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The strength of a magnetic field is proportional to current and decreases with distance based on the permeability of the medium it travels through. For small electromagnets, it seems it's obvious you want to minimize resistance to maximize current. But as an electromagnet gets larger, outer layers have a larger distance to travel through. At some point this will dominate. why aren't inner layers of very large electromagnets made of Iron, which will decrease conductivity a little bit, but increase permeability?

To be clear, I'm not talking about adding an iron core. I mean make the wires out of iron.

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Iron is not a very good conductor of electricity so such design would increase the power wasted in generating the magnetic field. Not to mention that in solenoidal type designs, the magnetic flux is not through the wires but through the coil's center, where it makes more sense to put the iron.

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  • $\begingroup$ I understand that Iron has higher resistance and that it wouldn't make sense if you've only got a few layers of wire, which is how most solenoids are designed. But isn't there some point when permeability dominates over resistivity? like, suppose your coils are several feet thick. $\endgroup$
    – Joseph Summerhays
    Commented Mar 15 at 0:16
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    $\begingroup$ It might happen, but those are cases where the electromagnet was already failing to do its job in the first place. If your coils are thick enough such that permeability is starting to matter a lot, changing to iron is going to raise the resistance by such a large factor that you'll have a very hard time driving the current high enough to generate a good magnetic field. $\endgroup$
    – Cort Ammon
    Commented Mar 15 at 4:31
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    $\begingroup$ @JosephSummerhays The permeability of iron as an external layer on an iron core is apt to reduce the field intensity in the center of the coil; it might spread the field, reducing the intensity, but that is rarely of interest. Iron pole pieces typically concentrate the field in a relatively small region instead of spreading broadly. $\endgroup$
    – Whit3rd
    Commented Mar 15 at 20:01
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This question was based on a misunderstanding of what permeability means. You can see my misunderstanding and it's correction here. In short, the permeability of a material has nothing to do with how the B-field "passes through" it. It only has to do with what the B field is inside the material. having inner coils be made of iron will not let the magnetic field "pass through" any better, because the H field is unaffected.

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