They're still not dirt cheap, mind you. I bought three $US20 grab bags on Ebay, and got 81 magnets as a result; they'd all easily fit inside a cigarette packet, if not for the fact that they'd roll it up into a ball if you tried. But it's a respectable collection, nonetheless.

There's an assortment of rods, discs, rectangular prisms and spheres, and there are only three of the usual flat-banana-shaped surplus hard drive magnets. Those magnets are massively strong, just like any other neodymium, but their odd shape makes them unusually fragile. Magnets of more Euclidean-solid shape are more useful.

The ForceField grab bags seem to be the cheapest way to get hold of these things, short of getting old dead hard drives for free and ripping them to bits. But you can do that, too. I'll show you how in a moment.

First - feats of strength.

Field strength
Any time people talk about super-powerful magnets they have to show pictures of big metal things dangling, so here some are.

That's a 15 inch spanner hanging there along with the other ironware. The sphere holding the main string of tools is only about two thirds of the way to holding its maximum load. Getting the four tools in that string balanced was slightly tricky, but not because of any lack of magnet power.

These are a couple of little 3/8th inch disc magnets at work. The one at the top couldn't hold a lot more. But whaddayawant from something smaller than the average button?

ForceField have some more heavy-lifting pictures on their demo images page.

Magnetic field strength is measured with two units, the Gauss (G) and the Tesla (T). 1T equals 10,000G.

The earth's natural magnetic field is about 0.5G, depending on where you are - it's weaker at the equator and stronger at the poles. It's also slowly declining at the moment, which is something that it does periodically; geological evidence shows that it's actually reversed several times over the planet's life. The mental giants at the Institute for Creation Research use the decline of the field strength to prove that the planet's only a few thousand years old.

In case you're wondering, this, like various other of their proofs, doesn't stand up too well.

But I digress.

The strongest cheap ferrite magnets have a field strength at their poles of around 1000G, or 0.1T. NIB rare earth magnets, on the other hand, have surface field strength of about 1T. Ten times stronger.

The size of a magnet has a lot to do with the perceived strength of its field, though. None of these magnets are very big, so that inverse-cube-law field strength reduction bites into their power quite quickly.