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Once upon a time, the Artificial Intelligence lab at Massachusetts Institute of Technology had a “magic switch” hooked up to their PDP-10. Every once in a while, you run across something totally random but far too interesting to put down. It’s not real clear where this tale originated but it caught our attention on social media and we thought you might like it.
Magic switch mystery
Back in the days when phone bills were printed on cards with holes punched in them to be read by computers, the PDP-10 was a stylish machine. Someone with the initials GLS apparently studied there back in the day. He discovered the magic switch but couldn’t keep his curiosity to himself. The power it once held is long gone along with punch cards, yet the switch remains immortal.
While “snooping around in the cabinets that housed the MIT AI Lab’s PDP-10,” the student “noticed a little switch glued to the frame of one cabinet. It was obviously a homebrew job, added by one of the lab’s hardware hackers (no one knows who.)”
He knew better than to touch an unknown switch. Especially one attached to a big hunk of hardware. Flipping it might crash the machine. “The switch was labeled in a most unhelpful way. It had two positions, and scrawled in pencil on the metal switch body were the words ‘magic’ and ‘more magic’. The switch was in the ‘more magic’ position.” He called in a consultant. “I called another hacker over to look at it.”
He had never seen the switch before either. “Closer examination revealed that the switch had only one wire running to it!” Basic electronics teaches a switch needs two wires to work. The mystery deepened when they traced down where the wire connected. A ground pin.
It looked like a joke, so they flipped it. The computer promptly crashed.
“Imagine our utter astonishment. We wrote it off as coincidence, but nevertheless restored the switch to the ‘more magic‘ position before reviving the computer.” About a year later, he told an associate about the magic switch. Of course, he wasn’t believed.
Is one ground the same as another
The magic switch was still there and set the same. After it crashed when flipped a second time, they “ran for Richard Greenblatt, a long-time MIT hacker, who was close at hand. He had never noticed the switch before, either.”
“He inspected it, concluded it was useless, got some diagonal cutters and diked it out. We then revived the computer and it has run fine ever since.” That’s the end of that mystery. Or is it?
While impossible to confirm, one possible theory is that because the switch body is metal, “suppose that the non-connected side of the switch was connected to the switch body (usually the body is connected to a separate earth lug, but there are exceptions.)”
“The body is connected to the computer case, which is, presumably, grounded.” Or else, it was simply magic.
“Now the circuit ground within the machine isn’t necessarily at the same potential as the case ground, so flipping the switch connected the circuit ground to the case ground, causing a voltage drop/jump which reset the machine.”
In any case, the author claimed, “I still have that switch in my basement. Maybe I’m silly, but I usually keep it set on ‘more magic.’“
