Tong Shiu-sing (Translation by Janny Leung)

### How does the Kelvin water drop electrostatic generator work?

When we talk about hydroelectricity, it is normal to think of huge dams or similar constructions. Scientists use hydropower to rotate turbines, generating electricity by the principle of electromagnetic induction. But please do not misunderstand that I am going to introduce those huge power plants here. Instead, I will talk about a tiny setup that generates static electricity; it accumulates electric charge by the principle of electrostatic induction, eventually resulting in a very high voltage. Those who see this setup for the first time would be surprised that this simple Kelvin water drop electrostatic generator produces such an amazing effect.

Take a look at Fig. 1. Water flows into the metal vessel via the dropper which is inserted into the little hole of the metal cover. The cover keeps a distance from the vessel without touching each other. Since there are many ions in water, when it falls, it may bring a very small amount of charge to the metal vessel. Such electricity should be extremely small at first, and it would be neutralized by opposite charge very soon, not being sufficient to produce a visible effect. However, somebody came up with a very special idea and successfully accumulated such charges bit by bit. The method is to create an identical setup, and then cross-connect the two with wires as shown in Fig. 2. Suppose vessel A carries a slightly negative charge, part of the charge will be transmitted to metal cover B' via the wire. Since opposite charges attract each other, metal cover B' will attract water that carries more positive charges into vessel B. And then vessel B will share these charges with metal cover A', which will attract water that carries more negative charges into vessel A, resulting in an increase in negative charges in vessel A! Via an inter-electrostatic induction between the two setups, the charges on the vessel and metal cover on both sides will increase continually, creating a special effect of generating electricity by water drop.

 Fig. 1 Fig. 2  Kelvin water drop electrostatic generator.

Water would be sprinkled instead of falling vertically into the vessels not long after the water drop electrostatic generator comes into operation, as if the sprinkler of a fountain is inverted. This is because the charges of the water drops are opposite to that of the metal cover, and the two attract each other. Besides, if the connecting end of a neon bulb is in contact with the metal vessel, the neon bulb will be lit up shortly, indicating that the metal vessel carries electricity. It has also been found that after connecting two conductors as electrodes to the vessels, and then drawing the two electrodes close to each other, they even discharge through the air with spark, showing that the voltage produced is very high. But it is by no means easy to produce such an effect: the setup has to be well designed, the metal parts which are in touch with other components have to be insulated properly, and a very dry environment is also needed. It is very humid in Hong Kong, making it difficult to be done here.

Just a small quiz at last. A water drop electrostatic generator can separate negative and positive charges, thus the electrostatic potential energy of the system must increase. You probably know that energy can neither be created nor destroyed. Where does the electrostatic potential energy of the system come from then? Can you write down the whole process of energy transfer?