I'm working on an installation that involves reading the resistance of lines drawn in pencil/graphite from one point to another. I'm completely new to the arduino and physical computing in general so I might be a little lost here, but as I understand it I should be able to attach the 5V from the arduino to one point then have one of the analog inputs and the ground (through a resistor) attached to the other point. My problem is that I'm not sure what value of resistor to attach to the ground. If anyone could offer some help I would greatly appreciate it and I'd be more than happy to clarify anything if it's unclear.
Probably the best start would be to draw some lines with your graphite pencil of various lengths and widths and measure them with a good ohm meter. This will establish the range of resistance you would be dealing with and a proper voltage divider network could be calculated. Have you thought how you are going to make electrical connections from the graphite lines to wire connections? This may be a critical part of being able to read your lines successfully.
You'll need to estimate the resistance of the circuit. That is, the resistance from the end of the pencil mark, through the pencil, to the connection to the pencil lead (which is actually graphite, of course). Do you have a multimeter that can measure Ohms?
Once we know (roughly) the resistance of the circuit, we can estimate the external resistance needed.
Another option is to use a "constant current circuit" which will send a fixed amount of current through the pencil, and allow us to measure voltage directly. We'll still need to know the resistance, so that we can set the proper amount of current (a few microAmps, most likely).
Sounds good in theory but I think you will come across the problem of the graphite lines fracturing and not forming a continuous circuit on the paper. You might get somewhere with a very soft pencil and soft paper like blotting paper but I think this will not work very well.
Of course I could be wrong, I was once back in 1986 ;)