About thirty minutes ago, I was sitting at my desk. It's a hot summer day, I'm upstairs, and sweaty. My iPod nano was sitting on the desk, connected to my Apple Cinema Display, happily syncing and charging.
I shifted my arm to get a better angle with my mouse, and suddenly OW!
That's odd. The sharp edge of the iPod nano must have hit some nerve in my arm. I picked up the nano and a sharp jolt went right through my hand. Yeeeowch!
That's an electrical jolt. Why is my iPod electrified!? Is my charging cable frayed or defective somehow?
More importantly, what am I connected to? I'm sitting in a plastic chair on a hardwood floor. Current should not be flowing through ME. Then suddenly I realized: I've got my Macbook Pro sitting on my lap, charging.
Now, I am wearing pants. I'm not even in shorts! But it's hot and sticky enough, (and my arm was definitely sweaty enough) that I conducted a current from my iPod to my Macbook.
The Macbook is plugged into the socket on the near wall, and the Apple Cinema Display (which was not connected to my Macbook in any way) is plugged into another socket across the room. Those two sockets must be on different household circuits for there to be such a potential difference! (Either that or my Cinema Display is electrified!)
Yikes!
Being a good electronics geek, I tried to measure the potential difference and I couldn't do it. I don't know why, but the probes on my meter just wouldn't make a good connection with the metal on the Macbook. Even when I tried to measure resistance from point to point on the metal case, it measured Inf. ohms. But if I angled an LED just right, I could get the LED to light up, with one leg connected to the Macbook and one connected to the metal case of the Cinema Display. Pretty cool!
A few minutes later, I had another, more serious realization: I just took an electrical current across my heart. Several times, actually.
Now, as far as I know I have a strong heart, and don't feel any ill effects. But if you do have heart trouble, be careful with your metal cased Apple products!