Shri Khalpada walks through the two main kinds of touchscreen, with interactive demos, at PerThirtySix.
Resistive screens stack two conductive layers with spacer dots between them. The controller puts a voltage slope across one layer and reads the other. Press down and the layers meet at one point, and the voltage read there says how far across the press was. Two readings, one for X and one for Y, happen a hundred or more times a second. Pressure is all it takes, so a gloved hand or a stylus works. Two fingers do not: the readings blend into one point in the middle.
Capacitive screens instead use a grid of sender and listener electrodes under the glass. Your finger, being salty water, pulls part of the field away and the listener gets less signal.
Khalpada notes a phone has only a few hundred crossings, not thousands, and that controllers find the strongest crossing then average its neighbors for sub-grid precision. Palms are ignored because their cluster is too big. Water causes trouble when your finger touches it, smearing the touch across the screen.
See more at perthirtysix.com.
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