A story about a strange “magic switch” at MIT | Boing Boing

A story about a strange "magic switch" at MIT |  Boing Boing

The Jargon File is “a comprehensive compendium of hacker jargon that illuminates many aspects of hackish lore, folklore, and humor.” The first version was compiled in 1975 by a Stanford computer scientist named Raphael Finke. It has been maintained by various people and hosted on various computers over the years, but in recent decades the canonical version was edited by Eric Raymond (author of the influential essay, “The Cathedral and the Bazaar”).

A few years ago, I (GLS) was looking through the cabinets that housed the MIT AI Lab’s PDP-10 and noticed a small switch attached to the frame of a cabinet. It was obviously a homebrew job, added by one of the lab’s hackers (nobody knows who).

Don’t touch an unfamiliar switch on a computer without knowing what it does, because you could crash the computer. The switch was labeled in the most useless way. It had two positions, and scrawled in pencil on the body of the metal switch were the words ‘magic’ and ‘more magic’. The switch was in the “most magical” position.

I called another hacker to look into it. I had never seen the change before either. Closer examination revealed that the switch only had one wire going to it! The other end of the wire disappeared into the maze of wires inside the computer, but it’s a basic fact of electricity that a switch can’t do anything unless two wires are connected. This switch had a wire attached to one side and no wire to the other side.

It was clear that this change was someone’s idea of ​​a silly joke. Convinced by our reasoning that the switch wasn’t working, we flipped it. The computer crashed instantly.

Imagine our absolute surprise. We wrote it off as a coincidence, but we nevertheless restored the switch to the “most magical” position before reviving the computer.

A year later, I told this story to another hacker, David Moon, as I recall. He clearly doubted my sanity, or suspected me of a supernatural belief in the power of this switch, or perhaps thought I was deceiving him with a bogus saga. To demonstrate this, I showed him the same switch, still attached to the cabinet frame with only one wire attached, still in the “most magical” position. We examined the switch and its unique connection and found that the other end of the wire, although connected to the computer wiring, was connected to a ground pin. This clearly made the switch doubly useless: not only did it not work electrically, but it was attached to a place that couldn’t affect anything anyway. So we flipped the switch.

The computer quickly crashed.

This time we ran into Richard Greenblatt, a longtime MIT hacker, who was nearby. I had never noticed the change before either. He inspected it, concluded it was useless, got some diagonal cutters and removed it. We then revived the computer and it has been working fine ever since.

We still don’t know how the switch crashed the machine. There is a theory that some circuit near the ground pin was marginal, and flipping the switch changed the electrical capacitance enough to upset the circuit as millionth-second pulses passed through it. But we will never know for sure; all we can really say is that the switch was magical.

I still have that switch in my basement. Maybe I’m silly, but I tend to keep it to “more magic”.

1994: Another explanation for this story has since been offered. Note that the body of the switch was metal. Assume that the unconnected side of the switch was connected to the body of the switch (usually the body is connected to a separate ground, but there are exceptions). The body is connected to the computer case, which is presumably connected to ground. Now the circuit ground inside the machine is not necessarily the same potential as the box ground, so flipping the switch connected the circuit ground to the box ground, causing a voltage drop/jump which resets the machine. This was probably discovered by someone who found out the hard way that there was a potential difference between the two and then connected the switch as a joke.


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