Magic Switch Story

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 knows who).

   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" position.

   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 instantly 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.

   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 {dike}d 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".

   {GLS}

   (1995-02-22)