To pull weapons away from their bearers, knights off their horses, or “sticky” shields that trap blows, etc.?
Have you ever tried to do these things, or anything even similar?
Magnetism is incredibly weak over even short distances. Sometimes I can’t even insert a second piece of paper under a refrigerator magnet that’s already holding up one page. And you think a magnet can pull an armored knight off his horse? Good luck with that.
It was hard enough for them to get a magnetized needle (a tiny mass floating in water with negligible friction, I would add) to point north. Forget about anything bigger!
I don’t think magnets strong enough for that kind of thing were even available until relatively recently.
I don’t know how strong they are. Would an entire bridge made from magnetic materials be enough to disarm or dismount people, for example? Or at least make it hard to walk across with metal boots?
Under such conditions, sure, especially since the boots would be in actual contact with the bridge. But have you ever seen such a big magnet, or even close to it? (I’m guessing that you’re busy writing some fiction and want to make sure you get your facts correct.)
The closest thing I’ve seen in real life would be the electromagnets used for security locks. If the door has a metal plate maybe 4 by 8 inches in size, being held to a powerful electromagnet, no one will be able to push that door open. But shut the current and a child can push it.
That application harnesses the fact that magnets are strongest in the perpendicular direction. But it is relatively easy to slide along the magnet’s surface. In your bridge, for example, if the bridge and the boots are smooth, then it will be very difficult to lift their boots off of the bridge, but somewhat easier to just push forward without lifting up. But the surfaces are rough, then that might increase the distance between the bridge and the boot, to a point where the magnetism is much weaker. Millimeters can make the difference.
Just curious why the OP restricts his query to “ancient” warfare. Why isn’t he asking about modern warfare? Is there a suspicion that modern weapons aren’t made of metal?
Perhaps he saw Mystery Men and thought “Hey, that worked pretty well; I wonder how well it would work versus swords?”
Or, um, take the boots off.
The OP would be best-advised to sneak into a car-breaker’s junkyard, and experiment with the giant electromagnets there.
I’m not writing anything, but if somebody else wrote about this in a fictional context, I’d love to read it 
I was just curious.
Although… it would be a fun mod for medieval warfare games.
Cool. Good to know. Has it ever been considered, even theoretically? Did some medieval scientist think about it, do the math, and realize how impractical it would be?
Because it’s trivial to do in modern times with electromagnets. And also impractical when a shell or precision munition would provide more lethality for less effort. As for magnetic shielding/redirection of projectiles, I haven’t done the math but I just assume that forces incurred by ballistic weapons are orders of magnitude higher than what a human leg or arm could provide.
Magnetism was known before electricity, though, and required only mining and processing technologies that would’ve been available then.
Is that a movie? A TV show? What happened?
He should be more focused on the weapon that shrank women’s clothes.
…portable clothesdryers?
This was my first thought. How strong a magnet would the ancients have had access to?
Although, some of Archimedes’ war machines (the Claw, the Death Ray) sound about as far-fetched.
No, it’s not.
I want to make sure you hear this, because you seem to have missed it when I and others said it before: No, it’s not trivial, even in modern times, even with electromagnets.
Claverhouse brought an excellent example, that of the car junkyard. That’s a damn powerful magnet, but it is in direct contact with the junk. There are also some very powerful industrial magnets which are used to sweep through stuff to attract the junk – I’ll concede that in those cases there is some distance between the magnet and the junk, but still, it is moving relatively slowly, and the junk is relatively lightweight. To jump from there to using a magnet to pull an armored soldier off of his charging horse is just absurd.
I did not know that. You mean even a junkyard magnet hovering above an armored knight – that’s what, 300 lbs total? – could not pull him off a horse without direct contact?
Movie. 1999. A fairly funny superhero spoof with a good cast.
Close to it. You can find videos of scrapyard electromagnets in action on YouTube. Here’s one that shows a magnet getting very close to relatively light-weight wheel rims before they hop up to the magnet.
The scaling of force with distance is very painful here. In the application you’re envisioning (a magnet hiding some distance away from the target knight, e.g. high up in a tree), the scaling could be as bad as distance to the seventh power, meaning that if it just barely worked at 2 meters it would need to be 80,000 times stronger to work at 10 meters. To be sure, the scaling isn’t quite as bad closer in to the magnet, but it’s still very bad.
Just did a little quick Google research, and found that magnetic fields follow an inverse cube relationship. The strength of the field decreases by the cube of the distance. If my math is correct, a magnet capable of lifting 5000 *tons *from a distance of half an inch would have to get within 16 inches of a 300 lb. armored knight to yank him from his horse.
That’s how the magnetic field varies with distance, not the actual force. The force depends on the product of the magnetization induced in the target object (third power) and on the gradient of the lifting field at the object (fourth power at large distances).