The Arduino board on my desk

Here's the board on my desk. I have the ground (on the right) and three connections coming off of the common (on the left): a jumper to analog pin 0 (voltage sense), a ~400Ω resistor to digital pin 11 (discharge), and a 1MΩ resistor to digital pin 12 (charge). Basically, when 11 is low and 12 is high, 5V is fed through the 1MΩ resistor and the probe's capacitance slowly lets the voltage increase; when 0 detects that at 3.15V (63% of 5V), 12 is brought low and 11 is set to input mode to discharge the capacitors back to zero. The time it takes to get to that voltage is divided by 1MΩ and we now know our capacitance...