How do I make a magnet?
May 12, 2011 8:32 AM   Subscribe

How do you make a permanent magnet if you don't have one? How do you make a permanent magnet that's more powerful than the one you have?

Okay, so let's say I accidentally got transported back in time 30,000 years. I want to impress the natives and/or build/do useful things that are known about nowadays. I've got my laptop with a snapshot of Wikipedia. There are definitely locals around. If I hang out with them for a couple months, I should be able to communicate with them.

Now, the problem is, my laptop only has like 4 hours of battery life. Not enough to read all about science, technology, and history. So my first job is going to be to build some way of getting power into the machine.

Gathering the actual energy isn't hard -- build a water-wheel or windmill, store it in a flywheel, that sort of thing. But converting the energy into electricity; that's the hard part.

Okay, sorry for the long set-up. That's what led me to think about this question, and I found it amusing so I thought I'd share.


Let's say I had some iron and some copper. If I heated the iron and moved it in the presence of an electric field, it would become a magnet. But I don't have an electric field. I could generate one if I had a magnet, but I don't.

How do I turn my iron into a magnet?
If I had a weak magnet, how could I make a stronger one? Like, if I move a weak magnet, but I move it a *lot*, does that generate a stronger electric field that I can use to make a stronger magnet?
posted by Galaxor Nebulon to Science & Nature (11 answers total) 5 users marked this as a favorite
Electrostatic Generators will be where you want to start with electricity.
posted by Loto at 8:38 AM on May 12, 2011

If you have a rod of soft iron, you can align it to north and south and hit it several times on the end with a hammer. This will make a weak magnet, but enough to get you started. I don't think you'd be able to get a strong enough magnet to generate electric fields to recharge your laptop without modern industry (i.e. Alnico magnets).
An electrochemical battery might be more effective. You'll need a bunch of lemons (, or alternatively a ready supply of vinegar.
posted by leapfrog at 8:50 AM on May 12, 2011

The wikipedia article on magnets is pretty thorough.
Ancient people learned about magnetism from lodestones, naturally magnetized pieces of iron ore...

Ferromagnetic materials can be magnetized in the following ways:

* Heating the object above its Curie temperature, allowing it to cool in a magnetic field and hammering it as it cools. This is the most effective method and is similar to the industrial processes used to create permanent magnets.
* Placing the item in an external magnetic field will result in the item retaining some of the magnetism on removal. Vibration has been shown to increase the effect. Ferrous materials aligned with the Earth's magnetic field that are subject to vibration (e.g., frame of a conveyor) have been shown to acquire significant residual magnetism.
* Stroking: An existing magnet is moved from one end of the item to the other repeatedly in the same direction.
(Note that the first bullet point is similar to your idea, but does not require an electric field)
posted by muddgirl at 8:58 AM on May 12, 2011

Response by poster: An electrochemical battery might be more effective. You'll need a bunch of lemons...

The hard part is getting the copper and zinc.

So, am I right that I can make a stronger magnet from a weaker one by generating current with it?
posted by Galaxor Nebulon at 9:08 AM on May 12, 2011

Brass is copper and zinc. They'd have had both.
posted by empath at 9:27 AM on May 12, 2011

Best answer: Well, the real problem here isn't generating electricity (and electrostatic setup will probably be your best bet) it's regulating the voltage so you don't blow up your laptop.

So, am I right that I can make a stronger magnet from a weaker one by generating current with it?

Yeah. In fact, any current generates a magnetic field. But unlike having a 'north and south' the magnetic field is actually circular around the wire as you can see here. But if you wrap your wire in a coil you can create a regular magnet (like in this diagram) you create a 'regular' bar magnet entirely with just electricity, you don't need any iron or anything else.

So you can use your static electricity from your static generator to create a purely electronic magnet and then use that to generate your AC. Jedilk's dynamo is an example of this principle (in fact it was the first dynamo apparently)
posted by delmoi at 9:32 AM on May 12, 2011

Electrostatic generators are high-voltage low-current. On the other hand, the magnetic field strength from an electromagnet is linearly dependent on the amount of current in the wire. I don't think an electrostatic generator is going to produce enough current to make an electromagnet stronger than, say, earth's magnetic field.
posted by muddgirl at 1:01 PM on May 12, 2011

Muddgirl, that is true but it is also linearly dependent on the number of turns in the winding of the wire so it is possible to compensate for a weak current. It's completely possible to make an electromagnet that is stronger than the Earth's magnetic field which is on the order of some tens of micro-Teslas.
posted by Loto at 1:09 PM on May 12, 2011

It's dependent on the turn density, yes. You can't just wind as many turns as you like - you have to pack them tighter and tighter into the same length.

It took me a while, but here's a calculator for solenoid electromagnets. The OP can play with realistic values - I used a current of 8mA (which is what we can produce today - not necessarily what a primitive electrostatic generator can produce), soft iron core with relative permeability around 60, and a target of 0.5 gauss. Remember that we can't stack wire, so we have to calculate the turn density and see if it's feasible. If we can only produce, say, 1 mA, we're talking about needing to fabricate a wire that's about 1mm in diameter.
posted by muddgirl at 2:15 PM on May 12, 2011

Yes, of course but my issue was with:

I don't think an electrostatic generator is going to produce enough current to make an electromagnet stronger than, say, earth's magnetic field.

Which is false. A small Van Der Graff generator (one of the worst electrostatic devices for maximizing current output) can manage 50 microAmps which is more than enough to produce an electromagnet with a magnetic field larger than that of the Earth's.
posted by Loto at 6:04 PM on May 12, 2011

Most Van Der Graaf generators use a DC charge instead of utilizing, say, the triboelectric effect. My overall point was that using Earth's magnetic field is easier than building an electromagnet, and on an order of magnitude just as effective.
posted by muddgirl at 8:33 AM on May 13, 2011

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