Kelvin water dropper

The Kelvin water dropper, invented by Scottish scientist William Thomson (Lord Kelvin) in 1867, is a type of electrostatic generator. Kelvin referred to the device as his water-dropping condenser. The apparatus is variously called the Kelvin hydroelectric generator, the Kelvin electrostatic generator, or Lord Kelvin's thunderstorm. The device uses falling water to generate voltage differences by electrostatic induction occurring between interconnected, oppositely charged systems. Its only use has been in physics education to demonstrate the principles of electrostatics.

A typical setup is shown above. A reservoir of water or other conducting liquid (top, green) is connected to two hoses that release two falling streams of drops, which land in two buckets or containers (bottom, red and blue). Each stream passes (without touching) through a metal ring or open cylinder which is electrically connected to the opposite receiving container; the left ring (red) is connected to the right bucket, while the right ring (blue) is connected to the left bucket. The containers must be electrically insulated from each other and from electrical ground. Similarly, the rings must be electrically isolated from each other and their environment. It is necessary for the streams to break into separate droplets before reaching the containers. Typically, the containers are made of metal and the rings are connected to them by wires.

The simple construction makes this device popular in physics education as a laboratory experiment for students.

A small initial difference in electric charge between the two buckets, which always exists because the buckets are insulated from each other, is necessary to begin the charging process. Suppose, therefore, that the right bucket has a small positive charge. Now the left ring also has some positive charge since it is connected to the bucket. The charge on the left ring will attract negative charges in the water (ions) into the left-hand stream by the Coulomb electrostatic attraction. When a drop breaks off the end of the left-hand stream, the drop carries a negative charge with it. When the negatively charged water drop falls into its bucket (the left one), it gives that bucket and the attached ring (the right one) a negative charge.

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