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Some years ago, I (GLS) was snooping around in the cabinets that housed<br>the MIT AI Lab's PDP-10, and noticed a little switch glued to the frame of one<br>cabinet. It was obviously a homebrew job, added by one of the lab's hardware<br>hackers (no one knows who).<br>You don't touch an unknown switch on a computer without knowing what it<br>does, because you might crash the computer. The switch was labeled in a most<br>unhelpful way. It had two positions, and scrawled in pencil on the metal<br>switch body were the words ‘magic' and ‘more magic'. The switch<br>was in the ‘more magic' position.<br>I called another hacker over to look at it. He had never seen the<br>switch before either. Closer examination revealed that the switch had only<br>one wire running to it! The other end of the wire did disappear into the maze<br>of wires inside the computer, but it's a basic fact of electricity that a<br>switch can't do anything unless there are two wires connected to it. This<br>switch had a wire connected on one side and no wire on its other side.<br>It was clear that this switch was someone's idea of a silly joke.<br>Convinced by our reasoning that the switch was inoperative, we flipped it.<br>The computer instantly crashed.<br>Imagine our utter astonishment. We wrote it off as coincidence, but<br>nevertheless restored the switch to the ‘more magic’ position<br>before reviving the computer.<br>A year later, I told this story to yet another hacker, David Moon as I<br>recall. He clearly doubted my sanity, or suspected me of a supernatural<br>belief in the power of this switch, or perhaps thought I was fooling him with<br>a bogus saga. To prove it to him, I showed him the very switch, still glued<br>to the cabinet frame with only one wire connected to it, still in the<br>‘more magic’ position. We scrutinized the switch and its lone<br>connection, and found that the other end of the wire, though connected to the<br>computer wiring, was connected to a ground pin. That clearly made the switch<br>doubly useless: not only was it electrically nonoperative, but it was<br>connected to a place that couldn't affect anything anyway. So we flipped the<br>switch.<br>The computer promptly crashed.<br>This time we ran for Richard Greenblatt, a long-time MIT hacker, who was<br>close at hand. He had never noticed the switch before, either. He inspected<br>it, concluded it was useless, got some diagonal cutters and<br>diked it out. We then revived the computer and it has<br>run fine ever since.<br>We still don't know how the switch crashed the machine. There is a<br>theory that some circuit near the ground pin was marginal, and flipping the<br>switch changed the electrical capacitance enough to upset the circuit as<br>millionth-of-a-second pulses went through it. But we'll never know for sure;<br>all we can really say is that the switch was<br>magic.<br>I still have that switch in my basement. Maybe I'm silly, but I usually<br>keep it set on ‘more magic’.<br>1994: Another explanation of this story has since been offered. Note<br>that the switch body was metal. Suppose that the non-connected side of the<br>switch was connected to the switch body (usually the body is connected to a<br>separate earth lug, but there are exceptions). The body is connected to the<br>computer case, which is, presumably, grounded. Now the circuit ground within<br>the machine isn't necessarily at the same potential as the case ground, so<br>flipping the switch connected the circuit ground to the case ground, causing a<br>voltage drop/jump which reset the machine. This was probably discovered by<br>someone who found out the hard way that there was a potential difference<br>between the two, and who then wired in the switch as a joke.

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