I'm theoretically aware of grounding issues that can come up when using an oscilloscope [e.g. as explained in this video], but I can't make sense of this one:
I have a device under test (imagine an Arduino board) plugged into my laptop, which is plugged into the wall via a brand new Apple power supply which has only two pins (no ground). If I connect my probe ground lead to the shield of the device's USB connector, and then probe the device's GND my oscilloscope shows a 152.0V sine wave at 60Hz, offset by -10V!
When I unplug my laptop, this signal goes away – unless I happen to be touching both my laptop and the metal end of its power cord! In fact, my oscilloscope shows this "110VAC" signal when I probe the metal end of the Magsafe adapter.
Why would the outer "ground" of my MagSafe adapter have mains-level VAC on it relative to my oscilloscope's ground? They are plugged into the same outlet — is this expected or could there be something wrong with this building's wiring?
UPDATE: I realized I had a three-prong extension cord handy to swap out for the "duck bill" on this Apple power adapter. When I use that, the AC voltage measured on the outer part of the MagSafe connector goes away. Still disconcerting that it would be there when using the two prong plug, or is it just some sort of harmless low current leakage when connected that way?