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(Full disclosure — I posted this first on Physics SE and it was suggested it might be a better fit here.)

If I place a ring on the screen of my phone, and then try to scroll with my finger inside the ring (first diagram), the ring and my finger move up and down, but the screen doesn't scroll. If I leave the ring on the screen, but put my finger to one side of it (second diagram), it does scroll.

Sketch of an iPhone with a gold wedding ring on the 'screen'. The tip of a finger is placed inside the ring, and arrows indicate it's being moved up and down. Sketch of an iPhone with a gold wedding ring on the 'screen'. A finger is on the 'screen' next to the ring, and arrows indicate it's being moved up and down.

I've repeated thing with several rings, and have found that if the ring is flat (a plain band) and touches the screen at every point around it, scrolling doesn't work. If the ring doesn't touch the screen all the way around (either because it's shaped, or has a large stone) my phone will still scroll. These results are consistent with both gold and silver rings.

Can anybody explain why this is? I think I understand the theory behind capacitive touch screens (which is what my phone has) but I can't understand why the metal would only have this effect when it completely surrounds my finger. It isn't working as a second 'finger', because if it were it would zoom rather than scroll in the second scenario.

The above was seen on my phone (iPhone SE 2020), but my Kindle Fire tablet doesn't display the same behaviour, nor does a Huawei mediapad, presumably because they use a different type of touchscreen.

BeginTheBeguine
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    Probably something in the software; the phone has some way of determining the difference between a finger pushing a button and your cheek pressing against the screen while you hold it up to your ear, and it's probably based on the size of the thing it sees touching the screen. The ring + finger probably just makes for too large of a thing. – Hearth Sep 17 '21 at 04:33
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    We can only speculate what kind of components your device has and how it might detect or reject a touch in hardware and software. I'm afraid the answer might be quite boring, either the hardware can't detect that as being a valid touch operation because it is an object with unexpected parameters going off the charts, or it is rejected in software because it does not look like a touch operation that is within reasonable parameters for being an expected for a touch operation.. So we can't know why your device works the way it does. – Justme Sep 17 '21 at 05:15
  • A fun game :-). what the others said - it's unknowable but probably software related. The cheek on screen suggestion has some likelihood. Faces are too large to be accepted. A finger in a full ring looks like a wide single contact A ring not making full contact is seem as a random unrelated touch || These guesses may be completely wrong :-) . – Russell McMahon Sep 17 '21 at 12:46

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