In L. Sprague De Camp’s Lest Darkness Fall, a modern educate person falls through a time warp and ends up in 6th century Rome. He introduces some modern innovations: the still, Arabic numerals, horse collar, the printing press, paper, and telescopes.
I’m wondering how easy it would be for him to have made permanent magnets? How exactly would one do this starting from what they had then? Could he have used the technique of heating iron above its curie point and then working and cooling the iron in a magnetic field? Could he have produced some better magnetic material than plain iron?
The classic method is to find them already naturally occurring. And once you’ve got one, you can make more by rubbing another piece of iron against it. But these won’t be very strong: Maybe strong enough to make a compass, but not much beyond that.
To make magnets of decent strength (say, strong enough to hold a picture on a refrigerator), you need electricity. You can get that electricity from generators, but then you’ve got a chicken-and-egg problem, because generators use magnets. On the other hand, provided you’ve got access to copper and zinc (both metals known to antiquity) and some decent acid (which you might need to find an alchemist for), you can make a battery. The batteries you make won’t be very good, either, but you can make up for that by using a bunch of them, in parallel (for current capacity) and series (for voltage). Technically, this is what a “battery” actually is: A whole bunch of cells connected together, just like a battery of artillery is a whole bunch of guns working together (most modern “batteries”, the 1.5ish volt ones, are actually just a single cell).
As mentioned, the easiest way to do this was to find natural magnets. People knew about these, and often treasured them, sometimes making elaborate “jackets” for them.
You can certainly make a magnet using electricity and an iron rod – but it necessitates making insulated wire to wrap around the rod and a source of electricity, either a battery or a generator. If your generator isn’t a dynamo-type, you need a magnet for it. If it IS a dynamo-type, then you generate the magnetic field by running wire coils around iron to make a magnet, which is what you’re trying to do in the first place.
Or, if you already have a magnet, you can magnetize another piece of iron by stroking it repeatedly and in the same direction with the magnet
There is another alternative, if you don’t have a magnet to start with – you can make a non-magnetic iron bar into a permanent magnet by aligning it with the earth’s magnetic field and striking
A compass may be the only real use of them back then. Well, if you had a strong one, you could play party tricks or something.
Actually, zinc was not known back then. While they did make brass, they did it by adding a zinc ore to liquid copper or something like that. They didn’t make zinc metal except by accident, and then didn’t know what they had. So if our time traveller wants strong magnets (maybe they need to fix a flux capacitor), they’re going to have to teach the local smiths how to make zinc or find a substitute.
You’d only need to generate a magnetic field for a limited time, just enough for the (probably small) piece of metal to cool. At least for compasses, the magnets don’t have to be very large. For flux capacitors, who knows… they probably require neodymium.
Huh, ignorance fought, then. So how does one make zinc?
And really, any two distinct metals will work, so you could also do (say) copper and iron. It probably won’t work as well as zinc, but again, that’s a problem that can be solved via brute force with enough cells.
Actually, you can obtain a weak magnet by simply taking an iron bar, aligning it north/south, and repeatedly hammering it. I seem to recall doing this with a railroad spike when I was a kid. Note that I said WEAK magnet. You get something that can pick up a few iron filings.
If the iron bar is heated above the Curie point and vibrated during cooling while aligned north-south it will magnetize. It would be a little better than just hammering cold iron. It may hold the field for a while but I believe something about the structure of the iron is insufficient to keep little granules of iron in the same magnetic orientation over time. Perhaps not a problem if the magnets can be kept aligned north-south.
I suppose they could have figured out how to make ceramic magnets made from ferrite but I don’t know how they could get a strong magnetic field in them without electricity.