Halo Effect - WTF !!!!

[simonbaker]

I agree with you that the magnetic fields created by the galvanic currents are static..... until the target and its field are zapped by the dynamic field of the metal detector. I am not sure what happens then.

Does something similar happen to a battery when you apply external power to recharge it?

Is it possible that this "recharge" takes a few micro seconds to decay and can thus be sensed?

Tinkerer

Magnetic fields don't interact with other magnetic fields per se, so zapping a field with another field should do nothing. Charging a battery with a field... ok, could have a physical basis. I would think the actual effect is that you would be inducing some current in a conductive circuit that flows in and out of a battery (batteries are usually like short circuits to AC), so it is the presence of the conductive path that really is being detected.

If the battery effect manages to set up conductive paths in the ground that weren't there before, then I see a real "aura" being created. But I believe it is the conductive path that your metal detector is detecting (by inducing some current at the TX frequency), not any DC current flowing from a battery effect.

To test if your metal detector can detect stationary magnetic fields, raise it well above the ground and swing it through the earth's magnetic field. If it doesn't detect it, then I think there is almost no chance it could detect any feeble mag field from tiny galvanic currents -- I can't work out the numbers, but that is my guess.

But physics/chemistry can be subtle and fool us, so I'm open to surprises and won't close my mind to any possibilities.

[Tinkerer]

Do magnetic fields interact?

Magnetic fields don't interact with other magnetic fields per se, so zapping a field with another field should do nothing. Charging a battery with a field... ok, could have a physical basis. I would think the actual effect is that you would be inducing some current in a conductive circuit that flows in and out of a battery (batteries are usually like short circuits to AC), so it is the presence of the conductive path that really is being detected.

If the battery effect manages to set up conductive paths in the ground that weren't there before, then I see a real "aura" being created. But I believe it is the conductive path that your metal detector is detecting (by inducing some current at the TX frequency), not any DC current flowing from a battery effect.

To test if your metal detector can detect stationary magnetic fields, raise it well above the ground and swing it through the earth's magnetic field. If it doesn't detect it, then I think there is almost no chance it could detect any feeble mag field from tiny galvanic currents -- I can't work out the numbers, but that is my guess.

But physics/chemistry can be subtle and fool us, so I'm open to surprises and won't close my mind to any possibilities.

Simonbaker,

Do magnetic fields interact? OK, maybe I have been using the wrong word. What I mean by interaction, is that a magnetic field can deflect or distort another magnetic field. If you take 2 magnets and force them together with the same poles, the fields of both magnets are pushed aside, forced into a different shape.

If you look at my Avatar picture, you see 2 distorted magnetic fields. The picture is a computer simulation of 2 coils with the current running in opposite directions.
You can see that the resulting field is totally asymmetrical. this is what I mean by the fields interacting. There is probably a better word for it.

Tinkerer

[simonbaker]

Simonbaker,

Do magnetic fields interact? OK, maybe I have been using the wrong word. What I mean by interaction, is that a magnetic field can deflect or distort another magnetic field. If you take 2 magnets and force them together with the same poles, the fields of both magnets are pushed aside, forced into a different shape.

If you look at my Avatar picture, you see 2 distorted magnetic fields. The picture is a computer simulation of 2 coils with the current running in opposite directions.
You can see that the resulting field is totally asymmetrical. this is what I mean by the fields interacting. There is probably a better word for it.

Tinkerer

(by the way I'm not arguing in any way, it's more like thinking out loud as we speculate about possible ways auras could happen)

Right, for sure, the two fields will combine, but the key point I was making is that they are not "aware" of each other. In other words, they don't bounce off each other or modulate each other in any way. It may look like they distort each other, but they really just superimpose. Now it is possible your detector is sensitive to a magnetic field other than the one it is transmitting, but it would probably be sensitive whether you are transmitting a field or not, in other words, if your receiver picks it up when you are transmitting, it would also pick it up if you are not transmitting*.

* Now I need to disclaim that. Most metal detectors use a piece of the transmit signal as part of detecting a receive signal, but that could be done theoretically without transmitting the magnetic field.

* Also, even though magnetic fields don't interact in space, they could interact in a material if it responds "nonlinearly". Usually to make physical things act nonlinearly takes a lot of "power", but I don't know enough about that, so I'd believe you could get some significant interaction of magnetic fields in some materials maybe. It would probably take some pretty unusual circumstances to be significant.

If we take your avatar for an example - if one of the coils were DC current and the other were 10Khz from your TX coil, I don't think the mag field from the DC current would bother the detector at all. What would bother the detector would be the second coil itself, because it would be like any other conductive object responding to the TX frequency in a big way.

That's how I see it but I'm going on basic school learning and I'm open to all kinds of secondary or hard to imagine effects coming in. I like to theorize about it all so I hope I don't sound argumentative, just throwing in my 2 cents (which is about what I found my first time out).

[Tinkerer]

Do magnetic fields interact?

Simonbaker,

take a look a the MIT WEB PAGE http://web.mit.edu/8.02t/www/802TEAL...tics/index.htm


Tinkerer

[simonbaker]

Simonbaker,

take a look a the MIT WEB PAGE http://web.mit.edu/8.02t/www/802TEAL...tics/index.htm


Tinkerer

Nice visualizations. They talk about the superposition principle which is basically what I was referring to. And then you add in the signal processing stuff inside the metal detector which is another whole ball of twine. But any knowledge of these ideas can help us use our detectors a little better I think. When it comes to building them I'm more of a tinkerer than a theorist! I hope to learn a lot from this forum.