Tingle or vibration from the back of the iPad while charging
You may or may not have noticed, but if you run your hand along the back of the iPad (or even an iPod touch) while it’s charging, especially when using a wall power adapter, you may notice a vibration or tingle sensation on your hand. You might be wondering what it is. This article is short and sweet, so let’s explore.
Charging your iPad
When you plug your iPad into a wall outlet (or any charger for that matter), you would think the current should go into the device alone. Well, it doesn’t. Some of the charge is dispersed along the metal case by design. Here is a comment on Apple’s forum quoted from Apple’s support team regarding this issue:
There is measurable AC voltage across the external metal parts when an iPad charges. The measured voltage is within the SELV (Separated Extra-Low Voltage) limit, which means that the iPad is safe to touch. Additionally, the touch current is within the safety limit according to UL/IEC 60950 (Safety of Information Technology Equipment).
So, there you have it. This is by design and nothing to be concerned over. Although, what Apple should have done is take that current being dispersed onto the case surface and run it to an LED to soak it up so you feel nothing. Of course, that means the iPad would need an external LED, but it wouldn’t be a bad thing to know when the device is charging without having to turn it on.
Checking your iPad with Apple
Note, if you get anything more than a mild sensation from the back of the iPad, then you should take it back to Apple. The current you feel from the back should be minuscule. If you see any sparks or feel anything more than a slight vibration, your iPad might be electrically defective. If you’re unsure, take it to Apple and have them check it out.
So, there you go.
 Apple’s Discussion Forum Comentary on this issue