I'm trying to understand why certain objects work (i.e. register a touch) on an iPhone screen, but others don't. From what I've read, the screen consists of two transparent conductive layers separated by an insulating layer. Furthermore, each layer is etched to form parallel electrodes, and the two layers are etched in orthogonal directions in order to create an x-y coordinate grid of capacitors. One layer has a voltage applied to it, and if the capacitance at any x-y point changes, the voltage at that point on the other layer will change from its nominal value. This is my understanding. Please let me know if I am in error. Based on this scenario, it seems to be that touching *any* conductor to the screen should work to change the capacitance between the two layers at that point. However, some things work and some don't. Reading online how to make your own "stylus" for the iPhone, I came across the suggestions of wrapping a conveniently sized object in aluminum foil and also trying a battery. These suggestions both seem to work, but others don't, like the end of a mini jack plug, a penny, and the metal tip of a mechanical pencil. I *think* that all of these last three should be conducting. So the experimental evidence seems to point to the idea that you actually have to apply an electric field to the screen. I assume that doing so helps, for the same reason why fingers work so well, and this could explain the battery, and the foil, which is thin and was in contact with my fingers directly. But based on what I've read about how this actually works, I cannot understand why it is *necessary* to do so and won't work at all otherwise. Another weird thing: why does only the negative terminal of a battery work, and not the positive? EDIT: In a test app (a doodling program), the penny works (draws stuff) when laid flat on the screen and moved around, but not when making contact with one of its edges. ??? EDIT 2: even that only works sometimes, not very reproducible.