Kelvin Water Dropper
Warning: This activity can be tricky!
Generating electricity with water might seem like a strange idea, but this experiment is designed to do just that.
The device we will build is sometimes called Lord Kelvin's Water Dropper - after its inventor.
The idea of the water dropper is to let two streams of water drops fall through a set of rings into two cans, building up electric charge as they drop. Each ring is connected to the opposite can below it (see the diagram).
You can use any conductive metal to make the rings and cans, but you will have greater success cutting the rings from small diameter steel cans and using larger diameter steel cans beneath. Be careful when cutting metal - ask an adult for help.
Mount the rings above the cans using coat-hanger wire, which is sturdy enough to support the rings and will allow you to adjust the rings easily. The coat-hanger wire also serves as the electrical connection between the rings and the cans.
When you connect the wire to the can or ring, you must ensure there are no sharp points that could cause electrical charge to leak into the atmosphere. The best way to prevent this leakage is to cover any join with aluminium foil and then tape the foil down.
Ensuring the water drops flow consistently is the hardest part of the experiment. Connect two hoses from a large container of water to two fixed eye-droppers or small nozzles. The wider the container, the more uniform the flow of water will be.
The drops should appear at a rate of more than one per second. If the flow of water is too slow, the device will charge up very slowly, or not at all.
Next, adjust the height of the nozzles and rings so the transition from a steady flow to drops occurs inside each ring.
Check where the coat-hanger wires cross each other. They should be about one centimetre apart from where they cross.
Once the water dropper is operating, watch for sparks leaping between the wires. This may take a few minutes to start up. Once it has started, you should see it sparking every few seconds. The sparking occurs because electric charge is building up on each half of the equipment. When the charge difference is sufficient, small lightning bolts jump from one wire to the other.
How does it work?
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.
