"How does this thing work?... basically, the principle behind Kelvin's Thunderstorm is 'like charges repel'. The bottomless cans at the top are charged positive and negative. The positive can repels the positively charged ions in the water. So what drops through the can is mainly negatively charged ions. These ion charges build up in the buckets which are linked to the bottomless cans in a sort of feedback loop. The charges build up until a spark jumps the spark gap." - William Anderson, Static Electricity, electricitybook.com, 28 Feb 2009.