Eddy Currents (Very Technical)

[Anonymous]

Consider the spin of electrons around the atoms in a piece of iron. The electrons are not aligned and there are some that are free to migrate in the iron. When an electromagnetic force is applied to the iron the electrons start to spin in the same direction, sometimes called magnetic domain alignment, and there are billions available. Permeability of metal is compared to that of air which has a permeability of 1.
Assume a piece of iron that is not magnetized has a permeability of 5000 in reference to the permeability of air which is 1. If an electromagnetic filed is applied to the iron and either the iron or field is in motion then force magnetic force is applied to the iron. This force will cause the electrons to spin in the direction of the lines of force in the electromagnetic field. When the electrons start to spin in the same direction the permeability begins to decrease from 5000 towards 1. More electrons spinning in the same direction mean there are fewer electrons to cause to spin and in the direction of alignment. The fewer electrons there are to align the closer we get to 1 so the permeability constantly decreasing as the spins align.
The electromagnetic force also causes electrons to move from the outer shell of an atom to the outer shell of another atom. The movement of electrons is what we call EDDY CURRENT and is very shallow or at the surface of the metal. Different frequencies of the electromagnetic field from the coil cause the eddy current to flow at deeper depth in the iron. These eddy currents flow in a circle around a piece of iron. The flow is around the axis on a nail shaped piece of iron. However this is sill quite small as far as dept of eddy current flow is concerned. We have tow primary consideration and they are the direction of spin of the electrons and the flow of electrons from atom to atom. If we reverse the electromagnetic field of force the iron has a permeability of 5000 again so the spin of electrons are reversed and the process is repeated but in the reverse direction. The process is repeated as long as the iron is in the field. What this means is the permeability of iron changes and is nonlinear.
The spin of the electrons and direction of eddy current depends on the rise and fall of the amplitude of the electromagnetic field that is the force to cause alignment and also to cause eddy current flow. This process causes self-inductance and that is the eddy current causes the iron to have its own electromagnetic field. This electromagnetic filed will be such that any decrease in the force of the field applied to the iron by the TX coils electromagnetic field will be apposed. The self-induced field will attempt to keep sustain eddy current flow and keep the electrons aligned or the iron magnetized.
Iron has a permeability that is nonlinear and so the rise and fall of the eddy current induced is nonlinear. It is the decay of eddy current that is of interest in the identification of a metal. The time of this decay is called a time constant. Metals will have a long time constant, medium time constant, sort time constant and is used to id metals. Iron has a long time constant and silver has a short one. The TC is on the order of microseconds. The explorer averages these time constants and compares the TC of a target to those stored in a lookup table for different metals. We then hear or see an indication of the predicted metal identification.
HH, Cody

 

[Anonymous]

Cody;
I would be interested to know the source the for theory of magnetism that states "<span style="background-color:#ffff00;">When an electromagnetic force is applied to the iron the electrons start to spin in the same direction, sometimes called magnetic domain alignment</span>"
The physics book that was used when I went to college, "Electricity and Magnetism" (published by Addison-Wesley Publishing Co. Inc. in 1954) by MIT professor Francis Weston Sears, states that:
"..ferromagnetic materials are composed of microscopic size but large enough to contain from 10^12 to 10^15 atoms. Within each domain the magnetic moments of all the spinning electrons are parallel to one another. In other words each domain is maganetized to saturation." The theory also goes on to say that in unmagnetized iron the domains are randomly organized, but in magnetized iron the domains are forced into some increasing degree of alignment depending of the strength of the applied magnetic field.
In the above theory it does not state that the electron spin of any atom is changed as a result of an applied magnetic field. It only states that the individual magnetic domains are brought into alignment with the applied magnetic field.
If there is a newer theory of magnetic domains, then I would be interested to read about that new theory.
HH,
Glenn

 

[Anonymous]

Glenn, the older theory was as you describe and one that I taught many years ago since it really is not that important one way or the other.
<span style="background-color:#ffff00;">"Modern theories of magnetism maintain that a magnetic field is produced by an electric charge in motion, and thus it is theorized that the magnetic field of a so-called "permanent" magnets such as lodestone is the result of electrons within the atoms of iron spinning uniformly in the same direction. Whether or not the electrons in a material's atoms are subject to this kind of uniform spinning is dictated by the atomic structure of the material (not unlike how electrical conductivity is dictated by the electron binding in a material's atoms). Thus, only certain types of substances react with magnetic fields, and even fewer have the ability to permanently sustain a magnetic field.
Iron is one of those types of substances that readily magnetizes. If a piece of iron is brought near a permanent magnet, the electrons within the atoms in the iron orient their spins to match the magnetic field force produced by the permanent magnet, and the iron becomes "magnetized." The iron will magnetize in such a way as to incorporate the magnetic flux lines into its shape, which attracts it toward the permanent magnet, no matter which pole of the permanent magnet is offered to the iron:"</span>
I took the above from this site instead of digging out the books that are stored way in the attic but have thaught this for years. I often simply talk of magnetic domains since I understand that is a common theory but think it is lacking in that is does not relate were well to electrons at the atomic level when it comes to induced eddy current flow. I just avoid the debate of a theory most of the time by simply stating the old theory and say magnetic domains. However, since I wanted to mention eddy current flow I used the modern theory.
<STRONG>
http://www.opamp-electronics.com/tutorials/dc_theory_ch_014.htm </STRONG>
HH, Cody

 

[Anonymous]

Cody,
Thanks for the link. I find the presentation interesting and well done. I admit that I have not yet studied it in great detail.
My take on what the auther is saying is perhaps a little bit different from what I understood you to say. My take is that the electron speed and direction (either clockwise or counter-clockwise depending upon position of the observer) remains the same relative to the other electron orbits of the atom. For example, consider a single isolated atom. It is possible that the net effect of all of the electorns of the atom in motion will generate a net magnetic field in the space surrounding the atom. Now when two atoms are brought very near one another. If the both atoms orient themselves properly, then the net magnetic field is strengthened.
If the above supposition is true, then the presence of an externally forced magnetic field, could be the motivation to induce the two atoms (or any number of atoms) to orient themselves so that all of the atom magnetic fields are put into a high degree of alignment. Then when the external magnetic field is removed, the orientatin of the atoms will remain about the same as before the external field was removed.
What do you think?
HH,
Glenn

 

[Anonymous]

You guys are talking about theory, and as I understand it, theory is just that, and unproven, so its a no win discusion, I would say. You both may be wrong, or one may be right, as what is a theory today will probably change next week. Well, maybe not that soon, but change, untill someone comes up with the truth about the matter.

 

[Anonymous]

Charles,
I appreciate your remarks and I am sorry if I have upset you with my post. Neither Cody nor I are trying to "win". We are sharing our thoughts in an attempt to broaden our understanding of the principle of metal detector behavior. The objective of the classroom is to learn.
Much of learning comes from understanding the current theory. I also realize that understanding the theory is not required to be a successful detectorist. Having said that, there is a significant number of people that are fascinated by "how things work".
HH,
Glenn

 

[Anonymous]

I understand what you are saying and thought I was saying that including the spin of the electrons. <span style="background-color:#ffff00;">The orientation of the atoms are critical as you point out and I do appreciate your clarifications of the original post.</span> I think this helps all of us to see this a little better. When anyone looks at my post and comments and point anything out to me then I go back and look at the post, patents, and if necessary look at the material I have collected over the years. If wrong then I am better off to know it and correct my thinking.
<span style="background-color:#ffff00;">I appreciate participants taking the time to read my posts. I believe I indicated I have been working on a book for several years about detectors to include basic and advanced theory and application. I constantly make corrections to better explain and correct any errous I find in my theory and and understanding plus learning how to use a use detector to be more effective in finding metal objects.</span>What I prefer not to do is to get a book out there and then find that is is full poor illustrations and awful explanations.
You know Glenn, I will be 64 in March and there are no mountains left that I feel I need to climb. I may just go lay in a chair if I feel like it and watch FOX. Ha ha It is tough but someone has to do it. How well I recall those 46 years of going to work.
Have a great evening,
HH, Cody

 

[Anonymous]

Well said and very true. I hope all participants do the same and understand this. You know I appreciate your input and that of others very much as I just posted above.
HH, Cody

 

[Anonymous]

You didn't upset me, I was just saying nether of you can really prove a point, when it is just theory. It is just what is thought at the time, but later it may be proved wrong. If you can prove it beyond all question, if it could be wrong, then it is no longer theory, it is truth, and a fact. But you are both saying what you are saying is a theory, is all I was saying. But I enjoy reading them, so keep them coming, even if it is just theory, it at least makes something for us to think about. And you are just a spring chicken Cody, I'm 71.

 

[Anonymous]

Charles,
I am not quite a "spring chicken". I am 67 and have been retired from the design engineering field for a couple of years.
You thoughts about theory are correct. A theory is an attemp to formulate an idea that explains why things behave the way they do. With that theory you can predict what will happen under certain circumstances. After much experimentation and verification of a theory, then it becomes what is called a "law", for example the law of gravity. Even after it becomes a law, it has only moved to the next level and can still be proven invalid.
In this particular case (of the principles of magnetism) I was referring to the theory that is more than 50 years old. I understand a theory for what it is and am continually looking for progress in the field of science. Cody acquainted me with an update to the theory that seems to make more sense to me. I am grateful to Cody for that.
HH,
Glenn

 

[Anonymous]

Way back when I was in school, they told me I should to into engineering, that I had the aptitude and brain for it. But I said to myself, I don't want to go to that much more school. By the time I had learned how little I really knew, it was kind of late to go back. So I guess you and Cody have gone to where I wish I had of. But I enjoy both of your posts, and hope you keep them up, I learn from them as I'm sure a lot of others do also, even if they may not post and say so.

 

[Anonymous]

[No message]

 

[Anonymous]

Add me to the thanks and I enjoy your posts very much. Glenn and I understand the other to some extent due to working with others that think like we do. As he said the interest in in the answer and not who has it which is when I really enjoy a discusson.
It can get fun with theory and very interesting if one likes those kind of discussions and or debates.
Have you thought of going to university or college and taking some courses you might enjoy. I had a lot of students do that and really enjoyned them. We have continuing education at the university where I taught and the campus is only a little over a mile from the house. Anyone can take the classes and regular classes can be taken for no credit so no testing is required. A person takes the courses for his or her own interests. There is a very large number of students that go only in the evening so night classes are nice.
Have a great one,
Cody

 

[Anonymous]

That all sounds good Cody, except my wifes health is very bad, and after 35 years together I couldn't leave her alone to go to school, even if I would enjoy it, I enjoy her more. I just live about 6 or 7 miles from Ole Miss, and they probably have courses. I used to live in Omaha, and that was when I guess I really could have gone, but things change.

 

[Anonymous]

We hunted the same parks when I lived in Bellevue. I found thousands of coins in those parks back in the early seventies as I mentioned to you before. I almost took a job with a larget company that did a lot of contract work for the military to just stay there for the coin hunting. My wife cried me back to Oklahoma.
I understand the probelems with getting out. I moved my 86 year old mom to our home when she started to have mini strokes, TIAs, so my wife or I have to be with her 24/7. That really cut back on my hunting as I feel I cannot ask my wife to do as much as I do since it is my mom. I also want to be with her as much as I can so retired from my position at the unviersity to do so. I had my wife retire from her teaching position. She taught Science to 6th grade kids. I have diabetes from Agent Orange during the Tet in Vietnam so have VA disability from that and have to watch the glucose. It is a real bother on very hot or cold days or if you let it drop or get higher than it should be.
It gets interesting!
Have a good one,
Cody