Nicely done.

I like this stuff with animation.

You could add an arrow for conventional current flow with 6amps stamped beside it.
Posted on 2003-04-10 19:53:21 by IwasTitan

And no, you don't seem any smarter if you insist it flows the other way, you'll just confuse peoiple until you too pretend it goes backwards.

Well put ;)
Posted on 2003-04-12 14:48:02 by NaN
There is another aside on ground, a supposedly true story I found on the internet which I can't find again anymore. Perhaps it wasn't true after all, or maybe the server deleted it after it wasn't being updated anymore, or maybe just the demons that plagued the MAGIC SWITCH did not want it to be known anymore...

One might point out that the electrical symbol for ground and chassis are different. Yet many computer designers connect the ground and the chassis... So why would their symbols have to be different?

Perhaps we should go back to the early, early days of computers, when they were behemoths the size of rooms, and programmers often entered the computer, literally, to fix it. The rooms were air-conditioned, of course, since computers just don't run well when warm. Let us go back to an assistant programmer in those days (call him John).

Now this John was exploring the internals of the computer to learn more about it when he encountered a peculiar metal switch taped to the chassis of the computer, with a single wire running from it to the inner parts of the computer.

The metal switch was labelled 'magic' and 'more magic'. It was currently set to the 'more magic' setting.

Out of curiousity, John flipped the switch to 'magic.'

Almost immediately the computer crashed. All running applications stopped, and John, scared, immediately flipped the switch back to 'more magic,' and started the long process of restarting the computer.

Time passed by and John kept the secret of the MAGIC SWITCH. And soon he became a head programmer, with an assistant programmer under him (call him Steve). And, like he had been made to do long, long before, John sent Steve into the computer to learn more about it.

Steve, as it happened, may have been just as curious as John had been when he was an assistant programmer. Or maybe the same goblins that lead John to that corner of the computer where the MAGIC SWITCH was now lead Steve to the same location. Regardless, when he came back, he asked John what the switch was.

John told him what it did. Steve, incredulous, did not believe it. He pointed out that the switch was only taped to the chassis. There was a single wire from the switch to anything: that single wire was connected to the GROUND line of the computer. The other terminal of the switch was not connected to anything at all. In electrical parlance, the switch was hanging - that is, it was connected at only one point, and the other terminal was not doing anything -- therefore it could not possibly have any effect on the circuit - on the computer. What was more, the switch was connected to the GROUND line - which made it seem even less likely to do anything at all.

They went back to the switch and flipped it to 'magic.' And again the computer crashed - all applications stopped in their tracks, the operating system did not even have time to panic. It just stopped.

Again John hurriedly flipped it back to 'more magic,' and, nervously, the two reset the computer, and finally called the director of the computer center (call him Bob).

Bob was as incredulous as Steve. The switch was hanging, after all, and could not have any effect on the computer. Connected to ground - it would be at zero voltage, and therefore could not possibly cause any harm. Together all three of them went to the computer, looked at the switch again, and Bob flipped it to 'magic.'

Nothing happened.

So Bob instructed the two programmers to cut the crap and cut the wires of the switch off. He tore off the tape of the switch itself while John and Steve carefully cut the single wire going to the ground line of the computer. The computer ran long and well after that, and never crashed again.

Inexplicable? Well... perhaps it can be explained.

The switch was metal. Now, some (cheap) metal switches are designed so that the casing of the switch is in fact electrically connected to one or the other terminal. Taped against the chassis, the switch in essence was connected to both the ground and the chassis - it wasn't hanging!

Apparently, the 'off' position was the 'more magic' position. When it was flipped to the 'magic' position, the switch turned on and connected the chassis to the ground.

Now after a long long time, any two things that aren't connected to ground, and aren't connected electrically to each other, will 'drift' in their voltages as a result of static electricity. While the computer has little to do that can cause static electricity build up, the people entering it did: wearing whatever clothes they have, they would move around in the computer room, increasing the static electricity on the computer chassis. And the place was air-conditioned: cold air is less humid, and the lower the humidity of the air, the less conductive it is. The less conductive the air, the more likely static electricity will build up.

And the computer, large and slow as it was, was kept on for months at a time; after all, a lot of people used the computer, and some of them might need to do several computations that might take overnight to finish. More likely than not, the chassis and ground were already a few volts apart after some months being left on.

Then when the switch was flipped to 'magic' or on, current flowed from chassis to ground. Now the current has to go somewhere: more likely than not it overloaded the computer circuits, causing it to stop.

And of course, when Bob himself flipped the switch, nothing happened! That was because he did it only a few hours after Steve and John did. This was not enough time to cause voltage build up between the chassis and ground.

Of course, that is IF the switch was, indeed, one of those types where one terminal was connected to it casing. But John still has the switch and refuses to let anyone see it. He swears that, as long as he kept the switch in the 'more magic' position, he hasn't encountered a serious computer problem...
Posted on 2003-04-14 22:59:37 by AmkG
The early days of computers abound in rumored stories like this. Mainframe computing was just strange, sysops seemed to be people who's job it was to prevent as many people as possible from using the precious hardware. Lord forbid if anyone actually SAW the computer too. It was usually kept secret behind a minimum of three locked doors.

Early 50's one rumor started at one site the computer just did not like women. Seems every time some fool let a woman inside the mighty (heavily air conditioned) computer room the beast would crash. Slowly, several computer hubs traded similar stories. Computers just didn't like women.

Supposedly it was traced down to their undergarments. Synthetics just weren't popular in clothing, most everything was cotton... except the newer wonder bras were nylon, which as we all know is a great static generator.
Posted on 2003-04-15 06:44:34 by Ernie
Reminds me of a floating ground.

PFM (Pure F**king Magic)
Posted on 2003-04-15 14:36:49 by alpha

Supposedly it was traced down to their undergarments. Synthetics just weren't popular in clothing, most everything was cotton... except the newer wonder bras were nylon, which as we all know is a great static generator.

sign outside computer room:
To all women: please remove your bras before entering computer room.:grin: :grin: :tongue:
Posted on 2003-04-15 20:35:41 by AmkG
Hi all
the story about the magic switch is documented in "the Jargon file" by Eric Raymond, you could find floating in the net (I found a copy in and there are several in text and html liying around, i recomend you to read it complete, its a funy and ilustrative document about our kind, now, aboout the ground, in electronics there is "electrical ground" and "chasis (or earth ) ground) and while they are considered 0 volts they could be at diferent potential, and unles by design they harae electricaly conected , you could get funy beavior if you do so, ( like a crased computer)

Posted on 2003-04-20 00:02:49 by Carlos
Ya, the idea, meaning, and most importantly, the implications of ground is something not to be taken lightly.

A person should really understand ground before moving on in any direction.

I reciently designed a series of Remote Radio unit located at pumping stations (mostly in country fields). Doing so, i have my transmitter in a Nema 4 cabinet on an aluminum post, with an antenna on top of it.

In protection against lightning strikes, there was ALOT of specific ground design to ensure that the electronics wont be affected by a direct strike.

Since voltage is a POTENTIAL DIFFERENCE. If the antenna takes a hit, the entire post turns into a big resistor, current flows, and voltage drop builds across it. I designed it so that whatever the voltage is at the Cabinet Ground, the incomming antenna ground will also be at this potential (with a surge surpressor of course).

In essence all electronics will RAISE and FALL 100000V from a strike, and all 5 volt signals will aslo raise and fall accordingly. This only done because of the gound is the reference for all electronic signals. My job was to design is to limit any posibility of higher voltages entering the cabinet ;)

The thing to note here, that for a piece of steel 20" high. The speed and intensity of a strike can easily create 100000V across it! So cable entering the cabinet at the top, and bottom may have largly different GROUND potentials. Not realizing this and alot of other stuff will destroy any electronics!

So far so good ;)
Posted on 2003-04-20 00:18:33 by NaN