[Thorin]

While using my new 12" RevB powerbook on a train the other day, I noticed something slightly strange. The powerbook was charging from a socket on the train, and I was resting my bare arm on the painted metal arm rest. I started to notice that I was getting a sharp pain in a tiny area on my arm, where it was in contact with the arm rest. I ran my hand along it and noticed that there were a couple of tiny scratches out of the paint, but that they weren't sharp in anyway, this left me slightly confused. I then noticed that I only got the sharp pain while I was touching the case of my powerbook.

Yesterday I set my powerbook down on my desk, plugged in the mains charger, and then went to plug a 3.5mm jack into the head phone socket. This jack is connected to the line in on the sound card of my PC. While doing this, I set my hand down on the powerbook, and happened to touch the metal part of the 3.5mm jack. This caused a very loud noise to emenate from my speakers. I was clearly conducting a charge from the powerbook case, through my body, and into the 3.5mm jack and ultimately my speakers. I tried this again having unplugged the mains charger, and couldn't replicate it this way. The powerbook's aluminium casing is clearly carrying a charge. I assume it must be very mild (as both my soundcard, and computer are still working, and I haven't been electricuted), but this isn't really a desireable situation is it?

Please note, I wouldn't recommened replicating the situation above, in case you course damage to yourself, or computing equipment.

[nate_02]

Originally posted by Thorin:
While using my new 12" RevB powerbook on a train the other day, I noticed something slightly strange. The powerbook was charging from a socket on the train, and I was resting my bare arm on the painted metal arm rest. I started to notice that I was getting a sharp pain in a tiny area on my arm, where it was in contact with the arm rest. I ran my hand along it and noticed that there were a couple of tiny scratches out of the paint, but that they weren't sharp in anyway, this left me slightly confused. I then noticed that I only got the sharp pain while I was touching the case of my powerbook.

Yesterday I set my powerbook down on my desk, plugged in the mains charger, and then went to plug a 3.5mm jack into the head phone socket. This jack is connected to the line in on the sound card of my PC. While doing this, I set my hand down on the powerbook, and happened to touch the metal part of the 3.5mm jack. This caused a very loud noise to emenate from my speakers. I was clearly conducting a charge from the powerbook case, through my body, and into the 3.5mm jack and ultimately my speakers. I tried this again having unplugged the mains charger, and couldn't replicate it this way. The powerbook's aluminium casing is clearly carrying a charge. I assume it must be very mild (as both my soundcard, and computer are still working, and I haven't been electricuted), but this isn't really a desireable situation is it?

Please note, I wouldn't recommened replicating the situation above, in case you course damage to yourself, or computing equipment.

The reason why your speakers made a strange noise is because it is a live wire, not because your powerbook has a charge to it. If you tap on the end of your speaker cable it will probably make a noise through your speakers.

[Thorin]

Originally posted by nate_02:
The reason why your speakers made a strange noise is because it is a live wire, not because your powerbook has a charge to it. If you tap on the end of your speaker cable it will probably make a noise through your speakers.

Nope. If I hold the 3.5mm jack, I get a very faint noise. If I touch the powerbook (that is not plugged into the mains) while holding the 3.5mm jack, no difference. If i plug the powerbook into the mains, and then touch the powerbook case, while holding the 3.5mm jack, I get a very loud noise, that stops as soon as I take my hand off the powerbook.

[iRebound]

Originally posted by Thorin:
I started to notice that I was getting a sharp pain in a tiny area on my arm, where it was in contact with the arm rest. I ran my hand along it and noticed that there were a couple of tiny scratches out of the paint, but that they weren't sharp in anyway, this left me slightly confused. I then noticed that I only got the sharp pain while I was touching the case of my powerbook.

I've had the same experience with my TiBook 550. weird.

[macplus]

I would have expected the cases to be insulated but it sure sounds like a ground-loop is being established between two potentials. When there's enough resistance between two common points (your speaker's ground and the PB case ground) you get a voltage difference. It only takes millivolts and an area of sensitive nerves to feel the sting.
Same deal with the speakers, sensitivity is typically in the micro-volt range so you get that great buzzing sound. (The slight hum typically heard when tapping an audio-in connector is actually the electric field surrounding your body being amplified, swamped with 60Hz.)

I've had the same thing happen when working on a car with a 12v battery system. My leg barely brushed against the door-sill plate and I was holding solder in one hand while working on a live wire. Bzzz.

Pointless trivia- the addage goes that if you ever have to go over an electric fence, you are far better off gripping the wire as hard as you can rather than brushing up against it.

[Spheric Harlot]

Yes, I'm seeing ca. 95V (not a typo!) AC between the 12" Powerbook case and any grounded object.

It is occasionally painful, and now that I've finally measured it, I think I need to be calling up the Apple store.

-s*