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So that I wouldn't accidentally touch the screen while downloading updates, my iPad was face down on the bed near me. When I inadvertently touched the back of it and moved my hand, I felt a vibration. I experimented, and found that if I did not move my hand, there was no vibration. And I thought I actually heard a faint sound when I moved my hand. After a while of examining this, I realized that it sounded like mains hum. So I unplugged the charging cable and the phenomenon stopped. Plugged the cable back in, and the phenomenon did not return. But it did return, though not as pronounced, when I picked up the tablet to enter this question.

So the question is, how can sliding fingers over the smooth outside surface of a device cause an internal vibration to come out when touching it with the same non-moving fingers doesn't? Hmmm, when I put it that way, it sounds more like a Physics question than an Apple question.

WGroleau
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  • Hmm, a related question is that if the vibration is due to the design of the switching power supply at the other end of the cable, why have I never observed this effect with other devices plugged into the same adapter? I have observed it on other occasions with the iPad. – WGroleau Mar 22 '17 at 21:08
  • Maybe the size at shape of the iPad makes it susceptible to resonance at 50 Hz and the finger motion provides enough additional energy to activate the resonance. But then why would it not happen after unplugging and reconnecting the cable? It was definitely not a Twilight Zone theme hum. :-) – WGroleau Mar 22 '17 at 21:11
  • Can you add the exact model of iPad to your question and clarify whether the charger and cable are genuine Apple products? As an aside, it'd be better to add your two comments above to the question itself - maybe as additional thoughts or something - rather than using the comments fields. – Monomeeth Mar 22 '17 at 21:51
  • My MacBook Pro does this as well when plugged in, so it is not just the iPad with this strange phenomenon. – jacksonwelsh Mar 22 '17 at 23:22
  • It's the switch mode PSU, lots of them do it. It's within design tolerance. I don't know why it's more noticeable when you move your finger, but it's something I've observed too, on many occasions with many devices. I actually use it as a simple 1st diagnostic at work for 'is this PSU working?' by grabbing the live [12v] end. http://apple.stackexchange.com/questions/72566/shocking-macbook-stubby-earthed-charger-uk/193509#193509 – Tetsujin Mar 23 '17 at 07:44
  • @Tetsujin: your answer sounds reasonable, but it is definitely a vibration and not current. I was lying on a foam mattress with a plastic cover, so I was definitely not a path to earth. And I could hear the hum; not a high-frequency hum either. Yesterday was not an Apple adapter, but I have observed the same effect with the Apple adaptor. – WGroleau Mar 23 '17 at 13:22
  • If it were pure vibration it would also work if you wore rubber gloves. I bet it doesn't, nor would it affect any small object placed there as a primitive motion-sensor. It feels like vibration, but it's not. – Tetsujin Mar 23 '17 at 13:25
  • I do not have any rubber gloves to try that. I worked in electronics for many years, but my last thirty were in software. However, my suspicions are that a current too small to feel like current¹ would (1) not be audible; and (2) would not exist without somewhere to go; and (3) would not cease when the contact surface is motionless. I was not wearing rubber gloves, but I was lying on a rubbery plastic. ¹Yes, I do know what current feels like, unfortunately, both hazardous ones and faint ones. – WGroleau Mar 23 '17 at 18:57
  • Whilst I fully agree with your suppositions, you've still to believe mine... or the hundreds of similar posts across the interweb, all worrying about a misinterpretation of what induced current feels like when holding the blunt end of a switch-mode supply. You don't need a path to earth, btw, you're 'ground enough' to feel the effect. My work boots are quite specifically no path to earth; if the phenomenon didn't work there would be little point in my using it as an 'is this live?' test. – Tetsujin Mar 24 '17 at 12:37

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