Lord Kelvin’s Thunderstorm is a type of electrostatic generator invented by British scientist William Thomson, better known as Lord Kelvin, in 1867. Kelvin referred to the device as his water-dropping condenser.
The device uses falling drops of water to generate voltage differences by using electrostatic induction, which occurs between interconnected, oppositely charged systems.
Water runs down from the top, with slightly positively-charged water attracted to the negative ring and slightly negative water attracted to the positive ring. The charged water flows through the ring and into a container.
The water traveling through the negative ring becomes H30+ and the water traveling through the positive ring becomes OH-. The charges then build in the ring connected to the container opposite it, attracting even more charge.
This results in a positive feedback loop. When the charge eventually reaches a certain threshold, a spark will cross the gap between the rings. The Kelvin Thunderstorm has been known to build a 20,000 volt charge with as few as 100 drops of water through each side in less than six seconds.
Keep in mind, this is done without any external power source, simply using the energy of the falling water drops. Electrostatic generators can be made to be very powerful.
The charge separation and build-up of electrical energy ultimately comes from the gravitational potential energy released when the water falls. The charged falling water does electrical work against the like-charged containers, converting gravitational potential energy into electrical potential energy, plus the kinetic energy.
The Kinetic energy is wasted as heat when the water drops land in the buckets, so when considered as an electric power generator the Kelvin machine is very efficient. The principle of operation is the same as with other forms of hydroelectric power.