Kelvin water dropper

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A diagram of the Kelvin water dropper.

The Kelvin water dropper,^[1] named for Lord Kelvin, is a type of electrostatic generator. Kelvin referred to the device as his water-dropping condenser. The device uses falling water drops to generate voltage differences by utilizing the electrostatic induction^[2] occurring between interconnected, oppositely charged systems.

Water runs down from the top, with slightly positively-charged water attracted to the negative ring and slightly negative water attracted to the positive ring. The charged water flows through the ring and into a container. The charges then build up in the ring connected to the container opposite it - attracting even more charge. This results in a positive feedback loop.^[3] The apparatus can be extended to more than two streams of droplets.^[4]

[edit] References

^ Sir William Thomson (Lord Kelvin), "On a self-acting apparatus for multiplying and maintaining electric charges, with applications to illustrate the voltaic theory," Proceedings of the Royal Society of London, vol. 16, pages 67-72 (20 June 1867).
^ Article in Journal of Chemical Education (PDF; subscription required). Division of Chemical Education of the American Chemical Society (November 2000 issue). Retrieved on 2007-04-02.
^ www.csiro.au/helix/experiments/dhexpkelvin.shtml.
^ Markus Zahn, "Self-excited a.c. high voltage generation using water droplets," American Journal of Physics, vol. 41, pages 196-202 (1973).

[edit] External links

This video is available on YouTube.com at "Lecture 10, M.I.T. 8.02 Electricity & Magnetism, Spring [term] 2002": http://youtube.com/watch?v=RQX8I9ZWtPQ&feature=related . See the last 6 minutes of this video for operation of Kelvin water dropper.

As is seen in the video, the water must be released as droplets, rather than as a continuous stream. Without the separation into isolated, self-contained droplets (of which there could be many simultaneous, as in a shower head), there is nothing to prevent the conduction and equalization of charges back up through the water stream, in effect shorting out the system. Properly operated Kelvin devices are able to generate high voltages, resulting in large, long, frequent and bright sparks.