Between smartphones, tablet computers and other gadgets such as GPS systems, touch screens are a common input method for devices without keyboards -- and thanks in part to Windows 8's updated interface, even some PCs include touch-screen monitors. Most touch-sensitive devices use one of two technologies: resistive touch screens, commonly used with styluses, and capacitive touch screens, operated by fingertip. Some specific devices, however, use alternate means to detect touch, such as drawing tablets and styluses that communicate via magnetism.
Resistive Touch Screens
Resistive touch screens contain two layers with an electrical current running through them. When you press on the screen, the top layer impacts the bottom layer, interfering with the current. The computer or device using the screen contains software that interprets the electrical change to locate the point of contact. This design limits resistive touch screens to recognizing a single touch at a time. Because resistive touch screens require pressure to work, they're usually used with a stylus, such as in a credit card reader at the store or in the Nintendo 3DS. A resistive touch screen's stylus doesn't contain any technology -- the tip of your nail works just as well, though you might scratch the screen if you press too hard.
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Capacitive Touch Screens
Commonly found in smart phones and tablets, capacitive touch screens only use a single layer with an electrical charge. When you touch the screen with your finger, your body temporarily absorbs some of the electricity, similar to how your body transfers electricity when touching a metal surface. A touch screen uses so little electricity, however, that you won't feel a shock. This change in charge indicates the place where you touched the device. This mechanism won't work with touch from nonconductive materials such as a plastic stylus or most gloves, but can support multiple simultaneous touches. Capacitive screens also have better visual quality than resistive touch screens, as the dual layers in resistive screens can reflect light.
Relationship to Display Technology
Both resistive and capacitive touch technologies are added on top of existing display components. In other words, the type of display used in a device -- LCD, LED, OLED and so on -- does not indicate the type of touch sensitivity the screen offers. Most computer monitors, for example, contain LCD screens with fluorescent backlights. Some of these LCD monitors also include capacitive touch technology, but most have no touch sensitivity at all.
Graphics Tablet Touch Screens
The touch-sensitive surfaces used in most drawing tablets do not rely on either resistive or capacitive touch screen technology. Instead, many graphics tablets use electromagnetic induction to generate a weak magnetic field from the tablet surface.The stylus picks up and returns this magnetism, providing the pen's location. Similar to how you can feel the pull of two magnets held closely together, the tablet can sense its stylus even before making contact, allowing you to position a cursor by moving the stylus above the tablet. Other types of drawing tablets use a battery-powered stylus that includes electronics for computing and broadcasting its own location.