Technology · Level 2 · 153 words

How a Screen Feels a Finger

Original passage © Studio AM, written for Fluency.

A phone screen has no moving parts. Nothing presses in when you tap. So how does the glass know where your finger is?

Under the glass lies a grid of wires, each thinner than a hair, and the phone keeps a faint electric charge spread across it. Your body is mostly salt water, and salt water conducts electricity. The moment your fingertip lands, it draws a little of that charge out of the wires directly beneath it.

The phone notices the loss. It scans the grid many times a second, asking where charge went missing, and that crossing of row and column becomes the spot you touched.

This one fact explains two small mysteries. A wool glove blocks your touch because wool does not conduct, so no charge can leave the screen. And rain makes a phone type nonsense because every drop of water draws off charge exactly the way a finger does.

Comprehension questions

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4 questions
1. What is the passage mainly about?

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B. The screen finds your finger by sensing where its electric charge is drawn away.
The middle paragraphs answer the opening question: the fingertip draws charge from the grid, and the phone locates the loss. The glove and the rain are consequences used as proof.

2. Based on the passage, why would the tip of a plastic pen fail to work on the screen?

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A. Plastic does not conduct, so it cannot draw charge from the wires.
The glove sentence gives the rule: no conduction, no charge leaves, no touch. A plastic tip fails the same test a wool glove fails.

3. In this passage, "conducts" means

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C. lets electricity pass through
Salt water conducts and a fingertip works; wool does not conduct and the touch is blocked. What passes in one case and fails in the other is electric charge.

4. According to the passage, why does rain make a phone type nonsense?

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D. Every drop draws off charge the way a finger does.
The last sentence states it: each drop of water draws off charge exactly the way a finger does, so the phone reads touches that nobody made.

Source: Written for Fluency. Original passage © Studio AM, written for Fluency.