Question

can any one describe me the working process of kelvin water drop?

Answer 1

Imagine that the left-hand can has negatively charged water drops landing in it. Some of that negative charge will flow up the wire to the right-hand ring. As the stream of water flows through the right-hand ring, the negative charge will induce a positive charge in the water stream.

The drops landing in the right-hand can carry a positive charge, which makes the left-hand ring positively charged. Drops falling through the left-hand ring become negatively charged. As the drops continue, more charge builds up in the cans.

How does one can start with a charge in the first place?

At the beginning there will always be some charge imbalance between the cans. The charge could come from ions floating in the air, cosmic rays or other sources.

If your device doesn't spark, it may be having trouble 'deciding' which side will be positive or negative. You can give it a 'kick start' by holding a charged object briefly near one of the cans while it is running (for example, a balloon or plastic bottle rubbed on your hair).

It is possible to build up huge voltage differences between the cans, sometimes as much as tens of thousands of volts. However, the current available to flow between the cans remains quite small. It can be enough to give you a fright, but not enough to hurt you.