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I recently came across this paper detailing a plan to use 12 superconducting rings around earth to strengthen our magnetic field. I'm planning on using this same idea for my own worldbuilding.

An advanced civilization is terraforming an Earth sized planet that has no natural magnetosphere. They use 12 superconducting rings to create an artificial one. The NIFS paper I linked to gives me a lot of good information that I need to use this idea, but one part of the paper, on page 10, in particular confused me. It says:

The finite magnetic field generated by a 6.4 MA superconducting ring would necessitate a 2.6 km safety zone adjacent to the cable to assure that the public exposure limit of 5 G is not exceeded.

I'm no physicist, but from what I've gathered G stands for gravitational constant. If I'm not interpreting that incorrectly, then my question is what is the relationship between powerful magnet fields and gravitational constant, and why is there a public exposure limit of 5 G?

Or, more concisely, what effects would a powerful superconducting ring have on a nearby human?

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    $\begingroup$ Hi, Welcome to Worldbuilding! As you're essentially asking about a physics report, did you try asking this on Physics Stack Exchange? It might be more suited to answering this question than here, so you may be able to get better answers there (assuming that it's not off-topic on physics) $\endgroup$ – Mithrandir24601 Jun 1 '17 at 22:43
  • $\begingroup$ Thank you for the advise! Like I said, I'm no physicist so I'm approaching all this from a fiction writing perspective, so learning how to best gather information is very helpful! $\endgroup$ – NathanR Jun 2 '17 at 2:22
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    $\begingroup$ Note that a 5 G (gauss) magnetic field is not particularly strong -- it's actually pretty weak, about ten times stronger than the Earth's magnetic field on the surface, and in practice observable only with a magnetic compass. I don't know where that regulatory limit comes from but it certainly is not linked to any biological effect on the human body. An ordinary refrigerator magnet (the kind you stick to the refrigerator door to hold a postcard) produces (near its surface) a field at least ten times stronger, and it doesn't come with dire warnings. $\endgroup$ – AlexP Jun 2 '17 at 15:11
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I'm reasonably sure that the $5\text{ G}$ there refers to $5$ Gauss. The Gauss is a unit of magnetic flux (named, of course, after Carl Friedrich Gauss). The $5\text{ G}$ figure makes sense, too; it lines up with the recommendations in this Cornell recommended safety guide for public areas (Section 5.3):

All public spaces are limited to less or equal to 5 G for static fields and less than or equal to 1 G for 50/60 Hz fields.

UC San Diego and the IEEE concur, coming up with similar figures for areas of regular exposure without shielding.

Also, let's do the calculations ourselves! The magnetic field outside a wire carrying a current $I$ varies with the distance from the wire, $r$:1 $$B=\frac{\mu_0I}{2\pi r}$$ where $\mu_0$ is the vacuum permeability. If we substitute in the author's $I=6.4\times10^6\text{ Amps}$ and set $r=2.6\times10^3\text{ m}$, we get $B\simeq0.0005\text{ Tesla}=5\text{ Gauss}$ - as the author claims.

High magnetic fields can of course wreak havoc with the heart and potentially other parts of the body, which is why we need to worry about this.


1 I should mention that the equation I used is really just an approximation in the case of a circular wire; it only fully holds in the case of an infinitely long straight wire. However, at such small values of $r$, it works well.

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  • $\begingroup$ Thank you for the reply! Those links should prove helpful, and having that equation will come in handy too! $\endgroup$ – NathanR Jun 2 '17 at 2:28
  • $\begingroup$ @NathanR I'm glad it helped! I should mention that the equation I used is really just an approximation in the case of a circular wire; it only fully holds in the case of an infinitely long straight wire. However, at such small values of $r$, it works well. $\endgroup$ – HDE 226868 Jun 5 '17 at 14:06
  • $\begingroup$ Just for interest, the magnetic field in an MRI scanner is on the order of 0.2 to 7 Tesla, where a Tesla is 10K Gauss. $\endgroup$ – jamesqf Jun 6 '17 at 4:20
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The biggest hazard will be flying metal. You can find many industrial injuries listed online where a hand or finger was caught between metal and a magnet. I'm not going to link to medical photos.

Magnets can effect people directly, possibly headaches and seizures, but for a person to be harmed at the cellular level by a magnetic field might be many orders of magnitude beyond what is being discussed here. It's hard for me to say, because the paper doesn't mention the strength or hazards of standing directly next to the magnetic cable. Reports of cellular damage is measured in thousands of Teslas, a Tesla is 10,000 Gauss.

G in this case stands for guass. 5 G is a safety standard for static magnetic fields. A safety zone is drawn at the 5 G distance. Beyond that line metal needs to be removed from clothing, tools secured, etc.

Magnetic Field Safety Guide

enter image description here

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  • $\begingroup$ I'd like to note that I agree, but linked to precisely the same guide in my own answer. $\endgroup$ – HDE 226868 Jun 1 '17 at 23:08
  • $\begingroup$ Oops, your post beat mine. $\endgroup$ – wetcircuit Jun 1 '17 at 23:09
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    $\begingroup$ Look up on youtube for this video of the world's most powerful electric magnet. There are image corruption problems and it's noted in the audio that it's due to the intense magnetic fields. It's been a while since I watched it, but I vaguely remember that the magnet was so strong it was causing image corruption from the next room, even while powered down. Magnets be crazy, yo $\endgroup$ – Draco18s Jun 2 '17 at 14:27

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