The reason why an MRI machine is a giant contraption that needs liquid nitrogen cooling, rather than a neat little metal-plated lump that you can buy over the Internet, is twofold. It's partly because the MRI machine is also a sensitive radio receiver, detecting the radio-frequency energy emitted by the magnetic nuclei in the patient's body when they interact with a strong magnetic field. But it's mainly because a 1.5 Tesla MRI machine is creating a 1.5 Tesla field over a large enough volume that a patient can be stuck into said field.

By the same token, junkyard car-lifting electromagnets only have about 1T field strength, but they generate that field over a big enough volume that their total lifting capacity, for conveniently steel-bodied cars, is massive. The coils under their protective armour draw at least a few kilowatts, and maybe considerably more - 20kW isn't out of the question for a big car-lifter.

You're not going to be lifting any Toyotas with a five buck magnet from anywhere. Nails will hop up only about an inch to hit the strongest of the magnets in the ForceField grab bags. In contrast, ferromagnetic objects of all types will fly across a room to make friends with an MRI machine, as occasional accidents attest.

Because of their limited field size, small neodymium super-magnets like these ones aren't actually much of a problem to deal with, at least as far as messing up your monitors and erasing your credit cards and wiping your video tapes and being hit by flying spanners goes.

Yes, when I had one in my back pocket, I at one point found myself unexpectedly attached to the washing machine. But the rapid diminution of the field strength means that you can hold the strongest of these magnets - the three spheres end-to-end, for instance - in your hand and wave them around a mere foot and a half from a computer monitor, and notice only slight image distortion and discolouration. Move the magnets further away and the effect vanishes.

Touch those same magnets directly to the screen, mind you, and they'll magnetise the heck out of the shadow mask and leave you degaussing until practically all of the world's cows have come home, had a nice sleep and gone away again. I own a degaussing wand...

...but I am not confident enough of my skill with it to deliberately Magna-Doodle all over a monitor just so you can see what it looks like. Sorry.

Quite big rare earth magnets can be had, if you want more field range. There's this one, for instance, which only has about 1.1 times the volume of a ping-pong ball, but which ForceField warn you not to buy unless you're confident that you know how not to crush, blind or mangle yourself with it.

As far as terrestrial magnetic fields go, 1T is quite strong, but it ain't much by the standards of the universe. Neutron stars and pulsars (which are spinning neutron stars) have magnetic fields. If they were made of nothing but neutrons then they wouldn't, but they've also got superconducting superfluid protons and various other exotic forms of matter, so they have.

They get just about the whole magnetic field of the normal star they once were, squished into their city-sized diameter.

The magnetic field strength on the surface of a pulsar has to be at least several million Tesla, and may range as high as a thousand million Tesla. That's more than strong enough to seriously deform electron orbits and make matter do very, very strange things, regardless of whether it's the sort of matter that normally cares about magnetic fields or not.

This magnetic field would certainly kill anybody who tried to land on a pulsar. Except for the fact that they'd have been very conclusively killed already by radiation and/or gravity gradient. Plus, landing on something that's spinning fast enough that its surface whips past at kilometres per second - in some cases, thousands of kilometres per second - presents a bit of a challenge in itself.

But, again, I digress.

And then, there's this I could tell you what this is, but you might as well just click here.

Looking for the ferrofluid? It's right here.

Drive magnets
Want free rare earth magnets? Got a dead hard drive? You're in business.

What, you might be wondering, are super-strong magnets doing inside a magnetic storage device? You're meant to keep magnets away from your drives, aren't you?

Well, yes, you are. A sufficiently strong magnetic field across the storage surfaces can destroy data in short order. And, since the drive platters spin rapidly, a magnet sitting on one side of them will present a nice oscillating field over the whole disk, from the platters' point of view.

But this is one of those situations like putting metal in a microwave oven. Popular wisdom is "Don't do it, ever", but that's only because the situations in which you can do it safely are complex enough that it's not a good idea to tell all and sundry about them, in case they get it wrong.

Hard drive manufacturers know it's OK to put magnets in a hard drive. Which is good, because no modern drive would work without one.

Old hard drives use stepper motors for their head positioning. Stepper motors rotate by one precisely defined small amount - one "step" - every time they're fed a current pulse. Which makes them a good way to make something like a hard drive read/write assembly move by the small steps needed to position it accurately over tracks on the drive.

Steppers are slow, though, and they're position-sensitive (the heads will end up in a slightly different place if the drive's tilted), and they're sensitive to temperature changes, and they wear out.

An alternative head motor design, which is used by all drives these days, is the voice coil. There's a coil next to a permanent magnet; when current's passed through the coil, it creates its own magnetic field, which interacts with the static field from the permanent magnet, and moves the head arm one way or the other depending on the direction of the current.

This motor design isn't at all precise, so there are "servo tracks" on the drive platters, which the heads read to allow the drive to tell where they are. That information lets the drive use a feedback mechanism to get very good precision. Presto, cheap super-high-track-density commodity hard drives.

Voice coil motors are better than steppers, because they have no temperature or position sensitivity to speak of, they're fast, there are no motor bearings to wear out, and they cost less.

The reason why these motors are called voice coils is that the early ones had the same straightforward cylindrical design as the voice coils in speakers. So that's the name they got. Then came various curved-magnet designs, but nowadays consumer drives all have simple swing-arm motor arrangements, with flat bent magnets that're magnetised lengthwise - with a pole at each end.

If you've got yourself a dud hard drive that you'd like to relieve of its magnets, it's easy enough to do.

You'll very probably need a screwdriver set to get into the drive. You can engage Torx screws with Allen keys or a flathead driver of appropriate width, but that's not an optimal solution. Fortunately, Torx quarter-inch hex bits will do the job, and are cheap; you need just one hex bit driver to match, and they're cheap too.

All it took to get the lid off this 1.2Gb Seagate was peeling off the sealing tape around the sides. Built for combat, this drive was not. But the parts inside are attached with Torx screws.

Commodity hard drives these days aren't the big-magnets goldmine that old voice coil drives were; if you can find yourself a 5.25 inch full-height voice coil monster drive, you'll get better magnets out of it. Advancing technology has allowed the manufacturers to make do with fewer and smaller magnets.

Inside the Seagate there's just one magnet - it's the thing above the coppery voice coil, here.

Here's the head assembly and magnet bracket removed from the drive. You have to remove the platters to remove the head assembly. Just unscrew everything in sight and it's easy enough to slide out the platters, which frees the heads and lets you lift everything out.

The two-piece magnet holder in this drive was, as usual, held together only by the strength of the magnet - but that's plenty. The holder effectively contains the magnetic field to the small gap that the voice coil sits in; there's a little magnetism on the outside of the holder, but it can barely hold a paper clip against gravity. This is why the magnet doesn't damage data, despite being right next to the platters.

The magnet in this drive is attached to its base plate with a few dots of glue, some little locating nubbins, and, once again, its own bodacious magnetic field. There's nothing wrong with leaving a hard drive magnet attached to its base plate, if you don't need both sides of it uncovered. The steel plate makes the magnet much less prone to breakage. NIB magnets are brittle, and flat thin hard drive magnets commonly snap while people are playing with them.

Here's another hard drive magnet assembly, with four separate magnets this time. These are neodymium magnets without any protective plating; under the shiny plating, all neodymiums look like ordinary ferrites.

Because it's so easy to get magnets out of hard drives, there are lots of these things in the surplus market. ForceField have a few of them.

Hard drive magnets have their uses, but less fragile shapes are better for many applications.

Like what, I hear you ask.

Stuff to do
Apart from making all of your magnetic science experiments work the way they're meant to (NIBs make it easier to push a grape, and they bounce better, too), there are useful things you can do with rare earth magnets.

No, really, there are.