Unconventional wisdom

Originally published 2002 in Atomic: Maximum Power Computing
Last modified 03-Dec-2011.


So I'm sitting here playing with astoundingly powerful magnets right next to my computer with four hard drives in it. And my nice 19-inch monitor that'll be permanently wrecked if the shadow mask gets magnetised. And I'm thinking about all of the things that people know you're not meant to ever do, but which aren't really that bad if you know what you're doing.

You can wave hand-crushingly grunty neodymium-iron-boron (NIB) magnets around quite close to magnet-sensitive devices in perfect safety, because of two things.

One, magnetic field strength falls off roughly with the cube of the distance. That's an approximation, but it's a useful one, and it means that the one-Tesla field from a NIB magnet the size of a marble isn't nearly that powerful 30 centimetres away.

Which is why my monitor is not permanently paisley.

(The management disclaims all responsibility if any readers of this column decide to paint an impressionistic landscape on the monitor of a workmate with a magnet out of an old hard drive, by the way.)

The other thing that it helps to know is that magnetic fields and magnetic storage media do mix, as long as the field isn't really amazingly strong.

The field has to be really strong, because it has to exceed the coercivity of the magnetic coating on the storage device. Hard drive platters have a coercivity of a few thousand Oersteds, which means a field of the same number of Gauss is needed to demagnetise them. The ferrite magnet on a computer's PC speaker, assuming it's not shielded, will have a surface field strength of only about a thousand Gauss, so it won't endanger hard drive data even if the drive's right next to it. Even 10,000 Gauss rare earth magnets can't wipe a hard drive if they're not sitting on top of it.

So just fiddling with magnets as you read Slashdot will not wipe your hard drive.

(If you find yourself getting aroused when I talk about magnets, by the way, check out this page. But don't e-mail me, you freak.)

Try explaining inverse cube laws and Oersteds and Gauss to the average Microsoft Outlook user, though.

In order to avoid mopping up drool from confused consumers, all of us Amazingly Knowledgeable, Windswept and Interesting Science and Technology Gods have fabricated a more easily explained conventional wisdom, which says: "Don't put a magnet near your computer, or, roughly speaking, your liver will catch fire."

Computing is full of conventional wisdom like this.

Electro-static discharge (ESD) precautions, for instance.

Some Computing For The Dumb books say that you should buy yourself an anti-static wrist strap and hook it up to something earthed while you work on your computer, lest your house turn to liquid manure.

Other books tell you to simply leave the computer plugged in and turned off, and touch the chassis metalwork periodically while you work.

And a lot of people fool with computer gear on top of an old newspaper on the dining-room table, and don't notice anything getting fried.

ESD effects can actually be quite subtle, and cumulative.

If you shuffle across a nylon carpet, jump in the air and strike a six-inch spark onto your well-earthed GeForce4 Unobtanium 9000 card, then yes, that trucker's probably toast.

And yes, it's possible to destroy hardware with static discharges smaller than a couple of hundred volts. You won't feel that, and you can build up that much of a charge on any fairly dry day, while sitting in a perfectly ordinary chair (as opposed to those Van de Graaff Industries plastic school chairs).

For this reason, telling people that just breathing makes them into potential chip-killers is not completely alarmist. Modern computers are riddled with highly static sensitive complementary metal oxide semiconductor (CMOS) chips. It is perfectly possible to kill your CPU stone dead just by touching its pins.

But, demonstrably, lots of people assemble and upgrade computers without taking any particular ESD precautions, and the PCs proceed to work.

Some of these people have just managed to luck out and not measurably damage anything at all. But most of them, including me upon occasion, haven't.

Static damage can be an all-or-nothing deal - take new gadget out of anti-static bag, instantly zorch it. But more commonly, the static damage is slight enough that the component in question is still well within tolerances, and works fine. Look at waveforms from said component on your oscilloscope and you'll see that what used to look like a Texas mesa in profile now looks like a really alarming ski-slope instead. But it works.

(You do have an oscilloscope, don't you? No, I'm sure you've got one. Look behind your plethysmograph. No, that's your sphygmomanometer, silly. Try again.)

Even if there's some static damage, the waveforms are still likely to be close enough for government work. The damage probably won't cause your computer to develop horrible stomach-turning instability problems.


In this case, the unconventional wisdom is "take reasonable ESD precautions, unless you can't, and if you can't, don't get too bothered about it".

That'd look as lousy on a T-shirt as the truth about magnets, though.

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