The Kelvin Water Dropper, also known as Lord Kelvin's Thunderstorm. Let's just watch it for a moment to see the thing in action and we'll explain as we go. There are streams of water on the left and right and there's a spark gap at the front center of the image and we'll see the sparks in a moment. But you also see an electrical charging and discharging whenever there's a spark. Here is a diagram which gives a first explanation of how the device works. Ordinary water comes from the top and is split into two branches that flow, as you can see, on the left and to the right, into ordinary buckets. The buckets are cross-connected to cans that are suspended, cans without bottoms, such that the stream of water flows through. There is an accumulation of charge on the two buckets that grows without bound until the electric field in the spark gap is large enough to cause a discharge, a spark. Before the discharge occurs, the electrical charge is up and deflects. With that preliminary explanation, let's watch the device in action some more. Here we see the electrical charging and there must have been a spark. Let's zoom in and see if we can actually see the spark. Charging, spark. Charging, spark. Let's get really close so we can really see the spark. Charging, discharge, spark. The device we built sparks every 10 to 15 seconds. Charging, spark. Now let's take a look at the left hand side. Here's the can through which the stream of water flows. The next item we can look at is watch the stream of water on the left. As the device charges, the stream of water gets attracted to the can. And when there's a discharge, the stream of water flows straight again. Charging up. The electrostatically charged water is attracted to the can that has the opposite charge. When the discharge occurs, the stream of water falls straight again. The falling water is polarized by electrostatic induction at the top before it starts to fall through, say, the positively charged can. So the negatively charged stream of water feels an attractive force to the positively charged can. That's enough to deflect it by several millimeters. If you watch closely from this distance, you can see the sparks happening in the image. The spark gap is adjusted to be about 4 millimeters in this case. 4 millimeters through air sparking from rounded electrode tips. One can estimate that the voltage associated with these sparks is about 10,000 volts. That's a lot of voltage, but not very much charge, actually. So these sparks are no more dangerous than static electricity that you accumulate on your body when you shuffle your feet across a carpet, for example. We can take one last look at the device in action and mention a few details of its construction. The buckets must be very well insulated on the bottom. They have to be on very good insulating blocks to avoid having the charge leak away before enough of it builds up to cause the discharge in the way we want. The tube at the top that divides the stream of water can be metal or plastic. Hours is made of metal. We use the siphon to draw the water out of a jug from the top of the device as shown. Here's the diagram of the apparatus again. The choice of which side becomes positively charged and which side becomes negatively charged arises from some random, tiny imbalance of the charge on the two sides. Once the imbalance is established, it continues to grow. The water is polarized by electrostatic induction such that, say, the negative droplets are preferentially dropping through the positively charged left hand can and vice versa on the right where positively charged droplets are attracted and fall through the negatively charged can. It's not hard to imagine that this effect works not just with separated little drops but indeed works for a continuous stream of water the same way.