[FotoPlasma]

Also, at 5V, a 1megohm potentiometer is only drawing 5uA (5 millionths of an ampere), unless I'm terrible at math. From my limited knowledge of E&M, at currents that low, I would be worried about interference from other electrical components. Jumping on the bandwagon, I'd go with 100k or 10k pots to get a reliable signal.