Theories about coil-shielding

[kt315]

When two objects in each other's vicinity have different electrical charges, an electrostatic field exists between them. An electrostatic field also forms around any single object that is electrically charged with respect to its environment. An object is negatively charged (-) if it has an excess of electrons relative to its surroundings. An object is positively charged (+) if it is deficient in electrons with respect to its surroundings.

Electrostatic fields bear some similarity to magnetic fields. Objects attract if their charges are of opposite polarity (+/-);objects repel if their charges are of the same polarity (+/+ or -/-). The lines of electrostatic flux in the vicinity of a pair of oppositely charged objects are similar to lines of magnetic flux between and around a pair of opposite magnetic poles. In other ways, electrostatic and magnetic fields differ. Electrostatic fields are blocked by metallic objects, while magnetic fields can pass through most (but not all) metals. Electrostatic fields arise from a potential difference or voltage gradient, and can exist when charge carriers, such as electrons, are stationary (hence the "static"in "electrostatic"). Magnetic fields arise from the movement of charge carriers, that is, from the flow of current.When charge carriers are accelerated (as opposed to moving at constant velocity), a fluctuating magnetic field is produced. This gives rise to a fluctuating electric field, which in turn produces another varying magnetic field. The result is a "leapfrog" effect, in which both fields can propagate over vast distances through space. Such a synergistic field is known as an electromagnetic field, and is the phenomenon that makes wireless communications, broadcasting, and control systems possible.

[kt315]

@Moderator:
who is that man to express freely not requested judgments on my school preparation?
---
yup. what technical university did you end? name and place.

[kt315]

Eddy current appear in the shield, which is in changing magnetic field. That is fact, given by the physics. Can you explain where is that nonsense?

i ask you AGAIN. why i must think about Eddy currents in my coil graphite layer?? FOR WHAT? is that give me the knowledge about MORE DEPTH and MORE GREAT SENS TO THE GOLD?

FOR WHAT?????? what RELATION has Eddy currents to shield of coil protecting from ElectroStatic Field Influence (ESFI)???

Eddy currents appear IN TARGET and JUST ONLY in target.

[kt315]

Maybe I did not express it clearly, sorry but English is not my main language.
When I spoke of a shield, I was referring to the coil shield. As you know, the variation of the EM field induces the Eddy current inside the conductors. This is what happens inside the coil shield. When I wrote: "The screen material should not interfere with the EM field" II meant to say that it's important to choose the right material to minimize the Eddy current inside the coil screen. There is no nonsense in what I said!
--
ok. you CHANGED your opinion. but changed after my replica.

[Altair]

Eddy current flows into the shield - nonsence. should not interfere with EM field - again nonsence. you need be back in to the school on 5th stage and learn
physics again FROM ZERO.

GUYS DO NOT SPREAD OUT THE NONSENCE.

Maybe I did not express it clearly, sorry but English is not my main language.
When I spoke of a shield, I was referring to the coil shield. As you know, the variation of the EM field induces the Eddy current inside the conductors. This is what happens inside the coil shield. When I wrote: "The screen material should not interfere with the EM field" II meant to say that it's important to choose the right material to minimize the Eddy current inside the coil screen. There is no nonsense in what I said!

Now, regarding coil shield material, tell me for what physical basis you said: "you have to use NOT good conductor but BAD conductor", Please.

Certainly having a low conductivity conductor means, with the same difference in potential applied, a low current through it; but if you think that the shielding of the coil is like a capacitor armature (the other is the earth), the low conductivity does not seem the best choice. I think there is a sort of compromise to have the best coil shield solution, that depends mainly on:
- Material;
- thickness
- form and execution;
- resistivity;

@Moderator:
who is that man to express freely not requested judgments on my school preparation?

[WM6]

If eddy currents appear in the shield, then shields should be detected (as targets).

Practically it depends on sort of shield and frequency of changing EM fields.

Par example: if you use as shield of PI coil kitchen Aluminum foil, and take very early sample, such one shield can be detected.

On other side, graphite shield are pretty immune to common used frequencies in VLF metal detecting or early PI sampling,
so we can say that there is no eddy currents in graphite shields. Graphite shields with proper taken conductance is not detectable.

You can take analogy from transformer core, between laminated Fe cores and ferrite cores and their implication to dissipative eddy
currents in dependence to transformer frequency used.

[Hyena]

for what you wrote nonsence??? what importance that has for/to THE SHIELDING??? WHERE the currents appear up?

Eddy current appear in the shield, which is in changing magnetic field. That is fact, given by the physics. Can you explain where is that nonsense?

[kt315]

Eddy current does not flow in shield? Prove or explain it

Eddy currents (also called Foucault currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor due to Faraday's law of induction. Except word "into", he was correct.

for what you wrote nonsence??? what importance that has for/to THE SHIELDING??? WHERE the currents appear up?

[Hyena]

Eddy current flows into the shield - nonsence. should not interfere with EM field - again nonsence. you need be back in to the school on 5th stage and learn
physics again FROM ZERO.

GUYS DO NOT SPREAD OUT THE NONSENCE.

Eddy current does not flow in shield? Prove or explain it

Eddy currents (also called Foucault currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor due to Faraday's law of induction. Except word "into", he was correct.

[kt315]

It could be but I think we need to avoid Eddy current flows into the shield; this is the main goal. Shield material should not interfere with the EM field. It's make like a capacitor armatures; that are made always of a good conductor!

Eddy current flows into the shield - nonsence. should not interfere with EM field - again nonsence. you need be back in to the school on 5th stage and learn
physics again FROM ZERO.

GUYS DO NOT SPREAD OUT THE NONSENCE.

[Koala]

Ah, so as to prevent static charge from building up in the first place, and only secondarily as a means of discharging static. A poor conductor implies resistance.
But now who can say what is ideal resistive properties of the graphite shielding. How many ohms per such an such. And how long(or short) drain wire make contact with the graphite shielding, longer drain wire contact make overall less resistance of the shielding.. And what about the gap? Is the shielding material frequency dependant(the limits to how much resistance per square area.
Seems like no exact science.
Conclusion: buy off the shelf coil and make life easier. Keep workshop clean.
"Excessive exposure to graphite dust over extended periods of time can cause a chronic and more serious condition known as Graphitosis, which is a form of pneumoconiosis. This condition arises when inhaled particles of graphite are retained in the lungs and bronchi"

One of the problems is we haven't found a reliable way to test the shield. I have charged up a Styrofoam cup and fetched it up to the coil nothing. Test fine on my lawn. Epoxed everything up and it fails on a field.

Never did find out why different grounds have different result. For example. My lawn with wet grass isn't as bad as uneven ground with wet grass. Dry sand and you can almost detect without a shield.

[Altair]

you have to use NOT good conductor but BAD conductor. mylar tape is GOOD conductor. guys study what is elecrostatic charge and discharge BETTER.
you have to have RESISTIVE conductor, 10-100 kOhm on 1 cm of lenght.

It could be but I think we need to avoid Eddy current flows into the shield; this is the main goal. Shield material should not interfere with the EM field. It's make like a capacitor armatures; that are made always of a good conductor!

[Krzychu_]

I present (I do not know if it's right) the illustration of what I wrote earlier, but when it comes to the conductive screen, in two attempts I managed to get a mixture of solvent-diluted wood varnish and powdered graphite, which, after a little careful application and drying, has resistances in depending on the thickness of the row layer 0.5 ... 1.5kOhm between the points of the meter spaced apart by 1 cm. The attempt to mix graphite with vicol wood adhesive, which is a water dispersion of polyacetate resin, ended in failure - the layer after drying almost does not conduct electricity and has a resistance of 20..35MOhm / cm, and polishing does not reduce this result. In the next step I'm going to apply a smooth, even layer with a paint roller and check the effect of detecting it with a metal detector.

[dbanner]

you have to use NOT good conductor but BAD conductor. mylar tape is GOOD conductor. guys study what is elecrostatic charge and discharge BETTER.
you have to have RESISTIVE conductor, 10-100 kOhm on 1 cm of lenght.

Ah, so as to prevent static charge from building up in the first place, and only secondarily as a means of discharging static. A poor conductor implies resistance.
But now who can say what is ideal resistive properties of the graphite shielding. How many ohms per such an such. And how long(or short) drain wire make contact with the graphite shielding, longer drain wire contact make overall less resistance of the shielding.. And what about the gap? Is the shielding material frequency dependant(the limits to how much resistance per square area.
Seems like no exact science.
Conclusion: buy off the shelf coil and make life easier. Keep workshop clean.
"Excessive exposure to graphite dust over extended periods of time can cause a chronic and more serious condition known as Graphitosis, which is a form of pneumoconiosis. This condition arises when inhaled particles of graphite are retained in the lungs and bronchi"

[Koala]

Mylar tape coming from cable shield works great for me.

Check my posts in the IDX thread. Foil over or under drain wire connected in the search head or separate wires and only grounded on the PCB its was never 100%
Even then it deteriorated over about a year
Tried many times.