What physical properties of materials trigger the capacitive touch screen? 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)
 A: Capacitive touch screens use a grid of transparent conductors on the inside surface of the glass to sense position.  Each conductor has some series resistance connected so that the capacitance from each conductor to ground can be measured by applying a current pulse to that conductor and measuring the rise and fall times of the voltages at these conductors.  By scanning the x conductors one at a time, the point of contact can be interpolated between the conductors with the slowest rise and fall times to get a precise y position, and the y conductors are similarly scanned to get the x position.  Because capacitance is proportional to the area of intersection between two plates (one being the conductor on the back of the glass, the other being the user's finger), pointed objects, even if good conductors, do not make good surrogates for fingers.  Try the same experiment with a rod of larger diameter (best if it's close to the size of a fingertip, but some touchscreens respond to smaller contact area than others), or some conductive material that deforms as it is pressed against the glass to cover more area, and it should work.  The conductivity of the material is much less important than the area it covers.
A: 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. ;)
