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.

Chisel the huge ferrite disc magnet off the back of a large dead speaker (if it wasn't dead before you started chiselling, it sure will be when you've finished) and you'll have a magnet with only about 1000G field strength, measured at the peak strength areas on its poles. It's a ferrite. That's all you get.

But big speaker magnets commonly weigh more than a kilogram and are several inches across. The peak strength areas at the poles are thus already a few inches away from the middle of the magnet's field. In this case, you can move another few inches away and still have 1/8th field strength.

So if you wave one of these big magnets over a pile of nails, they'll leap up to stick to it from several inches away.

Take a 1-Tesla-field-strength neodymium magnet the size of a button, though, and the peak field areas on the outside of the magnet will only be a couple of millimetres away from the middle of the field. Now moving just another couple of millimetres away gives you 1/8th field strength. Field close to magnet stronger; field far from magnet weaker.

That said, 1T power is still pretty darn impressive. Most current-model Magnetic Resonance Imaging (MRI) machines only have about 1.5 Tesla field strength, for comparison.