Run your finger across the Powerbook, for example just below the space bar, and sometimes it doesn't feel smooth - the finger vibrates as if there is a very fine ribbing on the metal. This is strange, because the same metal feels totally smooth to the same finger at other times. So we decided to investigate further.
It seems that sometimes if you're sitting with a Powerbook on your lap, your skin feels rough to someone else touching it. And you can give them small electric shocks. It probably helped that the other person also had a Powerbook on their lap. However, the most "interesting" thing you can do is rub your finger on the skin just in front of you co-experimenter's ear. Not only will it feel rough, but the will hear a low buzzing. We're not sure of the frequency, but we have this suspicion that it's 50Hz. (At least for experiments conducted in the UK). So what's up?
Apple power adaptors for Powerbooks consist of a small white "brick" into which various leads can be plugged. In the US a long earthed lead is supplied. In the UK, a long earthed lead is supplied, but also a small triangular unearthed plug. US leads can be used in the UK with a mains socket adaptor, but some of these only connect the live and neutral, leaving the earth unconnected.
It seems that if you're using the power supply unearthed, for various of the above permutations including the official Apple product for the UK, the aluminium skin of the powerbook is carrying a small current. I have a suspicion that it's earthed via the outher sheath of the power connector into the laptop, that earth cable goes all the way back to the brick, where it floats because there is no onward earthing, and that nice long wire gets a current induced from the mains hum, or possibly because some part of the AC circuitry is earthed to it. And if someone who is nicely earthed (for example using a Powerbook attached to an earthed power supply) touches the Powerbook, or a user "earthed" to it, you can create small shocks. My multimeter suggests it's about 0.4V AC between the earthed Powerbook. Shocking!