Nib asked
How do touch screens detect touch?
Most phone screens do not feel pressure or heat. They feel electricity: your finger conducts, and it disturbs a faint electric field above a grid of invisible wires.
8 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the guess
Most smartphones use capacitive screens, which sense the electrical effect of a conductive finger, not how hard you press or how warm you are.
Phones mostly feel electricity, not pressure.
a common mix-upMost phone touch screens do not sense pressure or heat. Older resistive screens did respond to pressure, but phones today mostly sense electricity.
2a finger is a wire
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.
3the grid
A capacitive screen has transparent electrodes, usually made of indium tin oxide, arranged as columns and rows. Each crossing forms a tiny capacitor that holds a small electric field.
Columns and rows of transparent wire; a field at every crossing.
4the dip
A conductive finger near the glass changes the electric field at nearby crossings, reducing their capacitance. That drop is what the touch controller measures.
A finger reduces the field at the crossings beneath it.
5finding the spot
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.
6bare finger
The sensing electrodes are directly beneath the cover glass. A bare fingertip is only a short distance away, so it produces a large change in the field.
Closer finger, bigger change.
7thick glove
A thick winter glove holds your finger too far away. The change is tiny, and the phone ignores you. Thin gloves often still work.
a common mix-upGloves are not blocked because they are cold or soft; a thick, insulating glove just keeps the finger too far from the sensor, so the change is too small to register.
8the whole idea
Capacitive touch: a grid of transparent electrodes holds a field at each crossing; a conductive fingertip reduces it nearby; a controller scans the grid and calculates the touch position.
Field, dip, scan, locate.
Every swipe is your fingertip quietly stealing a little electric field from a grid of wires you cannot see.