eli5: How does a smart phone know you are touching the screen with more than one finger, or doing a gesture?
In: Technology
Most modern touch screens are “capacitive” meaning they read the electric field above the screen to detect touch. The exact way that works involves a lot of physics, but the closest analogy is that it’s similar to how touching a magnet to a piece of metal makes that metal magnetic too. Putting your finger in the electric field makes your finger conduct a tiny amount of electricity which changes the electric field.
Now imagine you had a metal nail with a compass at one end and a magnet at the other. If the magnet were touching the nail it would be magnetic and the compass would point towards it, but if the magnet wasn’t on the nail the compass would point north. With that you can detect the magnet through the nail!
There is a big grid of capacitors in your phone that does the same thing to detect your finger through the screen. When you put your finger down, the processor can see which of those capacitors is affected by your finger and use that to determine where it is. With multiple fingers it would see a signal from two different capacitors and can interpret that correctly.
A side note, if you stick multiple magnets to the exact same nail it wouldn’t be able to tell them apart. That’s why tapping with multiple fingers up against each other doesn’t work. Also, don’t rub magnets on your phone, it could cause problems
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