I noticed my phone responds to my finger, but not to a plain graphite pencil.
Why is that, what is its physical reason?
How are styli able to approximate the human touch? What kind of conductive materials are being used there?
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$\begingroup$ this article explains how touch-screen works but not which materials it responds to computerworld.com/article/2491831/computer-hardware/… $\endgroup$– john mangualCommented Aug 26, 2016 at 20:27
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$\begingroup$ This is a question about technology, not physics. $\endgroup$– sammy gerbilCommented Aug 26, 2016 at 22:18
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$\begingroup$ what is technology as opposed to science or engineering? my question is about the materials science behind touch-screens. $\endgroup$– john mangualCommented Aug 27, 2016 at 0:38
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$\begingroup$ Have you read the article you linked to? The reason seems to be in the technology (how the screen is constructed) rather than any natural properties of glass. In essence you are asking "How does my touchscreen work?" This is technology, not science, and is explained in the article. $\endgroup$– sammy gerbilCommented Aug 27, 2016 at 1:06
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$\begingroup$ @JohnRennie The dielectric constants of the two materials have to be larger than a certain amount for it to register as a 'touch'. neither this nor the other answer contains too much detail just enough to get me started $\endgroup$– john mangualCommented Aug 27, 2016 at 14:09
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1 Answer
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Capacitive touch screens work by measuring capacitance at a number of locations on the screen, then interpolating between them to get a precise position. The simplest form of capacitor is two parallel conductive plates on opposite sides of an insulator, with the capacitance being proportional to the area of intersection of the two plates and the dielectric constant of the insulator, and inversely proportional to the distance between the plates. In the case of a touch screen, one plate is a transparent conductor on the back side of the glass, and the other is the user's fingertip. A pencil does not work well for this, even if made of metal, because its tip has a very small area. Styli made for this purpose usually use a tip that is at least 6 mm in diameter, which deforms when you press it against the screen, enlarging the contact area. Depending on the exact measurement technique being used (some touch screens measure mutual capacitance between two conductors on the screen, and others effectively measure capacitance to ground), your stylus may or may not need to be in contact with a human body to work.
