It's 9:20 A.M., and you're less than halfway through your physics lecture. You pull your iPhone out of your pocket, and instead of copying notes on Maxwell's equations, you start browsing Facebook.
As soon as you tap the keyboard, you are connecting to your profile, and you then zoom with the swipe of two fingers. Since you're not getting anything out of the lecture, why not take a few minutes to find out how your touch screen works?
The goal of all touch screens, whether they use a stylus or your finger, is to accurately measure where it has been touched. Naturally, on a flat, two-dimensional surface, that implies a coordinate system. Once these grid locations have been detected, the data can be sent to the processor, which correlates the touch to what was displayed on the screen and what the touch means for that software.
Some touch screens (though not iPhones) work on a 'resistive' principle. Above the LCD display are two conducting sheets, separated by either air or non-conducting supports. Current flows through each of the conducting sheets, but cannot normally cross between the sheets. One common method, known as 'four-wire' because of the type of circuit, uses current running through only the outer sheet initially.
When the screen is touched, the pressure causes the top screen to touch the bottom sheet at a specific point. The bottom sheet senses the change in voltage due to the top conducting sheet's transfer of current, and records this as one of the two necessary coordinate points. A current is then passed through the bottom sheet, sensing the location of the pressure in two dimensions.
On the other hand, iPhones and iPod Touches work by 'capacitive' technology. There are two subsets of capacitive technology, mutual and self-capacitance, and iPhones use both.
For mutual capacitance, there are again wires arranged in a grid, but they are rigid. When you touch the screen, there is a transfer of charge between one set of the driving lines and your finger. The second set of wires is sensing lines, which identify changes in current, and record the grid points of the touch.
In the self-capacitance system, there is an array of individual electrodes spread across the screen; changes in voltage can be perceived in each electrode separately, and the location is known. Because of the independence of these two methods, multiple touches can be recognized and processed simultaneously. A stylus or other non-conducting object is worthless to an iPhone because of the inherent reliance on the electric conductance of the human body.
Software built into the iPhone discerns the location of one or more touch locations, and calculates an exact point for the touch, regardless of the size of your finger. Once all of the data is gathered, the processor signals the software to give the appropriate feedback.
As you are commenting on your friend's status update, you don't consider the invisible layer of circuitry above the screen or the near-instantaneous reaction to your touch. You're just counting down the minutes until you can stop thinking about physics.
