A question for Carl_NC or anyone that might know.

[jmaclen]

I remember reading that gold has a different voltage? than aluminum when excited by an Eddy current. I figured a circuit that could read the value and compare the two. But then that sounds too easy to be possible.

Gold is mostly diamagnetic. Aluminum is mostly paramagnetic. However, density also matters. This may not be directly related, sorry.

 

[Carl-NC]

I remember reading that gold has a different voltage? than aluminum when excited by an Eddy current. I figured a circuit that could read the value and compare the two. But then that sounds too easy to be possible.

No, an EMF (~voltage) is created in the target but you can't measure it because it's infinitesimally distributed throughout the electrons. The EMF depends on the size of the target, and says nothing about its phase.

 

[Diga]

For once, I actually agree with Cherry Picker.

Seriously? Just once?

You've got some catching up to do buddy. I'm already at twice!

 

[Cherry Picker]

So values like these cannot be tested in the ground?

Material	ρ (Ω•m) at 20 °C Resistivity	σ (S/m) at 20 °C Conductivity
Gold	2.44×10−8	4.10×107
Aluminum	2.82×10−8	3.5×107
Calcium	3.36×10−8	2.98×107
Tungsten	5.60×10−8	1.79×107

 

[Diga]

I think those gold and aluminum numbers, are just too darn close for induction balance to separate CP.

 

[Carl-NC]

Resistivity is only one element in a target's response. If you have otherwise identical coins (diameter, thickness) made of gold and aluminum and you know your target is one or the other coin, then you can tell them apart. But when targets are mixed in size, shape, thickness, & alloy then you can't. Any metal can be made to exactly mimic a different metal.

 

[Diga]

Carl,

Is it the differences in domains from aluminum and a gold band, that sometimes can be audibly discerned between gold bands and aluminum trash? If so, does the domain number remain the same if the gold band was lying flat, at 45°, or on edge?

 

[Carl-NC]

Yes, I assume so. Generally band at 45° is just a weaker target, same phase. But a broad flat band can have cross-eddies in addition to the circular eddies, so it can start to look multi-domain as it's tilted.

 

[Rudy]

Is that the same process used by sewage plants?

This is more like what sewage plants use.

Eddy current separator - Wikipedia

en.wikipedia.org

Note that this is much different than "identifying" gold from aluminum or other metals.

 

[Tom_in_CA]

........uniform shaped piece of gold like a gold band........

The moment you add the slightest bit of tilt in, the ground, to a gold ring, is the moment you can add shape-showing good-bye.

And as I said in a prior post, you're not going to get a magical Television like image of coins, rings, foil blobs, etc.... Even if the resolution of the pixels got down to 1/100 of an inch. You would still have nothing more than a blotch of messy pixels.

 

[Tom_in_CA]

No, an EMF (~voltage) is created in the target but you can't measure it because it's infinitesimally distributed throughout the electrons. The EMF depends on the size of the target, and says nothing about its phase.

Resistivity is only one element in a target's response. If you have otherwise identical coins (diameter, thickness) made of gold and aluminum and you know your target is one or the other coin, then you can tell them apart. But when targets are mixed in size, shape, thickness, & alloy then you can't. Any metal can be made to exactly mimic a different metal.

The method described uses an inclined plane with magnets underneath. As metal slides down the incline, the magnets induce eddy currents in the metal which creates a Lorentz force that pushes the metal sideways. Light metal (aluminum) gets pushed farther than heavy metal (gold) thereby separating them. It's a fully mechanical process, not an electronic detection process, and is not something that can be used in metal detecting.

Carl, seems that people are saying, on the one hand, that they're not denying the laws of physics, and that : And they admit that Some things .... yes.... can simply be impossible . Yet they seem to exempt this gold vs aluminum thing. By bringing up links and lists of objections. But as soon as those are "put to bed" and shown to be impossible, it's as if .... we can't go there.


We simply CAN NOT bring ourselves to admit that this might be one of those things that's simply not the same as "something they heard" or "this thing they read".


We want this mousetrap SO BADLY that we apparently put critical thinking aside. Because, shucks, no one wants to be laughed at all the way to the bank. Eh ?


Anyhow, I am very much enjoying your contributions here on this topic. Thanx.

 

[Cherry Picker]

Looks like I stick to plan B for now.

Our city park has more than 130 years of trash, but the vast majority are pull tabs. I've identified 5 different ones and notched them out. There is only a small bit of gum foil trash as well as a small number of canslaw. Over 130 years of use I figure this park is plump with gold because for the last 40 years we've cherry picked silver and deep copper and ignored the gold numbers. My thoughts are I should have a pretty increased odds of gold digging everything that sets my Deus off. Now I just need to get out there and give my idea a test.

 

[Cherry Picker]

Carl, seems that people are saying, on the one hand, that they're not denying the laws of physics, and that : And they admit that Some things .... yes.... can simply be impossible . Yet they seem to exempt this gold vs aluminum thing. By bringing up links and lists of objections. But as soon as those are "put to bed" and shown to be impossible, it's as if .... we can't go there.

It is not a black & white, yes or no, world Tom. I admit there are physics that can't be ignored, such as the earth is round, but people are using Eddy current to determine aluminum from gold period. FACT. The question is, not was, is, can the process be made to function in a metal detector. I haven't put anything to bed, only got an opinion from Carl. Don't pat yourself on the back just yet LOL.

 

[Rudy]

So values like these cannot be tested in the ground?

Material	ρ (Ω•m) at 20 °C Resistivity	σ (S/m) at 20 °C Conductivity
Gold	2.44×10−8	4.10×107
Aluminum	2.82×10−8	3.5×107
Calcium	3.36×10−8	2.98×107
Tungsten	5.60×10−8	1.79×107

No, they can't be tested in situ.

 

[Tom_in_CA]

It is not a black & white, yes or no, world Tom. I admit there are physics that can't be ignored, such as the earth is round, ....

Do you spot the contradiction there ^ ^ ? On the one hand, you start with how we can't say yes or no, black or white, to anything in this world. Eh ? Yet in the very same breath you turn around and list something that we CAN say (drum roll) is : Black and white, yes or no.


The technology you dream of is possible. Westinghouse developed it in the early 1990s for a government contract (explosives detection, or something like that). You just need 10s of millions of dollars, you'll be wearing a lead suit, no one can be within a city block radius of you, and you'll need scores of government permits and clearances.

 

[longbow62]

How are Tone, Numerical I.D., and visual XY screen and or 2d Manticore screen related. Are they all the exact same thing just represented in a different way? Shouldn't the so called tonal purity of some targets also show with a more solid I.D. stability and a very consistent XY/2D screen? How did the White's Signagraph work? Did it do anything the current 2d and XY screens can't?

 

[Diga]

How are Tone, Numerical I.D., and visual XY screen and or 2d Manticore screen related. Are they all the exact same thing just represented in a different way? Shouldn't the so called tonal purity of some targets also show with a more solid I.D. stability and a very consistent XY/2D screen?

If it's true that gold bands have a very subtle tonal difference than most aluminum trash, then why isn't that gold band tone represented visually to the operator to indicate the high likelihood of a gold ring or something shaped very similar? The direct electronic representation would be much more precises than taking that precise signal and pushing it through a less precise electro-mechanical device known as the speaker, where that subtlety can be diminished or completely lost.

 

[Carl-NC]

This is more like what sewage plants use.

Eddy current separator - Wikipedia

en.wikipedia.org

Note that this is much different than "identifying" gold from aluminum or other metals.

This method is also using the Lorentz force to eject metal. It's pretty safe to say, this ain't gonna work with metal detectors.

Carl, seems that people are saying, on the one hand, that they're not denying the laws of physics, and that : And they admit that Some things .... yes.... can simply be impossible . Yet they seem to exempt this gold vs aluminum thing. By bringing up links and lists of objections. But as soon as those are "put to bed" and shown to be impossible, it's as if .... we can't go there.

I for one refuse to say it's impossible, because I don't know. While a Lorentz force approach is likely impossible, there may be something else. I've really never thought much about this, and now maybe I will.

If it's true that gold bands have a very subtle tonal difference than most aluminum trash, then why isn't that gold band tone represented visually to the operator to indicate the high likelihood of a gold ring or something shaped very similar? The direct electronic representation would be much more precises than taking that precise signal and pushing it through a less precise electro-mechanical device known as the speaker, where that subtlety can be diminished or completely lost.

See post #64.

 

[Diga]

"Very subtle" is the key phrase here. The audio takes a trained ear to hear the difference (I'm sure I can't do it) while the signals in the circuitry don't look different enough to act on. Also, it's a continuum, not a black&white difference. A simple gold band is single domain, a ring with a bunch of mounted diamonds is not.

Carl,

Thank you again for replying. I love this stuff!

1) When you said you're sure you couldn't hear the audio difference, then how do you know there's an audio difference?

2) Using an oscilloscope or some other instrument, you said that you can see the difference between single and multiple domains, but it's not different enough to act on. Given that the instrument recognizes the difference, then why can't the electronics in a metal detector (possibly combined with an algorithm) display the single domain of objects like a gold ring ? OR, are you saying that is possible to show, but the results would be too inaccurate to be of any value?

 

[Carl-NC]

1) It's all an assumption, based on the fact that quite a few people claim to hear slight differences, combined with the knowledge that single and multiple domain targets will theoretically generate subtle differences. I can't prove it.

2) It's like the audio, it might be possible to see the differences in an oscope response, but would be difficult to actually extract that difference in a meaningful way. I'm on the road, when I get back home I intend to look at this a bit closer, so for now I will say "I'm not sure."