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Imagine a setting where humans have to compete with another sentient race, which can only be harmed by magnetized weapons.

Humans have only basic technology, and magic is very very rare.

Would it be feasible for them to magnetize crude iron weapons with lodestones before a battle, or are lodestones not strong enough for that? In what other ways would they be able to fight back?

Edit: Any physical weapon can cause harm, as long as the magnetic field applies. Otherwise the weapons would phase through. Since iron can be magnetized easily, humans naturally picked it as their weapon of choice. The field itself doesn't have to be very strong. The strength of a fridge magnet would suffice, but on bigger stuff like swords, axes and arrows.

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    $\begingroup$ It depends a lot on HOW magnetic the weapons have to be to harm this other race. Is this like cold iron for fairies where it's mystically poisonous, or does it have to be magnetic enough to actually make nails stick to the blade in order to have the necessary effect? $\endgroup$ – Morris The Cat Aug 13 at 14:01
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    $\begingroup$ It would be very helpful if you added what effects the magnets have on them, and why it hurts them. Does it rip the iron from their blood, because they have so much of it? Are their bones naturally laced with metal? Do they have sensitive cybernetic implants? $\endgroup$ – Seymour Guado Aug 13 at 14:06
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    $\begingroup$ Please attach a number to the magnetic field, to allow us evaluate and compare. $\endgroup$ – L.Dutch - Reinstate Monica Aug 13 at 14:11
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    $\begingroup$ Um, what is wrong with just using lodestones as weapons? Slung pebbles and rocks-onna-stick are tried and tested devices, after all. $\endgroup$ – Starfish Prime Aug 13 at 14:33
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    $\begingroup$ (to answer my own comment: probably because lodestones are rare and valuable, and they'd be more useful making more magnets than being used as weapons themselves, and risk loss or destruction) $\endgroup$ – Starfish Prime Aug 13 at 15:04
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Yes this would actually work surprisingly well; lodestones have relatively weak magnetic fields compared to something like a Neodymium magnet but that still gives them between 6500 and 51000 Am-1 depending on how fresh they are, this gives them a magnetising force of 81-640 Oersted. This graph of induced magnetism suggests that this is enough to induce between 8,000 and 15,000 Gauss, that's 0.8-1.5 Tesla. You've indicated that the rating of a fridge magnet will be sufficient, depending on the source used that's either 0.01T or 0.005T. As reasonably pure Iron is slightly easier to magnetise than the steel in the induction example you can probably get acceptably magnetic weapons pretty quickly.

The hotter the metal is when you do your magnetisation the easier it is and the better the results. To avoid heating and demagnetising the lode the weapon needs to be hot but below the Curie Point; the best time to magnetise will be after the quench but before the piece is completely cold (this is sometimes referred to as being above the "critical temperature", the metal is hot to the touch and still somewhat flexible but definitely solid) and before the tempering is done.

Please do note that the initial magnetism induced in the object will not be completely held once the object is in use. The reason that Neodymium magnets are so strong isn't that they can be magnetised that much more than Iron but that they hold their magnetism better once it has been induced.

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    $\begingroup$ I'm reasonably certain that there's something critical missing here;you're implying that a natural lodestone can be used to make an iron magnet with the strength of a modern neodymium magnet, and that sounds dubious. Don't know enough about magnetism to see the issue yet, though. $\endgroup$ – Starfish Prime Aug 13 at 15:51
  • $\begingroup$ Even then, I'm dubious that you can use a weak magnet to produce a much stronger magnet. Your strongest example lodestone is something like 0.06T, I think, and so I suspect it could be reasonably used to create an iron magnet with a similar strength. That's a lot lower than your example, but certainly high enough for the OP's needs. Possibly I'd need to take that question to another forum though; prying a sensible answer out of google is proving difficult. $\endgroup$ – Starfish Prime Aug 13 at 16:11
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    $\begingroup$ @StarfishPrime Only if it was very fresh, and an extremely magnetised specimen, I had missed a thing that I've edited in. You're assuming a magnet can only induce a field of similar strength to it's own, based on the induction graph (a roughly hyperbolic curve) and some figures from this calculator that is certainly not the case. You only need a 500 Oersted field to create an initial 15000 Gauss permanent magnet with a 15000 Oersted field. $\endgroup$ – Ash Aug 13 at 16:11
  • $\begingroup$ Consider as a thought experiment, using two bits of iron, $A$ and $B$. $A$ has a magnetic field strength of $X$. Lets say I can induce a magnetic field in $B$ of $2X$. Now, why is $B$ not now inducing a field of at least $2X$ in $A$? If magnets could "boost" each other like this, just putting two bits of ferromagnetic material side by side would swiftly cause each of them to reach saturation, no? $\endgroup$ – Starfish Prime Aug 13 at 16:20
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    $\begingroup$ @StarfishPrime If you did it right they'd eventually reach an equilibrium state. Based on the apparent math they'd be more magnetic than they were, but well short of the material maximum, each pass of two pieces of the same permeability would disrupt the inducer as well as magnetising the inductee. The fields are at right angles to each other not parallel. The reason magnetite can increase the magnetism of iron without losing much of it's own magnetism is that the two materials have very different permeabilites. $\endgroup$ – Ash Aug 13 at 17:37
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Yes.

You'll likely find this paper of interest: The Early History of the Permanent Magnet

The first great systematic work on the magnet is that described by William Gilbert -- sometimes spelt Gilberd by contemporaries -- in the famous De Magnete Magneticisque Corporibus et de Magno Magnete Teure Physiologia Nova, published in 1600.

[...]

Gilbert gives three ways by which permanent magnetism -- or “verticity” as he termed it -- may be given to steel.

• The first method was by touch with a single load stone, which was to be drawn from the middle of the needle to the end, where the application was to be continued by a very gentle rubbing. [...]

• The second method was forging with the horizontal specimen pointing north and south in the Earth’s field [...]

• Gilbert was quite clear that a red heat destroyed permanent magnetism, but records that a redhot iron bar left to cool in the direction of the Earth’s field became permanently magnetic, and further that unheated iron bars left in the direction of the Earth’s field for twenty or more years -- such as window bars -- likewise acquired verticity.

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    $\begingroup$ I suspect that the second option (leave a red-hot bar correctly aligned) can't provide you with a field strength stronger than the earth's own, which is substantially weaker than a fridge magnet. $\endgroup$ – Starfish Prime Aug 13 at 14:55
  • $\begingroup$ If the second option can generate a strong enough field I can definitely work with it $\endgroup$ – Nightingale Aug 13 at 15:17
  • $\begingroup$ @ayrton from a bit of research on lodestones, it would appear that the magnetic strength of lodestones is substantially higher than could be caused by the earth's magnetic field alone. This implies that method number 2 could not produce a magnet as strong as any lodestone could be, and so is unlikely to be suitable for anything other than use as a compass needle (at best). $\endgroup$ – Starfish Prime Aug 13 at 15:22
  • $\begingroup$ @StarfishPrime I believe that the Earth's field can eventually produce a magnet of arbitrary strength if you leave it sitting long enough for the iron to sort itself out. Strong magnetism just require long timescales. $\endgroup$ – SRM - Reinstate Monica Aug 13 at 19:33
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    $\begingroup$ @SRM Some stones lie undisturbed for millions of years. They would have to have a magnetic field that is a million time stronger than a bar that you leave lying around for one year. In practice, naturally-occurring magnets have a much weaker strength. $\endgroup$ – toolforger Aug 14 at 13:00
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You can magnetize iron or steel by rubbing it with a magnet. It seems less than awesome, somehow.

Or you can magnetize iron or still by HITTING IT HARD!

MAGNETIZE BY HITTING

https://www.youtube.com/watch?v=GaD9vAuj20s

This is what you need for your story. Your Magnetizer will take each weapon, look it over while humming the incantation, place it on the ground, align it with the Earth's magnetic field, then whack it with his magnetization hammer. The owner needs to shout back when that happens. Magnetizer is good at it and usually just needs one hit.

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  • $\begingroup$ Can this provide a magnetic field stronger than the earth's own? If it can only manage .5 gauss, that's too low by a factor of 200 for the OP's needs. $\endgroup$ – Starfish Prime Aug 13 at 14:53
  • $\begingroup$ Striking cold steel, or iron can add stress to the weapon, especially repeated strikes. This is why forging steel that's too cold can be disastrous to the final weapon (also the steel/iron is a lot harder to manipulate when cold). $\endgroup$ – Artsoccer Aug 13 at 15:17
  • $\begingroup$ @Artsoccer - it seems like you would want a sword that could withstand repeated strikes when cold, because of the fighting thing. $\endgroup$ – Willk Aug 13 at 17:23
  • $\begingroup$ @Willk You might want a blade that withstands cold, but that's one of the downsides of steel swords -- they can shatter, and cold increases that possibility. Alloys and other techniques that reduce the susceptibility. But these techniques decrease the magnetism (usually a desirable thing since a magnetic blade builds up bits that make it hard to sheath and may interfere with its cut). $\endgroup$ – SRM - Reinstate Monica Aug 13 at 19:38
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As other answers have stated natural lodestones may be too weak to create the magnetism you require for "enchanting" your weapons.

We could consider however how lodestones themselves are created. From what I have (briefly) read on the subject they can be created by lightning strikes. If a lightning strike can magnetise an object, perhaps your primitive civilisation could base a religion around "The Thunder Gods".

They could construct obelisks or tall monuments to their version of Thor that are essentially just massive lightning rods. These places of worship would have the 'magical' abilities to bless iron weapons placed upon an altar at the base with the power to slay these demons plaguing your people.

When lightning strikes to monument it is carried down to the altar and into the object placed there, magnetising it in the same way lodestones are created. Now this may require some handwavium-supercharging of the numbers involved to create the field strengths you desire, but I think it would add some amazing depth to your civilisation as they would base a lot of culture and reverence around the thunder god's protection of their people. Storms would become times of great celebration and your warriors in training could complete a rite of passage during them, placing their personal armaments in the altar and holding vigil awaiting the "Blessing from the sky". To be "Thunder-Touched" would be a massive morale boost. Whilst rare for regular villages to have lightning storms, certain areas in the world do have higher frequencies of lightning strikes and your people could congregate in those areas, constructing multiple monuments and eventually becoming a religious hub akin to mecca that could be a destination for pilgrimages for those completing their warrior training. Also this would provide a natural deterrent to the enemies as the area would potentially be highly magnetised itself.

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It works! But even better (transferring off of @Roger's answer):

YOU DONT HAVE TO ACTIVATE THEM BEFORE BATTLE!

Simply letting the weapons cool after forging within a magnetic field (or rubbing them with magnets during cooling) would instill them with permanent mangnetism. [On any technology capable of forging iron weapons to begin with]

That way even rather primitive people could have permanently viable weapons against your monsters!

Sidenote: Shock and vibrations can/do diminish the magnetism, so a sword would loose its "power" against them after a lengthy duel (or over several years of dueling, I do not know how fast that effect is).

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As an alternate solution, you could embed the lodestone into the base of the weapons - the pommel for example. The metal of the weapon would guide the magentic field through it and essentially create a temporary magnet. Most people have tried something like this as a child - you can "extend" a magnet by placing a nail or some other ferrous item on the magnet, and then you can use the nail as if it were magnetic, until you take it off the permanent magnet.

Picking up string of paperclips with a single magnet

I'm reasonably sure this will allow for a more magnetic weapon than rubbing it with a lodestone. This could work as a sort of two-tiered approach where weaker magic could be counteracted with weaker magnetic fields, but for the strongest magic you need your own lodestone.

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    $\begingroup$ You would need an unfeasibly large lodestone, since a sword is a lot longer and more massive than three paper clips. On the other hand, if you could embed or attach a series of lodestones or permanent magnets along the length of the blade... $\endgroup$ – MJ713 Aug 15 at 3:01

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