The Kelvin water dropper, invented by Scottish scientist
William Thomson (Lord Kelvin) in 1867,
is a type of
electrostatic generator
An electrostatic generator, or electrostatic machine, is an electrical generator that produces '' static electricity'', or electricity at high voltage and low continuous current. The knowledge of static electricity dates back to the earliest civi ...
. 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
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
to generate
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
differences by
electrostatic induction
Electrostatic induction, also known as "electrostatic influence" or simply "influence" in Europe and Latin America, is a redistribution of electric charge in an object that is caused by the influence of nearby charges. In the presence of a charg ...
occurring between interconnected, oppositely
charged systems. This eventually leads to an electric arc discharging in the form of a spark. It is used in physics education to demonstrate the principles of
electrostatics
Electrostatics is a branch of physics that studies electric charges at rest (static electricity).
Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for amber ...
.
Description
A typical setup is shown in Fig. 1. 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)''. Each stream passes (without touching) through a metal ring or open cylinder which is electrically connected to the opposite receiving container; the left ring ''(blue)'' is connected to the right bucket, while the right ring ''(red)'' 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.
Principles of operation
A small initial difference in
electric charge
Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...
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 because it is connected to the bucket. The charge on the left ring will attract negative charges in the water (
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s) 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.
Once the right ring has a negative charge, it similarly attracts positive charge into the right-hand stream. When drops break off the end of that stream, they carry positive charge to the positively charged bucket, making that bucket even more positively charged.
Thus positive charges are attracted to the right-hand stream by the ring, and positive charge drips into the positively charged right bucket. Negative charges are attracted to the left-hand stream and negative charge drips into the negatively charged left bucket. This process of charge separation that occurs in the water is called ''
electrostatic induction
Electrostatic induction, also known as "electrostatic influence" or simply "influence" in Europe and Latin America, is a redistribution of electric charge in an object that is caused by the influence of nearby charges. In the presence of a charg ...
''. The higher the charge that accumulates in each bucket, the higher the electrical potential on the rings and the more effective this process of electrostatic induction is. 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. This is separate from the bulk flow of water that falls through the rings and breaks into droplets on the way to the containers. For example, as water approaches the negatively charged ring on the right, any free electrons in the water can easily flee toward the left, against the flow of water.
Eventually, when both buckets have become highly charged, several different effects may be seen. An
electric spark
An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an ionized, electrically conductive channel through a normally-insulating medium, often air or other gases or gas mixtures. Michael F ...
may briefly arc between the two buckets or rings, decreasing the charge on each bucket. If there is a steady stream of water through the rings, and if the streams are not perfectly centered in the rings, one can observe the deflection of the streams prior to each spark due to the electrostatic attraction via
Coulomb's law
Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that quantifies the amount of force between two stationary, electrically charged particles. The electric force between charged bodies at rest is conventiona ...
of opposite charges.
As charging increases, a smooth and steady stream may fan out due to self-repulsion of the net charges in the stream. If the water flow is set such that it breaks into droplets in the vicinity of the rings, the drops may 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 side of the system. In that case also, the buckets will start to electrostatically repel the droplets falling towards them, and may fling the droplets away from the buckets. Each of these effects will limit the voltage that can be reached by the device. 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 opposite charges which build up on the buckets represent electrical
potential energy
In physics, potential energy is the energy held by an object because of its position relative to other objects, stresses within itself, its electric charge, or other factors.
Common types of potential energy include the gravitational potentia ...
, as shown by the energy released as light and heat when a spark passes between them. This energy comes from the
gravitational potential energy
Gravitational energy or gravitational potential energy is the potential energy a massive object has in relation to another massive object due to gravity. It is the potential energy associated with the gravitational field, which is released (conv ...
released when the water falls. The charged falling water drops do
work
Work may refer to:
* Work (human activity), intentional activity people perform to support themselves, others, or the community
** Manual labour, physical work done by humans
** House work, housework, or homemaking
** Working animal, an animal t ...
against the opposing
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
of the like-charged containers, which exerts an upward force against them, converting gravitational potential energy into electrical potential energy, plus motional
kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...
. The kinetic energy is wasted as heat when the water drops land in the buckets, so when considered as an electric power
generator
Generator may refer to:
* Signal generator, electronic devices that generate repeating or non-repeating electronic signals
* Electric generator, a device that converts mechanical energy to electrical energy.
* Generator (circuit theory), an eleme ...
the Kelvin machine is very inefficient. However, the principle of operation is the same as with other forms of
hydroelectric
Hydroelectricity, or hydroelectric power, is electricity generated from hydropower (water power). Hydropower supplies one sixth of the world's electricity, almost 4500 TWh in 2020, which is more than all other renewable sources combined and ...
power. As always, energy is conserved.
Details
![cmglee_Kelvin_water_dropper_Cambridge_Science_Festival](https://upload.wikimedia.org/wikipedia/commons/b/b9/Cmglee_Kelvin_water_dropper_Cambridge_Science_Festival.jpg)
If the buckets are metal conductors, then the built-up charge resides on the outside of the metal, not in the water. This is part of the electrical induction process, and is an example of the related
"Faraday's ice bucket." 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
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate electric charge on a hollow metal globe on the top of an insulated column, creating very high electric potentials. It produces very high voltage direct ...
.
The discussion above is in terms of charged droplets falling. The inductive charging effects occur while the water stream is continuous. This is because the flow and separation of charge occurs already when the streams of water approach the rings, so that when the water passes through the rings there is already net charge on the water. When drops form, some net charge is trapped on each drop as gravity pulls it toward the like-charged container.
When the containers are metal, the wires may be attached to the metal. Otherwise, the container-end of each wire must dip into the water. In the latter case, the charge resides on the surface of the water, not outside of the containers.
The apparatus can be extended to more than two streams of droplets.
In 2013, a combined group from the
University of Twente
The University of Twente (Dutch: ''Universiteit Twente''; , abbr. ) is a public technical university located in Enschede, Netherlands.
The university has been placed in the top 170 universities in the world by multiple central ranking tables. I ...
(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.
A year later, they developed another version of a microfluidic Kelvin water dropper, using a microscale liquid jet (which then broke into microdroplets) shot onto a metal target, which yielded a maximum 48% efficiency.
Historical Background
In
De Magnete
''De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure'' (''On the Magnet and Magnetic Bodies, and on That Great Magnet the Earth'') is a scientific work published in 1600 by the English physician and scientist William Gilbert. A h ...
, published in 1600, 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.
Other early studies noting the interaction of static electricity with water and reported in the English language include:
* Francis Hauksbee "Physico-Mechanical Experiments on Various Subjects." (1719)
* William Watson, "Experiments and Observations Tending To Illustrate The Nature and Properties of Electricity". (MDCCXLVI) (1741)
* John Theophilus Desaguliers, "A Dissertation concerning Electricity" Innys and Longman, London MDCCXLII (1742)
* Joseph Priestley, "The History and Present State Of Electricity with Original Experiments by , Volumes I, II, and III (MDCCLXVII) (1747)
* James Ferguson, "An Introduction to Electricity", W. Strahan and T. Cadell, London MDCCLXX (1770)
* George Adams "An Essay on Electricity" London (1785)
* Tiberius Cavallo "A Complete Treatise On Electricity in Theory and Practice with Original Experiments" Volumes I and II (MDCCXCV) (1795)
* John Cuthbertson "Practical Electricity" J. Callow, London (1807)
* George John Singer "Elements of Electricity and Electro-chemistry" Longman, Hurst, Rees, Orme, and Brown, Paternoster Row 1814
* George W. Francis "Electrostatic Experiments" (1844)
* Henry Minchin Noad "A Manual of Electricity" in two volumes (1857)
By the 1840s it was able to be demonstrated that streams of water could carry electric charge, that streams carrying like charge were repelled and that streams carrying unlike charge were attracted. It could also be demonstrated that physical charge separation, that is, separation of charge into different regions, could be induced in a body of water by a static electric field.
William Thomson (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.
Experimental Studies
Investigations of the Kelvin Electrostatic Generator under various controlled conditions showed that it operated with tap water, distilled water (non-deionised) and a saturated solution of NaCl.
It was also found that the generator worked well even if the two liquid streams originate from different electrically insulated reservoirs. A model was proposed in which the electric charge results from the separation of the positive aqueous hydrogen ion and the negative aqueous hydroxyl ion as the water droplets form.
References
{{Reflist
External links
Kelvin Water dropper-
MIT OCW
MIT OpenCourseWare (MIT OCW) is an initiative of the Massachusetts Institute of Technology (MIT) to publish all of the educational materials from its undergraduate- and graduate-level courses online, freely and openly available to anyone, anyw ...
YouTube ("Reinhard Schumacher") - Kelvin Water Dropper: Implementation and ExplanationYouTube ("RimstarOrg") - Kelvin Water Dropper and How it WorksYouTube ("Veritasium") - Sparks from Falling Water: Kelvin's Thunderstorm*
ttp://www.splung.com/content/sid/3/page/lego_kelvin_water_dropper Lego Kelvin water dropper
Electrostatic generators
Physics experiments
Scottish inventions
Water dropper