The Kelvin water dropper demonstrates electrostatics by building up opposite charges in electrodes until it creates a spark. The system is designed such that positively- and negatively-charged water droplets are attracted in opposite directions and thus fall in opposing buckets, building up the charges within them. When the charges build up sufficiently, it creates a spark.
Kelvin Water Dropper
Like most spark photos, this image was the result of patience, timing, and luck.