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.
A Brief Overview
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?
How It Works
- To begin, there has to be some excess electric charge somewhere, either positive or negative. And there usually is. Let’s say that the right receiver has a slight net negative charge. Since it’s wired to the left inductor, the left inductor also has a slight negative charge.
- Here is where the real magic happens. Remember that the inductor is vertically positioned where the continuous stream breaks up into drops. Since the left inductor is negatively charged, it repels negative charge in the water. The continuous water stream from the reservoir to the inductor acts like a wire and that negative charge is repelled up that stream to the reservoir. But that leaves the drops falling below the inductor with a net positive charge. And since they are individual drops, they stay positive, all the way to the left receiver below them.
- The left receiver is a metal can and is in electrical contact with the water in it, and so the left receiver is made positive by those positively charged drops.But the left receiver is wired to the right inductor. That means the right inductor also becomes positively charged.
- That right inductor also has a stream of continuous water entering it from above and drops leaving it below. Since the inductor is positively charged, it repels positive charge up the solid water stream to the reservoir above. Meanwhile, the drops leaving it are left with a net negative charge and fall into the receiver below them.
- If you recall from step 1, the right reservoir was the one we started with, and was slightly negative. Now, as a result of the negatively charged drops falling into it, that receiver becomes even more negative.And since that right receiver is wired to the left inductor, that makes the left inductor more negative, which repels negative charge up the left stream, while the drops below it are made positive, and fall into the left receiver, making the left receiver more positive, and so on.
- So the right receiver becomes more and more negative while the left receiver becomes more and more positive. But remember, the two receivers are wired up to opposite sides of a spark gap. The electrodes at the spark gap also become more and more negative and positive. That is, until the voltage across the spark gap becomes so strong that it breaks down the air between them, and a spark crosses the gap, discharging the whole thing. But of course a little net charge is left somewhere and the whole process repeats.
Fanning Drops And Electroscopes
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.