A few weeks ago, I noticed that the aluminum enclosure of my unibody Macbook Pro had a strange when I brushed my hand across the surface. After some tinkering, I noticed that this only happened when the device was being used while charging and that it only happened when using my shorter, 2-prong, power cable — leading me to believe there was some sort of current leakage happening.

After some searching, I realized that this doesn’t seem to be a unique problem. Posts on Quora and StackExchange describe a similar phenomenon:

Whenever my MacBook Air is connected to the AC power and I have my hands on the palm rest, I feel it passes some electricity to me. If I unplug it from the AC power it stops doing that.

The consensus seems to be that this phenomenon is the result of an ungrounded charging circuit. Rather than pass excess current to the ground pin of an outlet, the charger is incorrectly leaking some of the current into the aluminum chassis of the device. Since the Macbook Pro (and many other Apple laptops) have a body made almost entirely of metal, the leaked current passes through the locations where a user typically rests their palms at a frequency roughly around 50Hz.

The strange vibrating sensation is the result of a phenomenon known as electrovibration — the rapid attraction and repulsion of the skin to an insulated ground plane (such as brushed aluminum), due to the changing magnetic field produced by the alternating current. While it is an unintended effect in my Macbook Pro, this same technique of creating electrovibrations has been used to create prototypes of touchscreens that are capable of modifying the texture and friction of the surface in different locations, such as TeslaTouch.

Originally published by Somatic Labs