Kelvin water dropper
The device uses falling water to generate voltage differences by electrostatic induction occurring between interconnected, oppositely charged systems.
A reservoir of water or other conducting liquid (top, grey) is connected to two hoses that release two falling streams of drops, which land in two buckets or containers (bottom, blue and red).
The simple construction makes this device popular in physics education as a laboratory experiment for students.
[2] During the induction process, there is an electric current that flows in the form of positive or negative ions in the water of the supply lines.
The voltages reached by this device can be in the range of kilovolts, but the amounts of charge are small, so there is no more danger to persons than that of static electrical discharges produced by shuffling feet on a carpet, for example.
The kinetic energy is wasted as heat when the water drops land in the buckets, so when considered as an electric power generator the Kelvin machine is very inefficient.
Also, the idea of bringing small amounts of charge into the center of a large metal object with a large net charge, as happens in Kelvin's water dropper, relies on the same physics as in the operation of a van de Graaff generator.
[4] In 2013, a combined group from the University of Twente (the Netherlands) constructed a microfluidic version of the Kelvin water dropper, which yields electrical voltages able to charge, deform and break water droplets of micrometric size by just using pneumatic force instead of gravity.
[5] A year later, they developed another version of a microfluidic Kelvin water dropper,[6] using a microscale liquid jet (which then broke into microdroplets) shot onto a metal target, which yielded a maximum 48% efficiency.
[7] In De Magnete, published in 1600, William Gilbert included studies of static electricity produced by amber and its interaction with water.
He observed the formation of conical structures on water which are commonly now called Taylor cones.
Lord Kelvin used this foundation of accumulated knowledge to, in 1859, create an apparatus involving the interaction of a stream of water with the Earth's static electric field to cause charge separation and subsequent measurement of charge to make atmospheric electricity measurements.
[10] It was also found that the generator worked well even if the two liquid streams originate from different electrically insulated reservoirs.
