Submitted By –

Singh,

Class XII ‘C’

Acknowledgement

I wish to express my deep gratitude and sincere thanks to the Principal, for her encouragement and for all the facilities that she provided for this project work. I extend my thanks to Ms. Toshi, H.O.D. Physics, who guided me to the successful completion of this project. I take this opportunity to express my deep sense of gratitude for his invaluable guidance, constant encouragement, immense motivation, which has sustained my efforts at all the stages of this Project work.

I can’t forget to offer my sincere thanks to my classmates who helped me to carry out this project work successfully and for their valuable advice and support, which I received from them time to time.

Index

HISTORY

Lord Kelvin (William Thomson) developed electrostatic theory and a range of novel and interesting electrostatic apparatus. Described below is a reconstruction of a water dropper electrostatic generator. Its capable of generating several 1000's volts but at very low current (few uA). It's a high voltage generator but develops no real usable power so it is really only to investigate static electricity. This design shown here is based on notes given in A D Moore’s wonderful book – “Electrostatics”.

 

Sideways Front

Design: The basic idea of the generator is very simple. There are two metal pots (I used stainless steel sugar bowls) supported on good (Perspex blocks) insulators. These are

wired, using well insulated connectors, to metal rings (Cu or Al pipe ca. 2cm length, 2cm diameter) above the pots. The left hand pot is wired to the right hand ring while the right hand pot is wired directly to the left hand ring. So the pot-ring connections cross-over.

Above the rings are two water spouts that produce water drops from a water reservoir. The water tank can be earthed or left floating (electrically). A compression clamp is used to adjust the water flow so that we have as greatest number of drops produced as possible without the water coming out as a continuous flow. Normal tap water seems to work fine.

Now at the start we assume that one pot will tend to be slightly more positively charged than the other. So let us say, for the sake of argument, that the left hand (LH) pot is positive and the right hand (RH) pot relatively negative. As the LH pot is wired to the RH ring the RH ring will become positive while the LH ring negative (because they are directly wired pot-to-ring).

If a drop of water falls through the RH ring the positive charge on the ring will induce a negative charge on the drop. As it passes through, this now negative drop will fall into the RH pot, which already negative will now acquire even more negative charge from this charged drop of water.

In the mean time a drop falling through the LH ring will have positive charges induced (by the negative ring) and thus will fall into the positive pot thereby increasing the positive charge on this pot still further.

As each pot has acquired more corresponding charge each of the crossed wired rings will be more charged and thus will induce more charge on the subsequent drops falling through them. In quite a short time then the two pots will quickly build up positive and negative charge in the same way money in a bank account increases in value due to more money being added in and compound interest being added on to the whole lot!

Notes on construction:After much experimentation and reading through various books and the web I decided to ditch the wooden support structure and make a plastic only construction. This worked correctly the first time it was assembled. It appears that wood is too poor an insulator to be even close to the crucial parts of the apparatus and has the effect of stopping the buildup of any charge, stalling the generator.

The two cross wires must be made of well insulating wire (we can use inner wire of TV coax or rubber coated wires) and even though insulated should not physically touch each other anywhere. Any loss of integrity of the insulators will stop any charge building up and consequently stop anything happening at all.

All insulators need to be clean and dry.

Emptying water full pots: The two pots sit on metal plates from which the cross wires and the output connections are made. The pots make connection sitting on the plates. When the pots are full with water they can simply be picked up and poured back into the tank (but you will need to start again as you will discharge the apparatus in the process).

Kick Start:The obvious problem with this generator is that it needs the two pots to have slightly different charges upon them in order for the generator to work. In order for this to be so the insulators supporting the pots need to be as good as possible, otherwise any charge difference will simply leak away too quickly.

If the insulators are slightly loss-causing, you can kick-start the charge build-up process by bringing a charged body (such as a comb, or plastic ruler suitably charged

by wiping in a cloth or clean hair) near to one of the pots. This will accelerate the process and might overcome the initial losses that stop the generator working.

I have an 2kV electrostatic voltmeter that goes full scale defection in a few seconds using this marvelous machine! If you connect one wire of a neon bulb to one pot and the other wire almost touching the other pot the neon will flash when the potential reaches high enough to jump the air gap between neon wire and pot.

How it works: The above description basically accounts for how the machine works but how exactly does the machine separate charge and what role do the drops play? As the water flows through the rings the voltage on the ring attracts (by coulomb attraction) the (oppositely charged) ions in the water stream. So if the ring was negative the stream of water flowing through would have positive ions drawn to the surface. When this stream of water brakes off into little water drops, each drop takes away some of this charge. Each drop adds this charge to a pot. The difference in charge between the two pots is the voltage we measure (the voltage is the energy of attraction between the oppositely charged pots).

Kelvin Water Dropper A Water Powered Electrostatic Generator

Figure 1. The Kelvin Dropper

We drew a few sketches of how a Kelvin Water Dropper might come together and then I

used that to build a test model. We had some plastic construction kit material lying there at

our homes and we thought there may be enough to build a Kelvin Water dropper. Plastic

construction kit parts are good for the structural components because they are easily

adaptable and plastic is also an insulator. Other parts used in the construction were found

around the house including plastic bottles which were used to contain cold-drinks. We also

scavenged some wires from aquarium tanks, sweets gift-boxes and detergent containers.

The final model didn’t utilize bottles but instead used a plastic box.

Incitor Rings

Figure 2. Incitors made from wire.

The rings: Originally, I was going to use old copper wire rings for these parts but when I tried

them, they didn’t provide enough charge collection.

Eventually, I settled on aluminium wires from coax cables to which were bent and soldered

to form rings. The Aluminium wires had sharp edges which would have probably been a

source of corona discharge. The advantage of this was that the wire was also structural

being strong enough to hold the rings in the desired position.

Water Droppers

Figure 3. Water Droppers.

The droppers were originally two holes in the base of the container but there was

considerable variation in diameters of the holes, leading to drops flowing at different rates.

Then I used parts scavenged from aquarium tanks and inserted into the fuel tubing. These

provided very good rate of flow of water and well formed drops

Charge Containers

The charge containers deserve particular attention as they required a lot of experimenting

with before they worked. The surface of the aluminium foil contains the charge that builds

up drop by drop. There must be nothing on the surface where the wires that form the spark

gap collect the charge. These two aluminium foil cylinders are placed inside the two

detergent containers.

Figure 4. Insulated base, charge collecting cans and spark gap.

The charge containers must be well insulated and this was insured by placing the entire

platform on insulated surfaces and then further insulating them from the platform using

another set of plastic parts. The cans are secured from falling over by balancing them.

Sparking

The flow of water starts off as a steady stream of fast flowing droplets slowly becomes more

charged and repels the other water drops causing the stream of water to spread out past

the inciter rings and even fly upwards. With a breakdown voltage of 33,000 Vcm-1 at

standard temperature and pressure, with a spark gap of around 5 mm, this estimates the

voltage at some 16,500 V across the spark gap. Of course the current is minute and hence

the power output is really quite low and not dangerous.

Troubleshooting

If you decide to build a Kelvin water dropper and it does not work then I can offer the

following advice:

The charge collecting cans need to be isolated from the Earth and if quite often the base can become wet when adjusting the droppers which can prevent the spark occurring.

The spark gap needs to be carefully adjusted otherwise corona discharge will cause the charge to leak away at the sharp edges of the device. Try a smaller gap before opening it wider.

The inciter rings need to be placed at a level such that they are just below the forming droplets of water in order to start the charging process in the beginning.

The solder can be filed down to avoid any sharp points on the cans which can cause the charge to escape.

Bibliography