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I have watched some youtube videos about capacitive touch screen

http://www.youtube.com/watch?v=ZHzaVzYEZbw

http://www.youtube.com/watch?v=GmCE18RMEeQ

The videos show that a conductor(finger), in which charges can be induced, can be used to trigger the capacitive touch screen. However, I tried to wear a plastic glove to hold a metal rod to touch the screen and no detection is triggered.

I really want to know the physics theory about the capacitive sensing.

Can someone tell me what physical properties or shape trigger the capacitive touch screen?

Is there any examples that something WITHOUT conducting to human body can trigger the screen?

(P.S. I have read other posts about capacitive touch screen here and do not find a good answer so this is not a duplicate)

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Here is an example of something that works without being connected to the human body. –  Nathaniel Mar 6 '13 at 15:13
    
What is the difference between sausages/fingers and metal rod for capacitive touch screen? –  Timespace7 Mar 6 '13 at 16:28
    
I'm not an expert, but are you sure the screen you tried was the capacitive design? After all, there are touchscreens that work by circuit completion, pressure, or heat. –  Chris White Mar 6 '13 at 16:58
    
Yes. It is a ipad –  Timespace7 Mar 7 '13 at 2:06

1 Answer 1

The basics of a capacitive touch screen are well covered on the web, you could also look at Wikipedia. But essentially, touch screens on phones and tablets measure the actual difference in capacitance, rather than just detecting any change in capacitance.

The dielectric constants of the two materials have to be larger than a certain amount for it to register as a 'touch'. And since they're mostly calibrated for skin, it'll expect a capacitance similar to that induced by skin within some tolerance. I suspect this is why sausages work, since it's a (somewhat) similar material that's coating them.

This helps reduce the wrong detection of touches from moisture sitting on the screen or any other dielectric causing a change in capacitance, and helps with your silky smooth iPad user experience. ;)

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