Why can we control a smartphone screen with our fingers?
The short answer
Many smartphones use a touch sensor that detects the change in electrical behavior caused by a finger nearby, allowing us to control the screen with our fingers.
In 30 seconds
A screen includes a touch-sensing layer with thin electrodes, separate from or integrated with the part that displays images. When a finger touches it, the electric field and capacitance around the sensor change. A touch controller measures that change and sends the touched position as coordinates to the operating system. The operating system and apps turn those coordinates into taps, swipes, pinches, and other actions.
What is capacitance?
Capacitance is a measure of how an electrical arrangement stores charge and responds to electrical changes. When a conductive object comes near an electrode, it affects the electric field around that electrode and changes the measured capacitance. The human body conducts electricity to some extent, so a finger can create this change.
How does the phone know where we touched?
Many touchscreens combine electrodes arranged in horizontal and vertical patterns to locate a touch. The controller reads combinations of electrodes in sequence and calculates where the finger has the strongest effect. A finger is not pressing each individual display pixel like a collection of tiny buttons.
Does electricity flow from the finger?
It is not quite accurate to say that electricity simply flows from the finger and presses a button on the screen. The important effect is that the finger acts as a conductive object and changes the electric field or capacitance around the electrodes. The controller compares measurements with and without the finger.
Why can gloves make touch harder?
Ordinary glove materials do not conduct electricity well, so they can weaken the electrical connection between the finger and the sensor. Capacitive styli and gloves made with conductive threads are designed to create a similar electrical change. The response still depends on the design of the device and the glove.
Can the screen sense several fingers?
If the sensor is designed to measure several locations at once, the touch controller can track multiple fingers. The operating system and apps use that information for gestures such as pinching to zoom a photo. The number of supported touch points and gestures varies by device and app.
A common misunderstanding
The screen is not reading the temperature or fingerprint of your finger. Not every touchscreen uses exactly the same method, but capacitive sensing is widely used in modern smartphones.
In fact...
A touchscreen does not decide what happened from one measurement alone. Electrical noise, finger size, and the way a finger contacts the surface can all affect the signal, so the controller and software process the measurements to distinguish an intended touch from noise.