A Story About `Magic'

A Story About `Magic'
=======================

Some years ago, I (GLS) 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 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.

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'.

1994: Another explanation of this story has since been offered.  Note
that the switch body was 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.  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.  This was probably discovered by
someone who found out the hard way that there was a potential
difference between the two, and who then wired in the switch as a
joke.