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The Kelvin Water Dropper

What You Will Need To Build Your Own

Did you know that you can build a very simple high voltage generator which has no moving parts and is powered by the energy of falling water? By dribbling water through empty tin cans, thousands of volts can be “magically” generated.

The water dropper was named for its inventor, the Baron Kelvin (1824-1907). A professor at Glasgow University (from 1846), Baron Kelvin also made important contributions to experimental electromagnetism and theoretical thermodynamics. With James Joule, he discovered the Joule-Kelvin effect. His name is also given to the unit of the absolute temperature scale, the kelvin.

To make a Kelvin Water Dropper, you will need:

1) Four tin cans. Two small (about 300 ml - the size of a tin of condensed soup - should be perfect) and two large (look for cans about one liter in volume).

2) Stiff wire, about one meter, cut in half. Alternatively, you could use two wire coat hangers - just untwist the tops and straighten them out carefully. Remember to remove any plastic sheilding from them!

3) Styrofoam. one or two pieces big enough to rest the large tins on so they are as electrically isolated as possible.

4) Electrical tape. Get the thick, insulating kind. You will need this to cover over any sharp edges/points on your dropper - for safety, aesthetics and, importantly, to control where electrical discharge occurs. Charge likes to collect at sharp points on surfaces, so if there are jagged edges uncovered, they may interfere with the operation of the water dropper.

5) Sandpaper.

6) If you aren’t in a rush, some glue (make sure it is the kind that will stick to metal), will help your dropper be sturdy and long lived

7) Once again, if you want to build a very sturdy, “high performance sports model” dropper, (and if you already have the equipment/skills) some solder to ensure good electrical contact will be useful.

8) You will also need a bucket (and, for a deluxe, professional dropper a couple of little taps/valves)

How To Build It

1) To Open and empty all the cans. Unless you bought them yourself, or have collected used ones, you should probably save the contents for the rightful owner. Using a can opener, remove the tops from all four cans, and the bottoms from the smaller two.

2) Using either the glue or just the electrical tape, connect a large and a small can to either end of each of the wires. In the diagram, the red wire connects the large and the small red cans, and the green wire connects the green cans. Ensure that there is a good metal to metal contact so that the electricity can flow, by removing paper wrappings, sanding gently the exposed surfaces to remove and insulating oxidization and, if you want, using a coating of solder to make the joint.

3) Cover over the ends of the wire and any rough surfaces with the electrical tape. At this point you should check that your wires are bent correctly, so that you can arrange each small can directly over the other large can. When you do this, the wires should be roughly 2-3 cm (about 1 inch) apart. Once it’s running, you will need to fine tune the separation, so don’t stress over the exact distances here.

4) Drill or punch two holes in the bucket, spaced correctly so that the water falls through them and then through the small cans. If you are making the deluxe version, make these holes so that you can fit the taps to them, then you can easily control the flow of water. However, I found that the first dropper I built worked fine without the valves.

5) Now that everything is constructed, you will need to arrange the components so that you can generate the electricity. Place the large cans on the styrofoam, with the small cans over them. Now, using a shelf or a table, hold the bucket over the cans so that the streams of water falling through the holes go through the small cans into the large cans.

What You Will See

As the water is falling, observe the space between the wires (where they are closest) and the water as it passes through the small cans. If things are arranged correctly, you should see two things. 1) If the wires are close enough together, you will soon see sparks jumping between them. If you don't see sparks almost straight away, the wires are probably too far apart. Being careful not to touch the metal directly (so you don't risk a shock) or with anything conducting, push the wires slightly closer together. When you get them close enough that the field between them is higher than the breakdown strength of air, you should see sparks jumping between them regularly. 2) Near the small cans, you will probably see small droplets turning around in mid air and flying sideways or even upwards away from the cans.

These droplets are charged the same as the cans and the Coulomb force between them repels the drops.

How it Works

Although water is full of charged particles - ions from dissolved salts and from the breakdown of water itself, the water that initially falls through the holes in the bucket will be uncharged on the average. However, the random ness of the universe will soon cause there to be a slight charge on one the can/wire systems. Perhaps a drop will fall that is slightly charged, or maybe a cosmic ray will hit the wire and cause a small charge to form on one of the little cans.

Let's say that the little can on the left (and hence the large can on the right) is slightly positive. Then, this positive charge will have three effects on the falling streams

1) Negative ions will be slightly attracted towards the left most hole, so that the water falling through it will, on average, be slightly negatively charged. Thus, the left most large can (and the right side small can) will become negatively charged

2) Conversely, positive ions will move to the vicinity of the right hand side whole, making the right hand side stream positively charged.

3) The positive charge on the can will repel any slightly positively charged drops on the left hand side - this will tend to increase the total negative charge falling into the left hand large can.

This spontaneous process will cause a negative charge to form on one wire and a positive charge on the other. What's more, this spontaneous charge separation will tend to reinforce itself - the positive charge on one small can will cause negative charge to collect on the large can below, which will charge up the other small can, which will attract positive charge to the big can below it - increasing the positive charge on the first small can, which in turn increases the rate negative charge falls through it.

The stronger this charge is, the more it tends to reinforce itself, in a process called positive feedback, that only ends when the charge becomes so great that a spark jumps between the wires, removing the charge difference (until it spontaneously builds up again).

Instead of using the charge to generate a spark, there are a variety of ways the droppers charge can be harnessed. The coulomb attraction can be used to cause a metal rod to strike a bell (you will need conducting wires to suspend the components), or the voltage might be sufficient to light a small bulb.

If you build a water dropper I would love to see pictures of them (perhaps I might put the best ones up for all to see. I would especially love to see any innovative ways you use the generated voltage.