There are several different kinds of capacitive sensing. But in general, they all work by using an array of sensors to monitor the electrostatic field around the screen. When your finger touches the screen, it changes the electrical capacitance of that portion of the screen. This is detected as a change in voltage at various locations. The microprocessor captures, filters, and analyzes the data. Then it calculates the coordinates of where the touch occurred. You can read more about capacitive sensing here: http://en.wikipedia.org/wiki/Capacitive_sensing
When a stylus is used, the same principles apply. The only difference is that the stylus is acting as a conductor to transmit electrical charge between your hand and the phone. In order to function properly, a capacitive stylus must meet several criteria.
1. A conductive surface: It must be able conduct an electrical charge between your hand and the screen. If the material is too resistive or if the distance between your hand and the screen is too great, the signal reaching the screen may be too weak to be detected.
2. At least 1/4 inch wide: When filtering data, the processor ignores areas that are significantly smaller than a human finger tip. This helps avoid unintentional activation. Having a stylus that is about 1/4 inch wide will ensure that there is enough surface area to be detected.
3. A relatively flat end: Having a flat tip ensures that the whole face can get close enough to the screen to be detected.
4. A smooth surface: This will ensure that you don't scratch up your screen.
Following these criteria, you can find a wide variety of common items that can be used to activate a capacitive touch screen. Here are five of my favorite designs for a DIY capacitive stylus.
Ok, I Didn't Want To Do This...
I am going to have to explain exactly what I need the stylus for because it is the only way to be fair to you guys for trying to help me. I know it is dorky, but here is the reason: I need a machine to tap my ipod for me so that I can collect points on a certain app while I'm at work. Obviously this means that no part of my body can make contact with the stylus. I already built the machine; it is a 1rpm electric motor that spins a wooden lathe. A rod lays across the lathe, one end hinged to the same platform as the motor, the other end clamping the stylus. The rod rests on the spinning lathe until it reaches a notch I cut in the lathe and drops (about half an inch). The stylus also drops, tapping the ipod touch screen every 1 minute. As I said before, the stylus I purchased for this setup (it was a capacitive stylus that works well when in my fingers) did not register a "touch." Some of the things you guys mentioned are probably on the right track. Coy-technorat... could you provide me with the exact model stylus you used to unlock your ipod touch? You mentioned bamboo... is that the brand or the medium? Thanks again to all who gave me their time. I really appreciate it. Even if I am a dork.