Kelvin Water Dropper
The Kelvin Water Dropper, also known as Kelvin Hydroelectric Generator, The Kelvin Electrostatic Generator and Lord Kelvin's Thunderstorm, is an electrostatic generator invented by Lord Kelvin in 1867. It uses electrostatic induction and positive feedback between two interconnected, oppositely charged systems, to generate high voltages.
How It Works
The water dropper consists of a single reservoir of water, with two outlets through which a small stream can flow. Each of these outlets may have a tap to adjust the rate of flow of water.
The jets of water fall through a conducting ring, into a bucket underneath. The left hand conducting ring is electrically connected to the right hand bucket, and the right hand conducting ring is electrically connected to the left hand bucket.
When the water is flowing, its rate must be such that drops of water break off from the jet as it is travelling through the conducting ring. This is where taps are useful to control the flow of water.
Suppose, in the left hand conducting ring, there is a slight random negative charge. As the jet of water falls through it, positive charge will be attracted along the jet of water, so the jet of water is positively charged. As the water drops break off, they will carry this positive charge to the left hand bucket below.
The left hand bucket is now positively charged, and because it is electrically connected to the right hand ring, this ring is also positively charged. Therefore negative charge will be attracted along the jet of water falling through this ring. Negatively charged drops of water will fall off into the right hand bucket. The right hand bucket is now negatively charged, and as it is connected to the left hand conducting ring, this also becomes more negatively charged than it was before.
This charge difference builds up and up, and therefore, a high voltage develops between the two buckets. Eventually, this voltage will be high enough to break down the electrical resistance of the air (approximately 3kV mm-1) and a spark will be generated across the buckets at their closest point.
How to Build One
Obtain the apparatus below, and set up as per the schematic picture above.
An excellent version, built by a Science Technician at Calderside Academy, can be seen in the gallery below. This should be used as a photo guide.
- Gavlenised Buckets (2)
- Conducting Rings (2)
- This can be, for example, a baked bean can with its lid and base removed
- A water reservoir
- Wire to connect buckets to rings (2)
- Stiff wire to form a spark gap between the buckets
- Insulating material to place on work surface
- Apparatus to support the set up, such as clamp stands and bosses
- Digital coulombmeter (2) (Optional)
- The jets of water must break into drops as they travel through the conducting ring, not before or after
- Ensure sharp points are removed from the buckets and rings, as they will spray off charge
- Even plastic and wooden benches can conduct enough to prevent charge building up, so ensure the buckets are insulated from one another
- A spark of between 4 and 5mm is reasonable to expect
- A digital coulombmeter attached to one bucket will show very quickly if charge is building up so is a good diagnostic tool
- A second digital coulombmeter attached to the other bucket adds to the demonstration as build up of equal and opposite charge can be seen, and the charge can be seen dropping when the spark is formed
- Two coulombmeters also demonstrate the initial randomness that allows build-up of charge, as sometimes one bucket will start building a positive charge, and other times the opposite bucket will start building a positive charge
If you build your own, please upload an image and add to this gallery!
--Tom Dudman 11:07, 26 July 2011 (BST)