In 1867, British scientist Kelvin invented a device for generating very high voltage, which was later called the Kelvin dripping generator.
The structure of the Kelvin dripping generator is quite simple. As shown in Figure (01), there is a water tank A at the top, and a switch that controls the water flow is connected to the bottom of the water tank, and then the water flow is divided into two paths through a three-way pipe. The water tank is grounded, so under modern conditions, the upper water tank can be directly replaced by a tap. Below the
two drip nozzles for water outlet are two conductor rings B and C, and water droplets can drip through these two conductor rings into the two conductor barrels D and E below. The two conductor rings B and C and the two conductor barrels D and E must be well insulated from each other. Ring B and barrel E, and ring C and barrel D are connected by wires. A
certain amount of ordinary water is injected into the upper water tank A, and the water in the water tank drips through the two drip nozzles, passes through the two rings B and C and falls into barrels D and E. The two barrels D and E will be charged and generate a relatively high voltage between them. It doesn't take a lot of water, just a few cups of water to make the voltage between D and E reach tens of thousands of volts or more, which can be seen from the two gold foils in the electroscope H. If the distance between the two conductor barrels D and E is relatively close, spark discharge can even be generated between D and E. If the distance between the two barrels and the distance between the two rings are relatively far, the water droplets may eventually fly around under the action of high voltage and fly onto the conductor rings B and C. The water splashing everywhere destroys the insulation everywhere, and the experiment ends. [
attach]437548[ /attach] Figure (01) Kelvin dripping water generator
How can a few cups of ordinary water generate such a high voltage?
To explain the cause of the high voltage, please see Figure (02).
Figure (02) Working principle of dripping water generator
Figure (02) omits the insulating bracket, leaving only the drip nozzle and water flow A, two conductor rings B and C, and two conductor barrels D and E. The figure also shows a water drop F dripping from the conductor ring B and a water drop G dripping from the conductor ring C.
Before the water drop drips, the voltage between the two rings and the two barrels is not zero, and there is always a small voltage between the two. This small voltage may be caused by wind (if anyone does not believe it, please ask the ladies with long hair whether they have ever had their hair stick together with electricity on a windy day), or by human hands touching it (don't forget that the human heart keeps beating, and the electrocardiogram shows that there is at least a few millivolts of voltage between different parts of the human body)... In short, like thermal noise in the circuit, the voltage between the two will not be zero.
Assume that the left barrel D (together with the right ring C) is slightly negatively charged, and the right barrel D (together with the left ring B) is slightly positively charged. In Figure (02), we use red to represent positive charge and blue to represent negative charge. Then, rings B and C will redistribute the charge in the water in tube A. The water drop F flowing out from the left side will carry a small amount of negative charge because ring B is positively charged, while the water drop G flowing out from the right side will carry a small amount of positive charge because ring C is negatively charged. The
falling water drop F carries a negative charge and G carries a positive charge. When F and G fall into buckets D and E, they will transfer their own charge to buckets D and E, causing the voltage between buckets D and E to increase. Naturally, the voltage between rings B and C will also increase. The water droplets that continue to drip will carry more negative and positive charges. When the water droplets that continue to drip fall into the bucket, the voltage between buckets D and E will increase further. In this way, the water droplets that continue to drip carry more and more charge, and the voltage between buckets D and E will increase. Obviously, this is a positive feedback process. This process will continue until spark discharge occurs between the two buckets or the two rings, or the water droplets fly to the rings due to electrostatic force and destroy the insulation.
Obviously, the device shown in Figure (01) has the ability to generate electricity and is a power source.
A power source is defined as a device that converts other forms of energy into electrical energy by using non-electric field forces to do work on electric charges. So, what force is doing work in the "Kelvin water droplet generator"? What form of energy is converted into electrical energy?
From Figure (02), it is very obvious that the charge carried by the falling water droplet has the same sign as the charge carried by the bucket. The water droplet is repelled by the bucket, and the direction of the electrostatic force on the water droplet is upward. Since the water droplet is subjected to an upward force, the speed at which the water droplet falls into the bucket will be lower than when it is not subjected to the electrostatic force. In other words, the gravitational potential energy of the water is not completely converted into the kinetic energy of the water droplet, but a part of it is converted into electrical energy.
The charged water droplets fall intermittently. Does
this intermittent movement of electric charges constitute an electric current? The intermittent movement of electric charges in the charged water droplets certainly constitutes an electric current. Although the charged water droplets are discontinuous, the definition of electric current is "the directional movement of electric charges", which does not require continuity. The directional movement of free electrons in conductors such as copper and iron is called conduction current; the current formed by the movement of charged objects such as water droplets is called transport current. The common point of these two currents is that they both produce magnetic effects. By the way: changes in electric fields can also produce magnetic effects, which Maxwell called "displacement current", but changes in electric fields do not cause charges to move.
Figure (03) Kelvin
Who is Kelvin? How did he come up with this kind of generator?
Kelvin's real name was William Thomson. He entered the University of Glasgow at the age of 14, and later studied at Cambridge University. He also studied in France. William Thomson made considerable contributions to mathematical physics, thermodynamics, electromagnetism, and elasticity mechanics, and presided over the technical work of the transatlantic submarine cable. For this reason, he was knighted in 1866. Later, he was knighted Lord Kelvin in 1892. It was because of his outstanding contributions to thermodynamics that the 10th International Conference on Weights and Measures named the thermodynamic temperature "Kelvin".
Kelvin made the above experimental instrument in 1867, when the transatlantic submarine cable had been completed. He also proposed the second law of thermodynamics as early as 1851. At the end of the 19th century, Kelvin was a leading figure in the field of physics. The reason why Kelvin made such an instrument that looks like a toy to us today is to experiment with a power generation device that is different from the generator that uses Faraday's law of electromagnetic induction to convert mechanical energy into electrical energy.
Today, it seems that this "Kelvin dripping water generator" does not have much practical use: it can indeed convert mechanical energy into electrical energy, and can generate a very high voltage (generating spark discharge at a distance of several centimeters), but the current generated is very small.
However, this seemingly useless "Kelvin dripping water generator" inspired the Dutch physicist Van de Graaff more than 60 years later and was put into practical use. For details, please see the subsequent "Investigation of Things to Gain Knowledge II Van de Graaff Generator".
This content is originally created by EEWORLD forum user maychang . If you want to reprint or use it for commercial purposes, you must obtain the author's consent and indicate the source
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The discharge process will be greatly shortened. Otherwise, you have to wait a long time to see the discharge phenomenon, and if you miss it, you have to wait again.
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