A touchscreen feels like magic — you touch glass and something happens. The secret is hidden just under the surface: a see-through grid of thin wires running across and down, each holding a tiny bit of electric charge. Here's the clever part. Your finger is slightly conductive, because you're mostly salty water, so when it comes close it pulls a little of that charge away from the nearest wires. The screen's brain is constantly scanning every row and every column, watching for a drop in charge. When it spots which row and which column lost the most, that crossing point is exactly where your finger is. It can even tell when two fingers touch at once, which is how pinching to zoom works. Try a glove and it stops working — the glove blocks the charge. In the simulator, drag the finger and switch to a glove to watch the touch vanish.
Most people think a touchscreen senses the pressure or warmth of your finger. In fact it senses charge: your conductive finger steals a tiny amount from a hidden grid, and the screen finds where the dip is biggest.
What's actually happening
The first touchscreens worked by pressure. Press hard enough and two layers squashed together to complete a circuit, which is why old supermarket terminals needed a firm jab and barely noticed a light tap. Modern phone screens don't work that way at all — you can wake them with the gentlest brush, they ignore a stylus that isn't conductive, and they sense several fingers at once. The mechanism behind all that is electric charge, and it lives in a layer so thin and transparent you never know it's there.
Bonded to the underside of the glass is a fine grid of conducting wires, transparent enough to see straight through, laid out as a set of vertical lines crossed by a set of horizontal lines. The screen keeps a small electric charge sitting on these wires. Now bring your finger close. Your body is a decent conductor, blood and skin are full of salty water, and it's effectively connected to ground, so when your fingertip nears a crossing point it siphons a little charge away from those particular wires. The amount is tiny, around a picofarad of capacitance, but the screen is built to notice exactly that. A controller chip sweeps along feeding charge to each vertical wire in turn while listening on each horizontal wire, building up a map of where the charge has dipped. The dip is sharpest right under your fingertip.
From that map the chip works out where you touched. It finds the column with the biggest drop and the row with the biggest drop, and the crossing of those two is your finger's position; by comparing neighbouring wires it can pin the spot down to far finer than the wire spacing. It does this sixty to a hundred and twenty times a second, which is why dragging feels instant and smooth. Because the whole trick relies on charge flowing to your conductive finger, anything that breaks that path defeats it — which is why an ordinary woolly or rubber glove leaves the screen dead, why a plastic pen does nothing, and why "touchscreen gloves" have conductive thread woven into the fingertips. Touch a screen with two fingers and the chip simply sees two dips and tracks both, and that is all a pinch-to-zoom really is: the screen watching two charge shadows move apart.
A touchscreen finds your finger by watching a hidden charge grid: your conductive fingertip steals charge from the nearest crossing, and the biggest dip gives your position.
- 1On a phone or tablet, try tapping the screen through a sock, a glove or a sheet of paper. Nothing happens — the insulator blocks the charge.
- 2Now wet a fingertip slightly, or lick the tip of a sausage (a classic cold-weather trick), and touch the screen. Suddenly it responds.
- 3The difference is conductivity: anything that lets charge flow to ground acts like a finger, while an insulator hides it. That single fact explains every quirk of capacitive screens.
Common questions
The trick relies on charge flowing to your conductive finger. An ordinary glove insulates your fingertip, so no charge is drawn off the grid and the screen detects nothing. Touchscreen gloves add conductive thread to fix this.
A controller scans every column and row, finds the ones with the largest charge dip, and takes their crossing as your finger. By comparing neighbouring wires it interpolates to far finer than the wire spacing, 60–120 times a second.
The grid maps charge dips, so it can track several fingers at once. Two fingers create two dips; spreading them apart is simply the screen watching the two dips move apart — that is all multi-touch is.