Magic Switch Story
Some years ago, I was snooping around in the cabinets that housed the MIT AI
Lab's PDP-10, and 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
You don't touch an unknown switch on a computer without knowing what it does,
because you might crash the computer. The switch was labelled 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"
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! The other end of the wire did disappear into the maze of wires inside the
computer, but it's a basic fact of electricity that a switch can't do anything
unless there are two wires connected to it. This switch had a wire connected on
one side and no wire on its other side.
It was clear that this switch was someone's idea of a silly joke. Convinced by
our reasoning that the switch was inoperative, we flipped it. The computer
Imagine our utter astonishment. We wrote it off as coincidence, but nevertheless
restored the switch to the "more magic" position before reviving the computer.
A year later, I told this story to yet 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 fooling him with a bogus saga. To
prove it to him, I showed him the very switch, still glued to the cabinet frame
with only one wire connected to it, still in the "more magic" position. We
scrutinized the switch and its lone connection, and found that the other end of
the wire, though connected to the computer wiring, was connected to a ground
pin. That clearly made the switch doubly useless: not only was it electrically
nonoperative, but it was connected to a place that couldn't affect anything
anyway. So we flipped the switch.
The computer promptly crashed.
This time we 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.
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-of-a-second
pulses went through it. But we'll never know for sure; all we can really say is
that the switch was magic.
I still have that switch in my basement. Maybe I'm silly, but I usually keep it
set on "more magic".
magic number « Magic Paper « magic smoke « Magic
Switch Story » Magma » Magma2 » MagmaLISP