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Comment by jordwest | Hacker News Reader

What's fascinating to me about this phenomenon is that you can actually feel the 60Hz AC by running your hand across the lid.

It feels as if the surface is slightly bumpy, but the width of the bumps changes as you adjust the speed your finger moves.

Unplug from mains, and the surface becomes smooth.

As the answer notes, this only happens when using the two-prong adapter without earthing. It also seems to be more noticeable when connected to older buildings.

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I felt this on an Apple laptop in California a couple of years ago, in an apartment with only two-prong outlets in it. I'd describe the feeling more like the laptop cover was vibrating, rather than bumpy. The feeling went away and the laptop cover became smooth and normal if I lifted my feet off of the ground.

Once all the outlets were rewired to be three-prong, with ground, I used a three-prong power cord and the problem went away.

Pretty scary feeling, and scary to think that two-prong outlets and appliances are legal anywhere. I'd also be surprised if Apple hasn't been sued at some point by someone getting electrocuted when using their laptop and a non-grounded, two-prong power cord.

> and scary to think that two-prong outlets and appliances are legal anywhere

In Japan, two prong outlets make up the vast majority of sockets, it kinda sucks.

I don't think I've ever seen a three-prong outlet in Japan (maybe only new installations?), but "earth screws" are quite common (any room prepared for an air conditioner will have one at least) http://www.hakusui-trading.co.jp/safety/img/img02.jpg

What? That's crazy looking! Why not just put the third prong in there if it's wired up to handle a screw?

What's the point?

Just guessing here but I think it's to allow easy grounding of old appliances and other equipment that might risk contact with electricity but are not grounded by default. Metal frame desk as example.

Very clever actually.

And the cable on the left here is what the included Apple MacBook power cable looks like, with the little earth wire sticking out to screw on http://rinare.com/wp-content/uploads/image_archive/110801_2....

I think it's just a legacy thing - historically, the only things that needed grounding were static appliances like fridges, washing machines and air conditioners. The manual in our fridge has this instruction for earthing http://mayoyo.tokyo/jIe.jpg

I need to revise my comment that "I don't think I've ever seen a three-prong outlet in Japan", because now that I have power sockets on my mind, I just saw a three-prong outlet at my gym. So I guess newer buildings have them.

They're starting to become more common in newer apartments. For some reason my apartment has one outlet panel with two of the three-prong outlets, but the rest are two-prong. Except for the air conditioner and laundry machine sockets, which have the earth screw like you described.

120v isn't going to hurt you, just scare you. You have too much resistance.

You can die with 50V. It's not the voltage that is important but the amps.

You can also die if you stick your finger for a long time (24H) in a small current.

http://electronics.stackexchange.com/questions/19103/how-muc...

This doesn't only happen with the two-prong adapters, I have noticed this on a few MacBooks, plugged in using the standard 3-prong UK plugs (sometimes with my phone too). An even weirder feeling is when someone else is touching the laptop, if you run your finger over their arm you'll notice the same bumpy feeling.

Oh my god, this is the feeling my ex and I could never figure out. One of us would touch the other, and there was a weird vibrating sensation as one would move their hand along the other's skin. I can't believe I never thought that it might be the laptop.

This effect can be used for tactile feedback on touchscreens:

TeslaTouch: Electrovibration for Touch Surfaces:

http://www.ivanpoupyrev.com/e-library/2010/teslatouchUIST201...

Video: https://www.youtube.com/watch?v=FiCqlYKRlAA

So that's what it is! I assumed the source of that effect is some micro vibrations from fans, or because of some weird electromagnetic effects from parts placement. It happens both a on an HP ZBook and an older Macbook Air. Thanks for solving this mystery - moving to a better connector helped.

Well that explains what the weird feeling I used to feel when touching a 2015 Macbook I was using last year. It felt exactly like that (as though the surface was bumpy) though I never got far enough into debugging it to figure out that it was actual charge leakage onto the case and that I was feeling the AC current. That's a bit scary.

Question: aren't laptops powered off DC (there's a rectifier/transformer combo in the power brick) so how would AC get to the case of the laptop?

Laptops are indeed powered off of DC, which means that there will be a constant voltage measured across the positive and negative pins of the MagSafe plug.

However, the voltage measured from either pin to ground might very well fluctuate. Without the grounding pin on the power supply, the DC side of the supply may 'float' with respect to earth ground. If you plot the voltages measured from each pin to ground, it probably looks like a pair of sine waves, one shifted 20 volts above the other.

Right, so there's no relative phase change between negative and positive terminals but there is a phase change between the terminals and ground (which can be interpreted as a frequency relative to ground)? And since I'm grounded I experience that frequency? Interesting. Is there a relation between the "float frequency" and the AC freqency? Is it the same because of how commutators work?

I suspect that the float frequency is generally equal to the AC frequency, but I suppose it may be possible for it to be some subharmonic of the switching frequency, assuming you've got a switching power supply.

The socket isn't grounded the laptop on your desk and you are, so you get socked, I had the same thing with my tower case when I moved a flat I discovered that the socket I was using had the ground and hot terminals switched.

This is a common problem, the power cable from the power adapter to your laptop is also usually connected to ground, if the ground is hot then you get shocked.

Easy way to cause it -- connect something that's got it's own cheapo AC power supply (eg. phone charger, speakers) to your computer via USB. The DC grounds are linked via USB, but the different power supplies end up with a wildly different DC ground wrt AC ground from each other, which can cause buzzing noises or the surface feel you're talking about.

Bring back transformers! (ok, don't, but...)

I had this in my iPod touch. It feels "bumpy" when plugged into the wall. I thought I was going crazy.

I've experienced this with appliances other than computers, such as an old fluorescent desk lamp with a two prong cord, and I wonder if it was common before the adoption of double insulation. I looked at the computers in my house, and realized that they all have plastic enclosures.

My old 2011 MBP and MBA do this when plugged into European 240V power

This has been happening to me in both UK and Ireland, which use three pin plugs as standard, with every metal-bodied Mac I've owned since the Titanium PowerBook came out in 2001

I discovered you can also switch the on and off by connecting and disconnecting your feet from ground.

Feet on the floor => shocks.

Feet on the beam of a wooden stool => no shocks.

I'm deeply curious about this! Is it possible to recreate the situation at home, or does this require something miswired with the mains power supply or power adapter, or an older MacBook Pro?

connect the two prong adapter, take your shoes and socks off, feet on the ground... feel the vibe! the best shocks you get at the edges or at the pointy edges in that groove to lift the display up.

> feel the vibe! the best shocks [...]

Users enjoying electric shocks. Reality distortion field operating at nominal.

right, it's so powerful it's also interfering with your sarchasm detector, apparently.

My macbook was stolen years ago, but I remember it doing this exact thing. I never really looked it up or thought about it until now. It is a totally weird feeling for sure.

I am sure that's a feature by Apple, they want to give you a nice 9V tDCS running through your brain to make you more productive ;-)

Or it is a side-effect of Apple's reality distortion field :)

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