The water battery

Menaus

"Induction" isn't restricted to the electromagnetic form, there is also electrostatic induction. In this case the voltage is induced through the electrostatic variant.

http://en.wikipedia.org/wiki/Electrostatic_induction

sophiecentaur

Electrostatic induction always seems to be ignored in 'elementary' explanations of electrostatic effects. The classic one is when a rubbed comb (charged by friction) picks up (uncharged) bits of paper and dust. How is this?
When the comb is brought next to the particle, there is some movement of charge in the particle (polarisation of the molecules, if it's an insulator). The unlike charges end up closer to the comb than the like charges. They are closer, so the attractive force is greater than the repulsive force on the similar charges, which are further away. Net Force is Attraction without the paper actually having a net charge.

At School, we were shown the 'Electrophorus' (Google it). This is a metal disc on an insulating handle. You place it on a (+, say) charged mat. '-' charges move slightly towards the mat and + charges go to the far side. You touch it with your finger and the excess + charges flow to earth, leaving an excess of - charges on the metal. Take it away and it is charged 'by induction'. Again, no 'rubbing' involved.

The Kelvin Water Dropper (and the Whimshurst Machine) work on a similar principle. There is very slight polarisation of the water in the reservoir (one nozzle is slightly positive and the other is slightly negative. As the drops fall through the rings into the tins, the two halves of the apparatus gradually build up an 'induced' potential difference between the two diagonal structures. No friction is involved here - all that's necessary is a very small (unavoidable) imbalance at the start, which just gets magnified. I have heard this described as Positive Feedback.

OmCheeto

I think I'm understanding how this machine works now. I re-read the paper again after studying topics hinted at from the various posts: Electrostatic induction, Electrophorus, Gold-leaf electroscopes, etc.

I ended up thinking that this reminded me of Millikan's oil drop experiment.


One could modify Kelvin's thunderstorm device, say, replace the bottom tubs with glass beakers such that you could use a high speed camera to film the smaller drops approaching and then reversing direction. I think that would be interesting.

btw, did anyone watch the slo-mo portion of the video from my original post? Many of the smaller droplets went into orbit around the loop! I found that quite entertaining.

Ah ha! That explains why the hoops are hoops.

I am definitely going to build one of these devices.

sophiecentaur

The drops in Millikan are actually charged, though, afaik.

NascentOxygen

An interesting point in Prof Lewin's video is that his set-up departs from the usual description of how this needs to be arranged. His set-up seems to differ from his own diagram, too.

Usually, we are told to carefully adjust the flow rate so that the stream breaks up into discrete drops just about where it enters the hollow tubes, this causing the drops to become insulated from the stream above. But his video (and here I acknowledge the camera work could do with considerable improvement) appears to show water streaming into and out of the hollow tubes in an unbroken stream. The stream does spread chaotically horizontally as potential builds up, but it is nothing like a procession of discrete water drops.

Bobbywhy

Here is a tutorial and an excellent description of the detailed theory of operation of the Kelvin Water Dropper at the website of the National High Magnetic Field Laboratory. It provides a clearly written and definitive explanation of how the electrostatic generator works. See:
http://www.magnet.fsu.edu/education/...per/index.html

Cheers, Bobbywhy

OmCheeto

Thank you. While I was waiting for Java to download, I checked out the link to the Leyden jar referenced by your link. They said a Leyden Jar could be used to store the energy from the Kelvin Water Dropper. It reminded me of an experiment my university professor once did for the class. It was of a dissectible capacitor. He said; "If you don't understand what is going on here, you will never understand electricity". Well, needless to say, I've never understood his experiment. But the Leyden jar link explained it. So now, after 23 years, I finally understand what was going on.

If anyone is curious as to what I'm talking about, skip to 1:30 in the following video:

http://www.youtube.com/watch?v=9ckpQW9sdUg

(embedding disabled by request)