I recently made a small version of Lord Kelvin's "thunderstorm" while I was bored at school one day, and I'm still mystified as to how such a large potential gets generated. I have two hypotheses, but I haven't had a chance to test them yet and I was wondering if anybody knew how the generator works. My first hypothesis was that perhaps ions in the water are pushed and pulled by the rings attached to the buckets and it is the ions that provide the potential difference. This seems improbable, but I was going to test the apparatus with deionized water just to see. My other hypothesis takes into account the potential difference that exists between the lower buckets and the water source above it. I remember reading in Feynman's lectures a while ago that there exists about a 100V potential difference for every meter above the earth (assuming flat ground with dry air or lots of ions present like above the ocean). Could the water actually be carrying this charge difference down with it? The water source is only half a meter (if that) above the lower buckets. The apparatus has a 1.5cm spark gap made of tin foil that discharges every 10 or 15 seconds while its running. I haven't done the calculation, but I'm guessing this is on the order of magnitude of thousands of volts. Any ideas on how such a large potential difference builds up so quickly?