In addition, the tip needs to be wide enough to generate enough capacitance for the screen to register. Usually the part you hold also needs to be conductive, to connect it to the greater capacitance of your body, but there are ways around this. They're commonly a conductive foam core covered with thin rubber. A stylus for a capacitive touchscreen has to conduct electricity, but be covered with an insulator, to give it the capacitance (like your finger). Styli for resistive screens need to be small for a precise touch, and can be any material (because they just need to apply pressure).
Because of how it reads the contact point from the sheets, it can't understand more than one contact point, or a bigger area of contact. When you apply pressure, it makes the sheets touch together, and the touchscreen hardware can find where that contact point is. The older resistive touchscreens just had two flexible conductive sheets with a gap in between them. That's why it can tell different fingers, and the more sensitive screens can tell fingers apart by their shape, and make a guess at how hard you're pressing. The screen can tell all the points where the charge is disturbed in this way, building up a picture of where you're touching it. The screen has a charge inside it, and when your finger comes near it, the capacitance of your finger attracts this charge. They work a bit like the way you can feel a static charge with the hair on your skin. Today's multi-touch screens are capacitive, unlike the resistive touchscreens from a decade ago. Capacitive and resistive touchscreen technologies
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