Ask Dr. Universe: How do touch screens work?

Dr. Universe: How do touch screens work? – Nicholas, 11, Florida

Dear Nicholas,

When I got your question, I decided to do a little experiment. First, I tapped my finger on a tablet and sent a message to a friend. Next, I put on a pair of wool mittens and started typing, but the screen did not respond. Finally, I used a banana to see if I could use it to swipe the screen. It actually worked.

I wondered what exactly was going on here and decided to take our questions to my friend Praveen Sekhar. He’s an associate professor in the Washington State University School of Engineering and Computer Science.

Sekhar told me touch-screen devices use electricity to work and that different materials can impact how the electricity flows. Some materials called insulators, such as the wool mittens, keep electricity from flowing.

Then there are objects such as your finger or a banana that allow electricity to flow from one place to another. We call these conductors.

When your finger touches the screen, it creates a sort of pathway for electricity to flow from your finger to the device. You read that right: You have electricity in your body – from your toes to your fingers.

Sekhar said you can think of how these touch screens work sort of like a battery. If you look at a battery, you will see it has a positive charge end and a negative charge end. Electricity will start to flow if both ends are connected to your device.

A touch-screen device on its own has a negative charge, he said. But once your finger connects with the touch screen, it becomes positive. The electrical charges can work together to help your device work.

This kind of electrical ability is called capacitive technology and is found in many touch-screen phones, tablets and computers.

Sekhar also told me about another kind of touch screen. These are the kinds of touch screens we see at ATMs and in grocery stores. These screens aren’t quite as bright as your computer or phone.

We call these resistive screens, and they are made of layers of glass and plastic with a chemical coating and a sheet of metal underneath them.

When you press these screens with your finger, you apply pressure to the material. Inside the material, the electrical charges start moving inside as they respond to pressure from your finger and allow the device to work.

Whether it is capacitive or resistive technology, touch screens have become part of many people’s daily lives.

With help from an adult, perhaps you can do a little investigation into touch screens, too. Collect a few small items from around the house to find out which ones are insulators and which ones are conductors.

Screens are quite fragile, so you may want to use materials that will be gentle to your screen, like a cotton swab, an eraser or a banana. Touch the objects to the screen to see if they allow your device to respond.

Make a list of which objects conduct, or allow electricity to pass through, and remember how electricity helps your phone do all kinds of amazing things.

Sincerely,

Dr. Universe

Submit a question at askdruniverse@wsu.edu.