Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
how do touch screens detect touch
You would guess the screen feels your press. It mostly does not. Most phones are not sensing pressure or warmth at all.
What it feels is electricity. Your body conducts, so a fingertip is like a bit of wire, and the screen can sense wire coming close.
Under the glass is a grid of see-through wires, columns in one layer and rows in another. Every crossing holds a faint electric field.
Bring a finger near and it steals some of that field at the crossings beneath it. Their signal dips, a little, just where you touched.
A chip scans every crossing many times a second, finds the dips, and blends the neighbours to find the finger's centre, even between wires.
Seen from the side, the sensing wires sit just under the glass. A bare fingertip is very close, so the field changes a lot.
A thick winter glove holds your finger too far away. The change is tiny, and the phone ignores you. Thin gloves often still work.
So: a grid of invisible wires, a field at every crossing, a conducting finger that dips it, and a chip that finds where. Electricity, not pressure.
Every swipe is your fingertip quietly stealing a little electric field from a grid of wires you cannot see.