Swipe, tap, scroll: how touchscreens follow your finger

Using a touchscreen is almost second nature, letting us zoom in and out, scroll quickly and tap links without a mouse. So how does your phone know that you want to zoom in on a part of a map or see the next part of a page?

There are a few different types of touchscreen technology. Some use infrared light or sound, but the two most common are known as resistive and capacitive systems. 

Resistive screens were first patented in 1975 and produced commercially seven years later. In principle, they are the exact opposite of light switches. 

A light switch is an example of an “open circuit”. Flipping the switch makes an electrically conductive material (such as a length of wire) bridge a gap between two separated conductive pathways, closing the circuit and forming a continuous route. Electricity flows along the newly created length, lighting up the globe at the end. 

In resistive screens, closed circuits are the default. Electricity flows through a conductive layer that coats the glass. On top of it is another layer, this time of flexible resistive material, separated by rows of insulating spacers. 

When the upper layer is pressed and touches the conductive layer below, it disrupts the current. A processor calculates where that electrical distortion occurs and sends a signal for the screen to behave accordingly. 

Resistive systems were used for the first touchscreen phones, but today they are mostly deployed in devices that require precision but not speed, such as automated teller machines.  

Because they are pressure sensitive, resistive screens will work not only at the press of a finger, but also if poked by a stick, a spoon or any hard-enough object. But they’re slow to respond and can only cope with one tap at a time – making them useless for devices that react to multiple stimuli simultaneously.

Tinder users note: you can’t swipe left on a resistive screen.

Today’s smartphones and tablets instead use capacitive touchscreens, which rely on electrical charge rather than physical pressure – which is why they can be exquisitely sensitive to the glide of your fingers, but don’t respond to being jabbed by a pencil.

Although the technology seems really new, capacitive screens were first developed in the early 1960s, and incorporated into bespoke equipment at the European Organisation for Nuclear Research (better known as CERN) in 1973.

It wasn’t until the invention of the smartphone that the technology became ubiquitous. The first phone to incorporate a capacitive screen was the LG Prada, released in 2006. It sold a million units it its first 18 months. 

The technology stores electricity in a conductive layer comprising a grid of tiny wires: rows of “driving lines” that carry current, and perpendicular “sensing lines” that detect that current when they intersect. The conductive layer sits between a glass substrate and a protective coating. 

When any object that carries an electric charge – such as a fingertip – touches the coating, voltage travelling through the driving lines below is disrupted. These changes are picked up by the sensing lines. 

Since a drop of water can also carry a charge, it can set off your touchscreen device. But trying to trigger the screen while wearing insulating woollen gloves, for instance, just won’t work. 

A microcontroller registers and processes these points of contact in nanoseconds, making it feel like interactions with smartphones and tablets happen instantaneously.