Neil's News

Magnetic Noise

2 October 2009

As we broke for lunch at DocEng, one of the engineers gathered his things in a neat pile. What caught my eye was that he put the power supply for his laptop on top of his laptop.

[Power supply on a laptop]

This made my heart skip a beat. Wouldn't the magnetic noise from the transformer rewrite the bits on his hard disk? This started a bit of a discussion, and nobody else shared my concern. So I pulled out my mobile phone, loaded the Tricorder application, switched to magnetic flux and set it down on the power supply.

[Tricorder readings of a power supply]

The result was visually striking. The power supply was leaking a powerful magnetic field, one which fluctuated significantly for no particular reason. Measurement of other laptop power supplies in the conference hall showed a distinct split: All the IBM ThinkPad power supplies were unshielded whereas all the Macintosh power supplies were shielded. The next question was whether this was enough to flip bits on magnetic media.

[Floppy disk under a power supply]

I loaded 720kb of random data onto three low-density floppy disks then read back the checksum. Two of these disks were placed underneath the power supplies of ThinkPad laptops, one was placed underneath the power supply of my MSI Wind web server. A week later I read back the data.

[CRC Error]

Both the disks under the ThinkPad power supplies could not be read at all, there was massive data loss. The disk under the MSI Wind power supply read back perfectly and the checksum matched.

Conclusion: It is not safe to place your plugged-in power supply next to your laptop. I wonder how common this corruption is. The tech support people at Google (whose help I needed to read and write the antique floppy disks) had never heard of such a corruption, but admitted that the resulting symptoms would just be explained away as one of Microsoft Windows' frequent system failures necessitating a re-imaging of the laptop.

I left Munich the day before Oktoberfest started. The combination of being a non-drinker and the skyrocketing hotel rates made a quick exit desirable. The culture shock hit me as soon as I landed in San Francisco. People barging onto the back of the bus without paying, disabled people begging on street corners, drugged out people wandering through traffic while shouting. It was enough to make me question whether I had chosen the correct country in which to live. However shortly after returning, Google threw its own Oktoberfest complete with wurst, schnitzel, pretzels and a very good Oom-pah band. Have I ever mentioned how much I love Google?

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