Tap, swipe, pinch, zoom: we use touchscreens hundreds of times a day. But how does a sheet of glass know exactly where your finger is?

Two main types

1. Capacitive touchscreens

Used in almost all modern smartphones and tablets. Beneath the glass is a nearly invisible grid of transparent electrodes, usually made of indium tin oxide, carrying a tiny electric field.

Your body conducts electricity. When your finger touches the screen, it disturbs the electric field at that point, changing the capacitance (the ability to store electric charge). The phone’s controller measures these changes across the grid many times a second and calculates exactly where you touched.

2. Resistive touchscreens

Older and cheaper. They have two flexible layers separated by a tiny gap. Pressing the screen pushes the layers together, completing a circuit at that point. They work with any object, such as a stylus or fingernail, but need pressure and are less sharp and responsive. You may still see them in some ATMs and industrial machines.

Multi-touch magic

Capacitive screens can detect several touch points at once. That is what makes pinch-to-zoom and two-finger gestures possible.

Why don’t touchscreens work with ordinary gloves?

Fabric and plastic do not conduct electricity, so they do not disturb the electric field. Special touchscreen gloves have conductive threads in the fingertips.

Why does a wet screen act strangely?

Water conducts electricity too, so drops can confuse the sensors and cause “ghost touches”.

Layers of a modern screen

• Cover glass: toughened glass that protects the screen.
• Touch sensor layer: the electrode grid.
• Display layer: LCD or OLED panel that creates the image.

In many newer phones, the touch sensor is built directly into the display to make phones thinner.