JargonFile/entries/A Story About ‘Magic'.txt

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.