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I'm making an organism which generates its own magnetic field - specifically, a magnetic field which pulses on and off repeatedly. Said organism uses said magnetic field to communicate with other such organisms; why it does so isn't within the scope of this question.

This magnetic field is generated by an organ within said organism. Said organ is made of hundreds of sub-organs. Each of those sub-organs is made of three separate parts.

The first part is a saline-filled tube approximately 1.5 meters long. Its length is relevant later.

The second part is a stack of electrocytes, like those in electric fish - biological voltaic piles. Each saline-filled tube has 1 such electrocyte stack attached to it.

The third part is the rather awesome-sounding medullary command nucleus. It's a cluster of nerve cells which controls the function of electrocytes. Medullary command nuclei are another body part unique to electric fish - at least, in real life. Each stack of electrocytes has 1 such medullary command nucleus attached to it.

Each medullary command nucleus is receptive to a single type of neurotransmitter - the chemicals which make synapses function. While the medullary command nuclei in electric fish use acetylcholine as its trigger neurotransmitter, each of these medullary command nuclei uses a different one. For instance, one sub-organ will have a dopamine-activated medullary command nucleus, one sub-organ will have a histamine-activated medullary command nucleus, one sub-organ will have a serotonin-activated medullary command nucleus, and so on and so forth.

Whenever one of these medullary command nuclei are set off by their corresponding neurotransmitter, they tell the electrocyte they're attached to to zap the saline tube that said electrocyte is also attached to. When electrified, the saline tube creates a magnetic field, since running electricity through a conductor creates a magnetic field. In other words, the saline tube is an electromagnet.

The medullary command nuclei shut off soon after activating. Then, they're reactivated by another neurotransmitter. Then, they shut off again. Then, they're reactivated. This repeats ad infinitum. This means that the tube of saline is magnetic, and then not magnetic, and then magnetic, and then not magnetic again, ad infinitum. It is essentially a very weak (I'm going for a strength of approximately 30 microtesla, rather than 100 tesla) pulsed field magnet.

Each medullary command nucleus tells its electrocytes to send out pulses at a different frequency. Dopamine might be represented by a 1-hertz pulse, serotonin by a 2-hertz pulse, and so on and so forth. This means that different magnetic frequencies represent different neurotransmitters.

This is intended to be an inter-individual, built-in communications device. I'm still working on the "receive" mechanism, but that's not within the scope of this question.

See below for a visual representation of one of these saline-tube structures.

ITSANIMAGELOL

Now, imagine hundreds of these elements bundled together; that's ultimately what the organ I'm making looks like. See below for a rough idea of what I mean:

mygodANOTHERIMAGE

ITJUSTDOESNTSTOP

The question: I want each of the (remember: 1.5 meters long) saline tubes in this organ to create a 30-microtesla magnetic field at a range of 10 meters from the tube. What are the optimum dimensions of each saline tube for minimizing power usage, and how much power needs to be run through them to get the current I want?

Question inspired by Join JBH on Codidact's answer to one of my previous questions.

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    $\begingroup$ This is a cool engineering question, but methinks you've stepped beyond worldbuilding because you're no longer asking for a suspension-of-disbelief answer. Now you're trying to make something based partially on fiction, real - and that's not really what we do. My engineering background suggests that there isn't an answer to this question other than, "you actually can't do this." To be honest, your Q is the basis of a great scifi explanation. I'd run with it and wave a vulgar hand gesture at anyone who complains. $\endgroup$
    – JBH
    Jan 29, 2022 at 5:56
  • $\begingroup$ A value of 30 microtesla is in the order of the Earth magnetic field (25-65uT) en.wikipedia.org/wiki/Earth%27s_magnetic_field $\endgroup$
    – Goodies
    Jan 29, 2022 at 11:37
  • $\begingroup$ @Goodies Right, but it's enough to be notably different from it. $\endgroup$
    – KEY_ABRADE
    Jan 29, 2022 at 19:38
  • $\begingroup$ @KEY_ABRADE yes when the field can be used for communications, there would be some kind of modulation.. but I wonder how to get rid of the range issue, if you need that strength, see my answer. $\endgroup$
    – Goodies
    Jan 29, 2022 at 20:05

2 Answers 2

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Issue: a salt water tube is a single path conductor.

You're asking for a 30-60 microtesla magnetic field.. I wonder if this can be done anyway, with a bundle of straight (?) salt water tubes ! Each tube is a singular conductor, your magnet does not have windings around a core, like electromagnets in machinery.. using a singular conductor like a salt water tube, the magnetic field will exist in a cylinder, around your conductor. As you said, electricity will invoke magnetism.. but you ask for a relevant and detectable amount of magnetism.

https://en.wikipedia.org/wiki/File:Electromagnetism.svg

Required distance of 10 meters is huge, even for electromagnets !

Magnetic field strength is inverse proportional to the square of distance.

The field strength you require is about equal to Earth's magnetic field.

Imagine an electromagnet 1.5 meters long.. what diameter, how many "windings" and what current is needed to affect a compass, at 10 meters distance ? you'd need something in the order of the Earth magnetic field to move the needle.

For the calculation part, try https://www.accelinstruments.com/Magnetic/Magnetic-field-calculator.html

.. when I look at my results, targeting 0.05 mT, this thing is going to be big. You won't get a field this strong, using a single (flexible, non straight) thread of salty water !

The only organism I can find using active magnetism is the Magnetosome bacteria, it actually moves its body to align with Earth's magnetic field using a magnetic iron crystal grain, embedded in lipin https://en.wikipedia.org/wiki/Magnetosome

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@Goodies gives the way to calculate the the field using salt juice concept and I would add that the durations of the pulses would need to be short, or heating can be a problem. Also as was pointed out 30 microtesla at one meter is pretty large.

A different way to think about the problem is to try to find an alternative way to produce the magnetic field. Neodymium are among the strongest commonly available magnets and if you want something to produce a magnetic field comparable to the earths, the graph below shows that you would be trying to come up with something comparable to the strength of a small neodymium magnet.

One thought is to have your creature (instead of trying to produce magnetic fields with electric currents) is to have the organism align smaller magnets in their cells, perhaps with each cell or structure containing a small magnet, and have the orientation of the magnet controlled by the electrical impulses. Depending on the arrangement of the smaller magnets you could control the strength of the magnetic field. One advantage of this is that once aligned no additional energy is needed.

To communicate the creature could also change its orientation. By changing its angle with respect to the receiving creature they would change the strength of the field. I guess you could imagine the creature wiggling or dancing to communicate.

The disadvantage is that for the creature to change it magnetic signature it would probably take time.

A nice paper on Magnetoreception in animals is

https://physicstoday.scitation.org/doi/pdf/10.1063/1.2897947

The figure is from research gate DOI:10.1145/2800835.2800938 and is of the Magnetic field strength of a small neodymium magnet (10x8 mm cylinder) as measured by the digital compass of a Google Nexus 7 tablet.

Figure 1. Magnetic field strength of a small neodymium magnet (10x8 mm cylinder) as measured by the digital compass of a Google Nexus 7 tablet.

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  • $\begingroup$ This graph needs some explanation.. the blue line states the measurement of your google tablet when the magnet is present. It is the combined strength of your magnet and the Earth magnetic field. On a distance, the contribution of the test magnet approaches zero (not 43uT !!) $\endgroup$
    – Goodies
    Jan 29, 2022 at 20:19
  • $\begingroup$ It is not a very good paper my suspicion is that the author did not subtract the background field from the earth from their measurement. But yes the field should drop rapidly and approach zero with distance. $\endgroup$
    – UVphoton
    Jan 30, 2022 at 4:04

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