How Do Touchscreens Work? Why Your Finger Works and Gloves Don’t

You tap, swipe and pinch hundreds of times a day without thinking about it. Yet the glass under your finger is not just glass. Hidden inside is an invisible electrical net that feels your touch, works out exactly where it is and can follow several fingers at once.

Two main kinds of touchscreen

There are several ways to make a screen sensitive to touch, but two have dominated.

• Resistive touchscreens were common in early mobile devices, car navigation systems and some ticket machines. They are made of two thin, flexible layers with a conductive coating, separated by a tiny gap. Pressing the screen pushes the layers together, and the device measures where the contact happened.
• Capacitive touchscreens are used in almost all modern smartphones and tablets. They do not need pressure at all; a light touch is enough.

How a capacitive screen feels your finger

Under the cover glass of a capacitive screen lies a fine grid of transparent electrodes, usually made of a material called indium tin oxide. It is so thin that you can see the display through it. One set of electrodes runs in rows, the other in columns.

Where the rows and columns cross, they store a tiny amount of electrical charge, a property called capacitance. A controller chip constantly sends signals through the grid and measures the capacitance at every crossing.

Your body is a reasonably good conductor of electricity and holds electrical charge. When your finger comes close to the glass, it disturbs the electric field and changes the capacitance at the crossings directly beneath it.

The chip scans the whole grid many times a second, often 60 to 240 times. By comparing the measurements with their normal values, it finds the patch where the field changed and calculates its centre with high precision, much finer than the spacing of the electrodes.

Multi-touch: many fingers at once

Because the controller measures every crossing separately, it can detect several separate patches at the same time. This is what makes multi-touch possible: pinching to zoom, rotating with two fingers or typing with both thumbs. The iPhone, launched in 2007, helped make this kind of projected capacitive screen standard on phones.

Software then turns raw touch points into gestures, working out whether you tapped, swiped, held or pinched, and ignoring accidental touches such as the palm of your hand.

Why gloves, nails and pencils don’t work

A capacitive screen only responds to things that disturb its electric field. Ordinary woollen or leather gloves, fingernails, pen tips and wooden pencils are poor conductors, so they barely change the capacitance and the screen ignores them.

That is why touchscreen gloves have conductive threads in the fingertips, and why simple phone styluses have a soft, conductive tip that behaves like a finger. More advanced active styluses contain their own electronics and send a signal to the screen, allowing pressure sensitivity and pinpoint accuracy for drawing.

Problems with water

Water conducts electricity too. Drops of rain or a wet finger can change the field at many points, confusing the controller and causing missed or phantom touches. Modern phones use smarter software to tell the difference between a finger and a film of water, but very wet screens can still be tricky.

In short

• Most phones use capacitive touchscreens with a hidden grid of transparent electrodes.
• Your finger, as a conductor, changes the capacitance at the grid crossings beneath it.
• A controller chip scans the grid many times a second to find each touch.
• Measuring every crossing separately allows multi-touch gestures.
• Ordinary gloves, nails and pencils don’t conduct enough to register.
• Resistive screens work by pressure instead and respond to any object.