Lord Kelvin's '80s-RAD party trick!

Lord Kelvin's '80s-RAD party trick!

Lord Kelvin, what a guy!

For an obscure way to generate high voltage sparks from dripping water, try the Lord Kelvin's Thunderstorm! This experiment was originally performed by Scottish-Irish mathematician and physicist William Thompson, later given the title Lord Kelvin. He lived from 1824 to 1907 and was an all-around thinker and innovator for his time. His achievements range over thermodynamics, electricity, navigation, philosophy, and the structure of the atom. Some of his ideas were off base, while others became the fundaments of modern physics. The unit for absolute temperature bears his name, Kelvin. 

Sparks from Water

Thompson was experimenting with the phenomenon of electricity, which, back in the day, meant you rubbed different things together and hung them from strings to see if they attracted or repelled. He was one of the first to note the effects of electrostatic induction. Induction can be seen to happen when a charged object is placed to an uncharged object; the portion of the uncharged object can be seen to take on an opposite charge. Fig. 1 shows the even charges within a neutral object. Fig. 2 shows how a nearby charged object will induce an opposite charge on the nearest part of the neutral object.


Thompson expanded on this idea by developing his 'thunderstorm' experiment to amplify this effect. The experiment starts with a single volume of uncharged water at the top. Water is allowed to flow out through two small nozzles... tiny faucets really. A pair of short conductive tubes is placed around the drippers right in the location that the water streams break into separate droplets. Finally, the drops of water from the two separate drippers are collected in two metal cups, which are on insulators.


Thompson noted that when opposite electrical charges are applied to the two rings, it causes the water in the vicinity of the rings to become charged. When drops break free from the body of water, those drops carry the charge down into their respective cups. Fig. 3 shows this arrangement.

In the final version of the experiment, Thompson set the system of rings and cups up so that the charge from one cup became the source of the charge for the opposite ring, and visa-versa, as shown in Fig. 4. This cleaver approach lets the induction effect self-reinforce as the charge grows. The cups are also placed further apart and the spark is allowed to jump between a pair of rounded electrodes. (Sharp electrodes let the charge bleed away before it can get big enough to spark!)

This is all quite interesting, but I still couldn't quite picture it *actually* working. I mean... come on, I can arrange this whole thing in my basement out of plastic cups and aluminum foil - It doesn't mean that high-voltage is suddenly going to come jumping out at me from some soup cans on foam blocks...??  Right??  

SURPRISE - SCIENCE!   Yep. It totally works!
With a little fine tuning, I was able to get a reliable 4mm spark every 4 seconds. This is a video of the contraption I made:

How much voltage is that?

Air has a dielectric breakdown voltage of 3x10^6 volts/meter. That is to say, it takes 3 million volts to jump across a meter of air. Any less than 3 million volts, and that meter of air is a pretty good insulator. Dielectric breakdown scales with length, so you can multiply like this to cancel meters and get volts:

(3 x 10^6 volts/meter ) x (4 x 10^-3 meters) = 12 x 10^3 volts, or 12,000 volts.

Naturalists take note..

So invite your naturalist friends over for a rousing good time! Brandy... smoking jackets... parlor shades drawn... check! This is absolutely RADICAL in the 1880s sense of the meaning!

 Note: It does work better in low humidity places, and the foam under the cups has to be absolutely dry! Also, remember, the droplets must break off from the streams right within the foil 'inductor' rings. It's those tiny separate drops that are moving the charge to the cup. If the streams make it all the way down to the cups, it's just shorting out and no charge is building anywhere. Have fun!

Here's a link to a project on Instructables that has more detail. 

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