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How a Screen Knows Where Your Finger Is

8th grade nonfiction passage about capacitive touchscreens: 273 words, six questions, an answer key and vocabulary.

8th grade nonfiction capacitive touchscreens 273 words

How a Screen Knows Where Your Finger Is

When you tap a phone screen, nothing seems to happen except a gentle touch. Yet within a tiny fraction of a second, the phone knows exactly where your finger is. Most phone screens use a design called capacitive sensing, and it depends on the fact that the human body conducts a small amount of electricity.

Under the top layer of glass lies a thin grid of transparent electrodes, arranged in rows and columns like the lines on graph paper. The electrodes are so fine and so clear that you cannot see them. The phone sends a weak electric charge into the grid, and this creates a faint electric field above the screen at every crossing point of the grid.

When your fingertip comes close, it disturbs that field. Your finger is a conductor, and it draws away a tiny amount of charge from the spot beneath it. Circuits connected to the grid measure the change at each crossing. The crossing with the biggest change tells the phone where you touched, and the phone can compare many crossings at once to follow several fingers.

This explains some everyday puzzles. A plain leather or woolen glove usually will not work, because the glove is a poor conductor and keeps your finger from disturbing the field. Gloves with conductive threads in the fingertips solve the problem. So does a stylus made with a conductive tip. A fingernail or a plastic pen fails for the same reason as the glove.

Older touchscreens often worked by pressure, with two flexible layers that pressed together. Capacitive screens need almost no force, which is why a light swipe is enough.

Questions

  1. What fact about the human body does capacitive sensing use? in the text

  2. How is the grid of electrodes arranged? in the text

  3. What does your fingertip do to the electric field? in the text

  4. How does the phone follow several fingers at once? in the text

  5. Why does a plain woolen glove usually fail on a touchscreen? think about it

  6. Why does a light swipe work on a capacitive screen? think about it

Words to know

capacitive
working by sensing tiny changes in stored electric charge
electrode
a small piece of metal that carries electricity
conductor
a material that lets electricity flow through it
transparent
clear enough to see through
stylus
a pen-shaped tool used to touch a screen
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One passage, three levels

The same topic and facts written three ways, so a mixed class can read and talk about one text. Each group’s sheet shows only a shape; the key says which is which.

● Easier · 243 words · Flesch-Kincaid grade 4.2

When you tap a phone screen, you feel only a soft touch. But in a tiny blink, the phone knows just where your finger is. Most phone screens use a design called capacitive sensing. It works because the human body carries a little electricity.

Under the top glass is a thin grid of clear electrodes. They are set in rows and columns, like the lines on graph paper. They are so thin and clear that you cannot see them. The phone sends a weak electric charge into the grid. This makes a faint electric field above the screen at each crossing in the grid.

When your fingertip comes near, it upsets that field. Your finger is a conductor, and it pulls a tiny bit of charge from the spot below it. Circuits measure the change at each crossing. The crossing with the biggest change shows where you touched. The phone can check many crossings at once, so it can follow more than one finger.

This explains some puzzles. A plain wool glove usually does not work. The glove is a poor conductor, so your finger cannot upset the field. Gloves with special threads in the fingertips fix this. So does a stylus with a conductive tip. A fingernail or a plastic pen fails for the same reason as the glove.

Old touchscreens often worked by pressure. Two bendy layers pressed together. Capacitive screens need almost no force, so a light swipe is enough.

▲ This passage · 273 words · Flesch-Kincaid grade 7.3

When you tap a phone screen, nothing seems to happen except a gentle touch. Yet within a tiny fraction of a second, the phone knows exactly where your finger is. Most phone screens use a design called capacitive sensing, and it depends on the fact that the human body conducts a small amount of electricity.

Under the top layer of glass lies a thin grid of transparent electrodes, arranged in rows and columns like the lines on graph paper. The electrodes are so fine and so clear that you cannot see them. The phone sends a weak electric charge into the grid, and this creates a faint electric field above the screen at every crossing point of the grid.

When your fingertip comes close, it disturbs that field. Your finger is a conductor, and it draws away a tiny amount of charge from the spot beneath it. Circuits connected to the grid measure the change at each crossing. The crossing with the biggest change tells the phone where you touched, and the phone can compare many crossings at once to follow several fingers.

This explains some everyday puzzles. A plain leather or woolen glove usually will not work, because the glove is a poor conductor and keeps your finger from disturbing the field. Gloves with conductive threads in the fingertips solve the problem. So does a stylus made with a conductive tip. A fingernail or a plastic pen fails for the same reason as the glove.

Older touchscreens often worked by pressure, with two flexible layers that pressed together. Capacitive screens need almost no force, which is why a light swipe is enough.

■ Harder · 289 words · Flesch-Kincaid grade 9.1

When you tap a phone screen, nothing seems to happen except a gentle touch against the glass. Yet within a tiny fraction of a second, the phone has determined precisely where your finger is. Most phone screens rely on a design called capacitive sensing, which depends on the fact that the human body is capable of conducting a small amount of electricity.

Beneath the top layer of glass lies a thin grid of transparent electrodes, arranged in rows and columns like the lines on graph paper. The electrodes are so fine and so clear that they are completely invisible to the eye. The phone sends a weak electric charge into the grid, and this creates a faint electric field above the screen at every crossing point of the grid.

When your fingertip approaches, it disturbs that field. Your finger is a conductor, and it draws away a tiny amount of charge from the spot directly beneath it. Circuits connected to the grid measure the change at each crossing. The crossing with the largest change tells the phone where you touched, and by comparing many crossings simultaneously, the phone can follow several fingers at once.

This explains some everyday puzzles. A plain leather or woolen glove usually will not work, because the glove is a poor conductor and prevents your finger from disturbing the field. Gloves with conductive threads woven into the fingertips solve the problem, as does a stylus with a conductive tip. A fingernail or a plastic pen fails for the same reason as the ordinary glove.

Older touchscreens often worked by pressure, with two flexible layers that were pressed together. Capacitive screens need almost no force, which is why a feather-light swipe is enough to register.

Download as one PDF

How to use it

  1. Press Read it big (or B) to put the text on the projector, or Full screen (F) to fill the whole screen.
  2. Ask the questions one at a time and use Show answer when the class has decided. A shows or hides every answer.
  3. Print student copy for the class, or Print with answers for yourself.

Ideas for class

  • Before reading, put capacitive, electrode, conductor, transparent and stylus on the board with their meanings from the Words to know section and have students predict what the text will be about.
  • Read it aloud once while students follow along, then have them reread it alone or with a partner. At a calm 100 words a minute it takes about 3 minutes to read aloud.
  • Use the Read it big button (or B) to put the text on the projector, and Show answer on one question at a time so the class commits to an answer first.
  • Question 5 asks the reader to infer. Ask students to point to the words in the passage that led them to their answer.
  • Ask students to write one question the passage did not answer, then look for the answer in a book or on a classroom screen.
  • Print the student copy for independent work and keep the answer-key copy for yourself.

Related

Questions

What reading level is "How a Screen Knows Where Your Finger Is"?

It was written for 8th grade. It has 273 words and works out to a Flesch-Kincaid grade level of about 7.3. That is a formula's estimate from sentence length and syllables, and it is not a Lexile or a guided reading level, so check it against your students.

What do the questions ask?

There are six. 4 ask for something the text says directly and 2 ask the reader to work something out from clues. Each has an answer in the key, and the answers are written as a teacher would accept them, not as the only possible wording.

How long does it take to read?

About 3 minutes read aloud at 100 words a minute. For a one-minute fluency check, paste it into the fluency timer and use the word count.

Can I print it?

Yes. Print student copy gives the passage, the questions with lines to write on, and the word meanings. Print with answers gives the same page with the answers filled in. Both are made in your browser.

Is this passage from a published book?

No. It was written for Peabody Teach, so there is no book to buy and no login to read it.

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