Is it possible using capacitors or otherwise to increase the capacitance of a given circuit? For example, lets say we had a wire with a piece of foil connected to it, running to the arduino ground. Connecting a capacitance sensor to the foil would give a certain reading. What could be connected to the foil wire so that the capacitance reading is higher on the meter (permanently)
You can add parallel capacitors as many as you like to increase the capacitance.
Do they have to be discharged in some manner at some point though?
All the capacitors connected in parallel will be charged or discharged in the same way by whatever circuit you have connected them to - they act like a single bigger capacitor.
Cool, what about reduction of capacitance? Seems like I can use resistors. Would it be feasible to have a switch that runs betweens capacitors and resistors and in theory allowed you to toggle your capacitance readings very, very low or very, very high?
If you put two capacitors in series you get a reduced capacitance.
A reduced level compared to there being no capacitors at all? Why is that?
It might help if you can tell us what you are trying to do.
I was basically just experimenting around to understand how this stuff works. I have the capacitance sensor sample from the Arduino Uno Projects book created (project 13, the touchy feel lamp).
I wanted to understand more about capacitance, so I wired up another piece wire and foil on the bread board and was trying to see how I could modify the capacitance of the 2nd wire. So the two pieces of foil are touching and one wire runs to the capacitive sensor and the other is just stuck in the breadboard. From there I was just messing around to see how I could modify levels to understand more about it.
For example, if the 2nd foiled wire is connected to a 3rd wire that I hold in my hand, capacitance levels go through the roof. If that same wire runs through a strong resistor, the values go up, but less so.
So I started thinking, could I create a board the effectively reduced the 2nd wire/foil’s capacitance to 0 and could I also increase it without having to introduce my own human contact.
I suggest you Google for tutorials about capacitance and how they behave in series/parallel. Once you get your head around the concepts it's fairly straight forward and will give you a good grounding for further learning about electronics.