A story about a strange ‘magic switch’ at MIT

The Jargon Paper is “a comprehensive compendium of hacker slang that illuminates many aspects of hacker tradition, folklore, and humor.” The first version was written in 1975 by Raphael Finke, a computer scientist at Stanford University. It was maintained by different people and hosted on different computers over the years, but in recent decades the canonical version has been edited by Eric Raymond (author of the influential article “The Cathedral and the Bazaar”).

A few years ago, I (GLS) peeked around the cabinets of the PDP-10 at the MIT Artificial Intelligence Laboratory and noticed a small switch stuck to the frame of one of the cabinets. This is apparently a homemade job, added by a hardware hacker in the lab (no one knows who).

You don’t touch an unknown switch on your computer without knowing what it does, because you could crash your computer. This switch is labelled the most useless. It has two positions, and the words “magic” and “more magic” are scribbled in pencil on the metal switch body. The switch is in the “more magical” position.

I called another hacker to come and have a look. He had never seen a switch before either. On closer inspection, the switch has only one wire! The other end of the wire does disappear into the maze of wires inside the computer, but the fact that the switch can’t do anything unless there are two wires connected to it is a fundamental fact of electricity. The switch has wires attached to one side and no wires to the other.

Clearly, the switch was someone’s idea of ​​a silly joke. We were convinced of the reasoning that the switch didn’t work, and we flipped it. The computer crashed instantly.

Imagine our complete surprise. We wrote it as a coincidence, but returned the switch to the “more magical” position before restoring the computer.

A year later, I told the story to another hacker, and I remember David Moon. He clearly doubts my sanity, or my supernatural belief in the power of this switch, or thinks I’m fooling him with a false legend. To prove it to him, I showed him that switch, which was still glued to the cabinet frame, with only one wire attached, still in a “more magical” position. We scrutinized the switch and its individual connections and found that the other end of the wire, while connected to the computer wiring, was connected to the ground pin. This obviously renders the switch useless: not only is it not electrically functional, but it’s connected to a place where it shouldn’t affect anything anyway. So we flipped the switch.

The computer crashed quickly.

This time, we ran for Richard Greenblatt, a veteran MIT hacker who was close at hand. He had never noticed the switch before either. He inspected it, decided it was useless, took some diagonal cutters and scooped it out. Then we restored the computer and it has been running fine since then.

We still don’t know how the switch crashed the machine. There is a theory that some circuit near the ground pin is fringe, and flipping the switch changes the capacitance enough to perturb the circuit when a millionth of a second pulse passes through it. But we can never be sure; all we can say is that the switch is magical.

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

1994: An alternate interpretation of the story has since been proposed. Note that the switch body is metal. It is assumed that the non-connected side of the switch is connected to the switch body (usually the body is connected to a separate ground lug, but there are exceptions). The body is connected to the computer case, which, presumably, is grounded. Now the circuit ground inside the machine is not necessarily at the same potential as the case ground, so flipping the switch connects the circuit ground to the case ground, causing a voltage drop/jump, which resets the machine. This might be the hard way for someone to spot a potential difference between the two and playfully connect the switch.

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