NARRATOR: How does swiping your finger across a touch screen make your phone or tablet work? It's all because of invisible electric fields that pop up out of the glass screen in a grid pattern.
Shot of electric fields on glass screen.
NARRATOR: Those electric fields are monitored by sensors at the side of the phone. And when you poke your finger in an electric field, the field changes shape. When the sensors detect that change, the grid pattern lets your phone zero in on your finger's location. So what causes those pop-up electric fields, and what are they doing with your finger? The fields form around skinny wires that surround a layer of glass inside the phone. Whenever a pair of wires gets switched on, an electric field forms.
Shot of electric field.
NARRATOR: One of the wires becomes positively charged because the phone battery's positive terminal sucks electrons out of it.
Shot of electrons being sucked out.
NARRATOR: The other wire becomes negative because the battery's pumping electrons into it. The wires are separated by the sheet of glass, and whenever positive and negative charges are separated, you get an electric field. And it's strongest where the charges are closest — where the wires cross over.
Shot of wires crossing over each other.
NARRATOR: Electric fields will push and pull on anything with an electric charge, and that's exactly what your phone's electric fields do to your finger. Your finger has got lots of positive and negative charges floating around in it — the ions from the salts that are dissolved in your blood and cells.
Shot of molecules of salts.
NARRATOR: The electric field around a positive wire pushes the positive ions in your finger away, and pulls the negative ions closer.
Shot of negative ions being pulled from finger.
NARRATOR: The negative charges at the surface of your finger balance some of the positive charge in the wire… ..and that lets the battery suck more electrons out of it. As that wire gets more positive, the battery pumps more electrons into the negative wire to balance the charge. So with your finger near the phone, the wires are more charged than they are without it. Once the wires are switched off, the electric field disappears so those electrons can flow back through the wires, evening out the charge. The sensors at the side measure how many electrons flow through the wires. If there were more than normal, they know something was touching the screen, and your device's processor can use the wire grid to pinpoint exactly which spot was touched. So the sensors can't actually tell what a finger looks or feels like, they just know how good at conducting electricity fingers are. They're programmed to ignore lousy conductors, like plastic or gloves, and really good conductors, like metal. But they'll respond to anything that conducts like a finger. For smart phones, they're pretty easy to fool.
