Getting Sparks From Water With Lord Kelvin’s Thunderstorm

In the comments to our recent article about Wimshurst machines, we saw that some hackers had never heard of them, reminding us that we all have different backgrounds and much to share. Well here's one I’m guessing even fewer will have heard of. It's never even shown up in a single Hackaday article, something that was also pointed out in a comment to that Wimshurst article. It is the Lord Kelvin's Water Dropper aka Lord Kelvin's Thunderstorm, invented in the 1860s by William Thomson, 1st Baron Kelvin, the same fellow for whom the Kelvin temperature scale is named. It's a device that produces a high voltage and sparks from falling drops of water.

Lord Kelvin's Thunderstorm is build around the concept of water droplets falling through inductors. Two streams of water fall from small holes in reservoirs at the top. Those streams fall through two metal cylinders, called inductors, not making contact with them. A stream of falling water will change from a continuous stream into individual drops at some point, if it falls far enough.

The inductors are vertically positioned such that this change from continuous stream to individual drops happens as the water is falling through the inductors. To see these drops, the photo on the right, above, was taken with a fast shutter speed. Finally, the drops fall into metal cans, called receivers, at the bottom.

The receiver on the left is electrically connected to the inductor on the right, and the receiver on the right is electrically connected to the inductor on the left. You can see this in the photo above in the form of the crossing yellow and red wires.

Also, a wire is connected to each receiver and goes to either side of a spark gap. Every now and then, a spark crosses the gap. Magic. Or is it?

Besides watching the sparks, there are some additional fun ways to observe this repeated charging and discharging. One such way is to watch the drops fanning out as they fall, and then suddenly falling straight again.

Why does this fanning-out happen? The drops coming out of the inductors have the opposite charge of the inductors. For example, the drops falling below the left inductor are positive, while the left inductor is negative. Note also that the bottom lip of the inductor is to one side of the drops. Since unlike-charges attract, there's a horizontal attraction between the inductors and the drops falling below them that imparts some sideways movement to the drops.

The result is a visible fanning-out of the drops. This fanning-out gets wider and wider as the inductor becomes more and more charged. That is, until the spark occurs and discharges everything. At that time the drops fall straight down again only to start fanning out as the charge builds back up.

Another fun way of observing the charging and discharging in action is to place the terminal of an electroscope near an inductor or receiver. As the charge builds up, the electroscope leafs will spread apart. But when the spark occurs, the leafs fall back together

Rather than have just one stream falling through each inductor, some builders use something like a shower head to have multiple streams fall through. Below is a video from science YouTube channel, Veritasium, showing a large and very sci-fi looking Lord Kelvin water dropper that uses shower heads.

It also has a pump to keep the water flowing continuously. You might wonder what would happen if you did use a pump, since you’d be electrically connecting the two receivers. To avoid that, the receivers are just meshes through which the water drops fall. As the drops falls through, the mesh takes their charge. So I guess you can say these receivers receive the charge but not the water.