Kelvin Water Dropper: A Battery Mystery!

Imagine a battery that is powered by just falling water! Sounds odd? Traditional chemical batteries use an electrolytic solution, that generates ions(charged radicals), that create a potential difference and so a flow of current. But, this crazy experiment dazzles our mind, by creating a battery, exclusively out of water drops falling inside metal containers. There are no other batteries or components. Any guesses how? I first saw this experiment in a lecture video a few days ago(link in the end), by MIT’s Walter Lewin, and it blew me off! The explanation is so simple and yet so unimaginably twisted to guess, it had me confused for days until I finally saw the solution in another of his videos. This is why this man is one of the best physics teacher, because unlike others, his lectures are filled with such interactive, crazy experiments, building concepts rather than just mugged up knowledge. Here is a clue, it has to do with electrostatics and induced charges. The moment I saw this, I decided to share it on my blog.

Prof. Walter Lewin

Here is the experiment(pictured below): Water from reservoir falls through two nozzles into two metallic containers. Before reaching the metallic containers, just outside the nozzles, are two hollow metallic tubes/rings, one above each container, through which the water falls down, but does not touch it. Now, one container is connected to the tube/ring of another container, and the other container is connected to the tube/ring above this container using wires. Each container has a metallic contact pointed towards the other, both separated by a small distance, this is called the spark gap. When the water is allowed to fall slowly, after small regular periods of time, sparks are seen between the contacts, which points that a potential difference and thus a battery has been created. Explanation?

Kelvin Water Dropper Battery

We know matter is made of atoms, comprising negative electrons whirling around positive nucleus. Now, at an average the whole body as a whole is neutral, but sometimes it so happens due to friction or some other cause that the positive and negative charges aren’t exactly balanced, and so a body has a slight charge, of unknown sign and magnitude. Let us suppose, that the conductive ring and container on left are of group ‘A’ and those on right, of group ‘B’. Suppose there is some residual charge on ring A, say negative( this is a guess, we don’t know what sign the charge is, but this is unnecessary information as you will see later), this negative charge will induce charges on the water drop that is to fall from the nozzle above, and accordingly the drop’s area nearer to the ring will have positive charge induced and the part father will have negative charge induced. Now, the negative charge of ring ‘A’ attracts the positive charge induced on the surface of drop nearer to it(the lower portion) and repels the negative charge induced on the above portion of the drop, and this unequal force in opposite direction, cause the drop to split, and the positive part that was being attracted, falls down due to gravity in the container ‘A’ below. Now, since positive charges are falling in the container through water drops, the container also becomes positive because of conduction. Since this container ‘A’ is connected with ring ‘B’ through metal wires, ring ‘B’ also gets a positive charge. With a similar mechanism as above, ring B induces charges on the water drops falling from the nozzle above, and the container ‘B’ below becomes negatively charged, because of the negative charges from the water drops. Since container ‘B’ is joined to ring ‘A’, this increases the magnitude of negative charges already present on ring ‘A’.


Thus, a cycle of charge transfer is formed in the circuit, and as more and more water falls in each container, the magnitude of charges increases and accordingly the potential difference between the opposite charged spark contacts connected with the respective container increases too. After a certain limit, the potential cause a breakdown, which is seen as a spark(pictured above) between the contacts, and means the excess charge is released, because the potential difference became more than the critical value required to cause an electric breakdown of the air in between. Once the excess charge is released, the potential falls down, and the process continues till the water drops.


One way of predicting when this breakdown of charge will happen is observing the water stream falling down. As charges are induced in the respective streams, due the like charges, the water droplets start repelling each other, and thus the single stream, breaks off into tinier ones or spreads, and as this spread increases it represents the increasing voltage, and after a certain amount of spreading, the voltage is sufficient to cause a breakdown, and a spark is seen between the contacts and the stream goes back to being one single stream. Thus, by watching the spread of falling water, a spark can be predicted.

Isn’t this so amazing ? A process so simple and yet made us so confused as to how falling water created a battery! All the secret was in the small amount of unbalanced charge on ring ‘A’ and it started a cycle of an avalanche of charges, and created a battery. Physics is one hell of an interesting and amusing subject, with many such crazy experiments that go against our intuition. Teachers like Walter Lewin really know how to make us think and grasp the concepts. His lectures are filled with such amazing demonstrations of physics, and I would love to advice them to anyone who is struggling with his concepts or just wants them to brush up. He can make even the dullest of us love the subject. Wish we have more teachers like him. You can watch his lectures online, and many such lectures by amazing teachers around the world. It really helps me with my concepts and base, and know my subject better, especially his lectures on electricity and magnetism is helping me a lot with my second year. As a physics graduate I would recommend his and similar videos to everyone, so after leaving your reviews ┬áhere, do go to youtube and see for yourself. Got any other interesting ideas and experiments? Share below. Happy reading!

-The Cosmogasmic Person


Prof. Walter Lewin first demonstrates Kelvin Water Dropper

Prof. Walter Lewin explains Kelvin Water Dropper



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