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, DavidMoon 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".

GLS

(1995-02-22)