electricity - Why can touching an iPod charger sometimes cause one's skin to tingle?

I was visiting Shanghai recently. One afternoon, while sitting on the sofa, I inadvertently brushed a family member's arm. To my surprise, I felt an unnatural tingling from his skin—quite like a mild version of the shock from a (toy) shock pen or the slightly numbing vibration from an electric shaver. I could consistently feel the tingling by running my knuckle lightly across the family member's skin.

The family member had been using an iPod Touch plugged into a Chinese power strip via the stock Apple charger. The tingling persisted as long as at least one part of his body contacted the iPod and even when he touched the metal end of the charging cable directly with his fingertips.

More observations:

• The tingling persisted when he stood on a plastic stool instead of the sofa.


• The tingling did not appear when we plugged the iPod into a power outlet at a hotel we later visited in a different city in China.

Unfortunately, I am no longer in China, so I will not be able to conduct additional tests. Does anyone have a scientific explanation for this phenomenon as well as what might cause it to occur only occasionally?

Answer

You are feeling an electric current because the "live" wire of the electrical outlet is (capacitively) connected to the "ground" of your phone.

The same effect can sometimes be felt in the US (or wherever "back home" is for you) with an improperly wired desk lamp - or one that has an unpolarized plug (so you can plug it in "backwards" - that is, with the wire that should be neutral getting the AC fluctuations of the live wire and vice versa).

The electric field induces a small current in you because your body has a capacitance relative to "the world". You don't need to close the circuit - it is enough to touch the object (a conductive part like the external antenna). That is why standing in a plastic chair doesn't make it go away.

The amount of charge flowing is small (because your body's capacitance is small and the voltage is not that high) - but the effect is real. And it usually means something is off with the wiring: in a well designed and built system this should not happen.

Most recently I noticed the same thing using an Apple MacBook Air plugged in to an outlet in Europe. The frame of the laptop gave the same buzz. And this was a "genuine Apple component".

I believe it is harmless. But real.