Touchscreens

You tap your phone hundreds of times a day and it responds instantly. But have you ever wondered how the glass actually knows where your finger is? The surprising answer: it does not feel your pressure. You can press as hard as you like with a plastic pen, nothing happens. The display responds to something completely different, the electric charge of your body.

Your finger changes the electric field

Under the glass lies a wafer thin, transparent layer of conductive material. A small voltage is applied to it, creating a weak electric field. Your body is mostly water with dissolved salts and therefore conducts electricity. When your finger comes close to the glass, it draws away a little charge and distorts the field at exactly that spot. The display measures this tiny change. This is called capacitive, because your finger and the layer together form a small capacitor, a charge store. How much charge such a capacitor holds is called its capacitance, measured in farads. Your finger changes it by only a fraction of a picofarad, and a picofarad is itself already a trillionth of a farad. Exactly that is enough for the chip.

A grid finds the finger

Sensing that a touch happened is not enough, the phone also needs to know where. That is why the conductive layer is built as a grid: invisible conductor lines run across the display in rows and columns. At every crossing point, a chip checks about 60 to 120 times per second whether the charge there has changed. If, say, column 12 and row 7 report a change, the chip knows the finger is at the crossing of 12 and 7. And because every crossing point is measured individually, the grid can also detect two or more fingers at once. That is exactly what multi touch is, and pinch zooming would not work without it.

Why gloves get in the way

Now you can explain for yourself why your phone goes on strike in winter: wool and ordinary leather do not conduct electricity. The glove acts as an insulating layer between your finger and the field, your body can no longer draw away charge, and the display simply does not notice you. Special touch gloves therefore have conductive threads in the fingertips that connect the screen to your skin. For the same reason a plastic pen does nothing, while a special stylus with a conductive tip works fine.

The counterpart: the resistive touchscreen

There are also touchscreens that really do respond to pressure: resistive displays. There, two conductive foils lie on top of each other with a tiny gap. When you press the upper foil, it touches the lower one at that spot, and the circuit reveals the position. This works with gloves, pens and any object, but it is less precise and usually detects only one pressure point. That is why you find resistive displays today mainly on older ticket machines and on industrial machines, while capacitive technology has won in phones.