A diagram of the Kelvin water dropper.

The Kelvin water dropper,^[1] invented by Lord Kelvin (1867), 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 using positive feedback and the electrostatic induction^[2][3] occurring between interconnected, oppositely charged systems.

[edit] The Setup

The simplest setup for this is as pictured at right. A reservoir has two holes that drip water (or other liquid). The streams of dripping water each pass through a conducting ring, and land in a bucket. The buckets must be electrically isolated from each other and from their environment. Similarly, the rings must be electrically isolated from each other and their environment. The left ring is electrically connected with (wired to) the right bucket. And the right ring is wired to the left bucket.

If the buckets are metal (conducting) the wires may be attached to the buckets. Otherwise, the bucket-end of each wire can just sit in its bucket, as long as it is contacting the water in the bucket.

[edit] Principle of Operation

Everything starts out virtually uncharged, but there is usually some tiny imbalance of electric charge on every object. So one bucket will probably have a slightly more positive charge on it than the other. Alternatively, the water drops might pickup some small charge from rubbing against the spout or the air as they fall. If nothing else, the natural pH of the water result in a few more positive or negative ions randomly living on any given water droplet. Whatever charge lands in a bucket gets distributed over that bucket and the attached ring.

One way or another, one bucket and ring develops a little bit of charge. For the sake of example, lets say that the left bucket has a bit of developed positive charge. Now the right ring also has some positive charge since it is connected to the bucket. The charge on the right ring will attract negative charge to the right spout by electrostatic induction. The next water drop to come out of that spout will also have a negative charge. When the negatively charged water drop falls into its bucket (the right one) it gives that bucket and the attached ring (the left one) a negative charge.

Once the left ring has a negative charge it attracts positive charge to the left spout, so positively charged drops fall from the spout to the positively charged bucket, making that bucket even more positively charged.

So positive charges are attracted to the left spout by the ring, and positive charge drips into the positively charged left bucket. Negatively charges are attracted to the right spout and negative charge drips into the negatively charged right bucket. The positive feedback^[4] of this process makes each bucket and ring more and more charged. The higher the charge, the more effective the electrostatic induction is, so the whole process speeds up with time. Eventually, both buckets become highly charged, and a few effects may be seen. A spark may briefly arc between the two buckets or rings, decreasing the charge on each bucket. Or if this isn't allowed to happen, the buckets will start to electro-statically repel the droplets falling towards them, and may fling the droplets away from the buckets. The water drops might also be attracted to the rings enough to touch the rings and deposit their charge on the oppositely charged rings, which decreases the charge on that ring. Each of these effects will limit the voltage that can be reached by the device.

As with other forms of hydroelectric power, the energy here ultimately comes from the gravitational energy released by letting the water drops fall. Most of the energy is wasted as heat when the water drops land in the buckets.

The apparatus can be extended to more than two streams of droplets.^[5]

[edit] Other Names

The Kelvin water dropper sometimes is called the Kelvin hydroelectric generator or the Kelvin electrostatic generator

[edit] References

[edit] External links

• Video demonstrating Kelvin water dropper in operation: "10, M.I.T. 8.02 Electricity & Magnetism, Spring [term] 2002". See the last 6 minutes of this video for operation of Kelvin water dropper. Printed material related to this video: See "MIT Open Courseware" website; specifically, assignment 4 of course 8.02, which is available here: http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-and-MagnetismSpring2002/DownloadthisCourse/index.htm . As is seen in the video, the water must be charged as the stream breaks into droplets. If one attempts to charge the stream before it breaks into droplets — and the outlet can have several streams, as in a shower head — any charge that's induced in the stream can flow backwards through the stream and into the reservoir instead of flowing onto the droplets, so that in effect, the system is short-circuited. Properly operated Kelvin devices can generate high voltages, resulting in large, long, frequent, and bright sparks.
• Detailed description of device and how to build your own Kelvin water dropper.