science continues to ruin everything


I’ve been meaning to write about the Breaking Bad season premiere all week; now with the next episode about to air, my window for having anyone care about the viability of last week’s (spoiler alert!) magnet caper is rapidly closing. Nevertheless!

I was curious to know whether the episode’s scheme — which centers around the use of a salvage yard electromagnet to erase a laptop drive from outside a police station’s walls — was at all plausible. I imagine the Mythbusters will tackle this in highly entertaining fashion a year or so from now, but I wanted quicker answers. Conveniently enough, my friend M works at a related job, figuring out the chemistry that allows hard drive platters to be coated with various metals. So I wrote her and asked if she knew anything about whether this was plausible. Not specifically, she said. But! Her all-around science talent and experience provided some promising leads:

I’m not sure how feasible this is. You would need to generate a strong enough field, get the field close enough, and also sustain the field long enough. I think that article mentioned that the power from the batteries were an issue and I think that is the biggest obstacle for a “portable” system. Not sure if the car batteries could sustain the juice long enough for the hard drive to get completely erased. Also distance is an issue. I think the field drops off exponentially away from the source, and other materials in the way (like the building wall) can dissipate the field depending on its dielectric properties. I don’t know how well computer hard drives are shielded or what strength field you’d need to erase one, but it could be possible. Do you know what you would need for that in terms of strength and time? Seconds? Minutes?

We never erased a computer when I was in grad school, but we always kept metal and electronics outside the 5Gauss line when working near magnets. In the fringe field of a couple Gauss outside this, it was strong enough to erase subway tickets but not credit cards and definitely not a computer. To get a feel for lengthscales, a magnet of ~90,000G had dissipated to 5G by about 7-9ft away from the magnet. Metal would not get pulled from our hands toward the magnet unless we were within ~3 feet away.

I would be curious to find out how strong a field you need to lift a car though. I thought those junkyard magnets you have to be really close to the surface before it actually sticks?

This led me to some productive Googling — well, productive in a certain sense — that turned up a few more interesting details. The following has been written by a guy who never even took enough physics to get through Maxwell’s equations. Still, I think it’s not too hard to reach a plausible conclusion through some back-of-the-envelopery.

There are basically two considerations that M is pointing to: field strength and how easy it is to erase a given type of magnetic media.

On the field strength side, the news is not good for Walt and Jesse. Unlike most emissive sources (light bulbs; radioactive materials), magnetic field strength declines with the cube of distance rather than the square. Exactly why has something to do with the nonexistence of magnetic monopoles (outside of Star Control 2 anyway) and seems to be one of those mind-bending situations where reality knuckles under to some particularly inescapable math. But the upshot is that magnetic fields get weaker very, very quickly as your distance from them increases — faster than your experience with other radiative sources might make you think.

But how strong would the field be at its source, anyway? Here it’s tough to say: salvage magnets seem to be specced by how much scrap iron they can lift, not the precise attributes of the fields they generate. But MRI machines top out around 30,000 Gauss. Is a salvage magnet more powerful? M subsequently warned me about reading too much into the fancy cryogenic cooling of an MRI’s superconducting magnet versus the air-cooled conventional tech in the salvage magnet. They’re different machines built for different things, with very different field shapes, she stressed. All this is true. Still, to me it seems at least unlikely that a salvage magnet could outpace an MRI machine. And judging by the example distances and field strengths in M’s email, it would clearly need to.

Then there’s the question of how much of a field it takes to erase a hard drive. I know a little bit about the considerations here, having looked into magstripe reader technology back when I was fooling around with Metro’s farecards. The ease with which a magnetic medium can be altered is called its coercivity, and as M hints, there are high- and low-coercivity magstripe standards (for any that care, WMATA’s farecards are low-coercivity, and I think not even digital; based on my abortive experiments with them, I believe that they use an acoustic encoding scheme, though I’m not positive).

Anyway! How hard is it to flip a bit on a hard drive platter? Things get tricky here — coercivity is measured in Oersted rather than Gauss, and concerns the B component of a magnetic field rather than the H component (actually, neither are measured in those pre-SI units any more, but “Gauss” and “Oersted” sound a lot cooler than “amp-meter”). (H and B are linearly related based on some constants specific to each material (the fields are functionally identical outside the domain of a given magnetic medium), so all the above business about field strength still applies). Quantifying the coercivity of a typical hard drive — to say nothing of the magnetic shielding effect of the case and other junk around it — is not something I’ve been able to do.

But we have some circumstantial evidence. For one thing, any dedicated nerd will tell you that a broken hard drive is a great source for extremely powerful neodymium magnets. These have nothing to do with flipping the bits on the disk (they’re in place for the voice coil that positions the read/write head over the platter). But it does seem safe to say that having a very powerful magnet — powerful enough that, given a pair of ’em, you’ll have a hell of a time separating them with just your hands — mere centimeters away from a hard drive platter is not enough to influence the data on the disk one bit, even as it whirls through the magnet’s field at several thousand RPM. It therefore also seems pretty safe to say that you would need a noticeably strong magnetic field outside the device before data loss became an issue. In the show, of course, stuff flies all over the place, so in this respect, at least, Breaking Bad’s verisimilitude isn’t in question.

Finally, I am a little more optimistic about the viability of a battery power source than M is. This kind of project is a great way to wind up with a bunch of burning, half-melted plastic tubs of acid and lead (a horrifying clean-up problem, but I suppose Walt’s seen worse), but an array of lead-acid batteries really can deliver an impressive amount of juice (turning over an engine takes quite a lot of it). Judging from the afore-linked salvage magnet vendor’s site it looks like the show’s creators settled on a realistic voltage; and indeed, Vince Gilligan has said that this was something the writers wasted a bunch of time worrying about.

All in all, though, I think Walter and Jesse probably should’ve stayed in the chemistry lab rather than wandering over to the physics department: for all of Mike’s talk about the evidence room’s impregnability, it sure looked like it was just a cinderblock wall. I suspect some explosives and incendiaries would’ve done a better job of killing the data on that hard drive than an electromagnet could. After all, there’s a reason why geeks tend to talk about degaussing wands for sanitizing videotape, and thermite for securing old hard drives:

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Tom Lee


  • Explosives might do a better job, but it’s not exactly discrete in the same way as an electromagnet would be.

  • Actually, in the Breaking Bad scenario, the likelihood of success comes not from the magnetic flux density, but from the physcial impact. I have found that the quickest way to sanitize a reasonably modern hard drive is to hurl it against a concrete sidewalk, which tends to shatter the drive platters into a many little shards, like broken glass. If the screen was broken as the officer doing inventory said, the impact may have been sufficient to shatter the platters.

    I’m sure that two reasonably able-bodied men, with adrenaline coursing through their veins could have put the truck back on its tires and drove away, but hey…

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By Tom Lee