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In capacitive touch they lay down a grid of Indium Tin Oxide (ITO) and connect it to some sensors. Then when your finger which comes either in contact or really close the capacitance of the circuit changes and a touch is detected.
Actually, capacitive touch screens do not require to be touched by a body that can source current, rather, they sense when contacted by any conductor (see wikipedia article on capacitive sensing). Since wire is a prototypical conductor, you do not need to touch it to use it as a makeshift stylus.
Basically, a capacitive touch sensor works by detecting the change in capacitance in a "capacitor" that consists of a sensor plate and "ground", where this ground could be your ground plane, or a large nearby conductive plate, or water, or something.
Capacitive touch screen systems react to a local change of capacitance on the touch panel. Human finger as well as conductive material like a coin are working because electronic charges attracted by them. Putting a conductive surface on top of an insulator (the touch screen cover is like an insulator) is making a capacitor, right?
While developing, the device is connected via a USB-cable or debugger with the PCB. When I check the capacitive touch functionality on the display, it works perfectly. Every time I touch the display with my fingertip, the capacitive touch sensor triggers an interrupt on INT.
However the capacitive touch screen must require a bridge capacitance across the matrix grid of the electrodes beneath the thin glass surface. To make contact with the simulated finger is easy but to modulate its local bridge capacitance is not easy without motion.
Finally, can a capacitive stylus work without being held by a human hand? Of course. All you need is a conductive plate. You can cut a little square of aluminum foil and put it on the screen. It'll be detected as a touch-and-hold event. How do capacitive touchscreens really work? According to what it says on the box, er, according to their name.
On a small scale, just connectors in the corners would be enough to detect a change in capacitance across the screen. There are probably more lines to handle the relatively large area as well as multitouch. You should probably look up touch screen controllers, those will probably give you the relevant details. \$\endgroup\$ –
A capacitive touch screen is one half of a capacitor. To make it work, the equivalent of the remaining capacitor plate is needed. This will be a conductive element that is capable of absorbing charge. Your finger is connected to you and your body is large enough to absorb the small amount of charge to be sensed by the screen.
A much more definitive test could be done with one of those flexible rubber topped capacitive touch-screen stylus pens and a robot or 3-axis pen holder. It should be reasonably straight forward to modify a cheap 3D printer to a 3-axis CNC.