several millimeters across with a coating of gold). I can rationalise the lack of detector response if perhaps the gold on the surface is not continuous. That is, the surface is actually comprised of many small, discrete gold particles that give the illusion of a surface. However, the small piece of twisted wire has me stumped. Any ideas?
Lachlan

 

[Anonymous]

Hello Lachlan,
The twisted coil of wire is acting like an inductor. The current induced from the detector into this twisted wire coil would have alternating polarity eddy currents which end up cancelling the energy out as th epolarity of the current would be opposite on each of the twists.
I am pretty sure this is why it is difficult to be seen by a detector.
I am sure Eric is more well versed in understanding and definately in explaining this phenonomon(sp?)
I have seen first hand where a spring can be difficult to detect.
Good trick
Tony

 

[Anonymous]

Hi Lachlan,
You are about right as to the gold. I made a boat towed underwater coil once, and to make it heavy so that it stayed on the bottom, I mixed lead shot with the potting resin. I could have as much lead as I wanted and it was not detectable. Same with the gold, you can have a thousand small pieces whose individual signal decays are too fast to detect, and there is still no signal. Even if the particles are touching one another or have thin whiskers to another particle, it doesn't help much. Try the same test with a bag of 5mm stainless steel washers and win some beer yourself.
As to the twisted wire. Do you mean twisted as in a piece of lamp flex, rather than a coil? Any piece of wire is generally undetectable unless you join the ends to form a ring. The eddy currents induced by the transmitter need a circuit to flow round. You can, of course, detect a straight piece of wire if the cross section is great enough to support detectable eddy currents.
Eric.

 

[Anonymous]

Similar to what is inside a retractable ballpint pen, but on a larger scale. The coil is about the size of a US Quarter in diameter and maybe 3 inches long that I have seen used to demonstrate its inability to be detected.
Tony

 

[Anonymous]

I have no problem finding small springs with my Aquastar. I could see that you would not detect them with a PI if the springs are made out of a good grade of stainless steel.
Terry in Hawaii

 

[Anonymous]

Eric and Tony, thank you both for your responses.
Eric, as I understand it, the twisted wire is similar to the short section of intertwisted wire that can be found on a fishing swivel (only without the loop at the end). So, yes, twisted as in a piece of lamp flex. Sorry for not being clearer on that initially.
Thanks for the tip on the bag washers. I'll never go thirsty again!
Cheers
Lachlan