Touch screens have totally changed the way we use mobile phones. But how does wiping your finger on a glass screen make things happen inside your phone?

By Bernie Hobbs

 
Touch screens on phones and tablets really have the X factor. Being able to text, phone or film something just by swiping your finger on glass almost makes up for all those other failed sci-fi promises of the 60s.
But considering how futuristic touch screens seem, they rely on a bit of physics that's almost as old as Newton — capacitance — and the fact that your finger is three parts salty water.
If you stick your finger on a regular piece of glass, the most you can hope for is a smudge.
But if there's an electric field on the other side of the glass, some serious rearranging of electric charges goes on in the glass, in your finger and in the field itself. (Read more about electric fields).
And if there are dozens of small electric fields forming and disappearing in a grid formation on the other side of your glass screen, your phone can not only tell when a finger is touching it, it can pinpoint exactly where on the screen that finger is. Here's how.

Journey to the centre of the smartphone

The touch detection part of a smartphone is in the top part of the phone, above the LCD screen and the battery and circuits.
It's made up of two sheets of glass and a bunch of wires that are so skinny they're see-through. The top sheet of glass is the one you touch — it's mostly for protection and to keep your finger away from the business end of things, which happen on the layer of glass below. This second layer has got the skinny wires running over both sides: across it on one side, and up and down on the other. Together they make up a grid pattern.

The wires on one side of the glass are hooked up to the battery's positive terminal, and the ones on the other side are hooked up to the negative terminal. But there's only ever one pair of wires — one above and one below the glass — switched on at any one time. The switching happens really quickly, so every possible pair of wires gets charged up heaps of times in the same order every second.
In every one of these pairs of wires, the one that's hooked up to the battery's positive terminal gets electrons sucked out of it, and the negative terminal pumps electrons into the other wire. So you always have one wire (the one hooked up to the negative terminal) being more negative than the other. That difference in charge causes an electric field between the two wires, and it's strongest where the wires are closest — where they cross over.
These electric fields are really small, but they still affect nearby charges — like the electrons in the layer of glass.
Glass is an insulator — its electrons are held tightly by its atoms, so they're not free to flow as an electric current. But the electric field between the wires pulls the electrons a little bit towards the positive wire. No current flows, but pulling all those electrons closer repels electrons in the positive wire, and attracts more electrons from the battery to the negative wire. So the positive wire gets a bit more positive (fewer electrons) and the negative wire more negative than it would be without the help of the glass. And that means the electric field gets stronger.

Devices that can store charge in conductors separated by an insulator like this are called capacitors. Man-made capacitors first appeared in the 18th century, but nature had the jump on us by a few billion years. Lightning is made by thunderclouds and the ground acting like a giant capacitor, and your cells control what goes in and out of them by keeping an electric field across their insulating membrane.
If a dud conductor like glass can increase the electric field at the intersection of the wires, you can imagine what a bag of salty water like your finger can do to it. Better still, your finger doesn't have to be between the wires — the electric field around intersecting wires pokes up and out of the top layer of glass, right out of your phone. So when you touch your screen you're putting your finger right into an electric field.
The blood and cells in your finger are full of water with heaps of charged atoms dissolved in it — positive ions like sodium (Na+) and potassium (K+), and negative ions like chloride (Cl-). When your finger enters an electric field, the field gets to work organising those charges — sucking negative ions towards the positive wires and pushing positive ions away. And with all that extra charge getting organised in your finger, that particular electric field gets stronger so it can suck more charge from the battery to balance things out.
The turbo-charge your finger gives to the nearest electric field doesn't go unnoticed by the phone. The black border around all touch screens covers up a bunch of sensors that constantly measure how much charge is stored at the intersection of every pair of crossed wires. Once the power to a pair of wires is cut, the electric field disappears, so there's nothing to hold the built-up electrons in place. They leak out from the negative wire into another circuit, causing a small current to flow in it. The hidden sensors measure how long the current flows for — the more charge stored in the grid lines, the longer it takes to leak out, the longer the current lasts.
Stick a finger on your phone and the electric field at the nearest intersecting wires grows, so more charge is stored there. Depower the wires and the sensor notices that while all the other wires are producing the standard amount of current, one pair — the wires that intersect near your finger — are high scorers. It's a tactic straight out of the Battleship playbook, and it works a treat.

It's a stylus ... it's a finger ... it's a banana!

For all its smarts, your phone isn't detecting a finger — it just knows that something that's about the same conductivity as a finger is touching it. Metals will shoot the electric field through the roof, and cause an outflow current that's way too long. Non-conductors, like gloves, won't have nearly enough effect. But anything that's got about a finger's worth of free-moving charge — like a humble banana — will do the trick. Capacitance has a very democratic sense of touch.
Thanks to Dr Ben Buchler from the Department of Quantum Science at the Australian National University, and Professor David Jamieson from the School of Physics at The University of Melbourne.