About me

For my professional website, with information about my research, publications and teaching, see www.sites.google.com/site/rmlevans.

Monday, 14 May 2012

Lord Kelvin's Thunderstorm


This week, I wholeheartedly embraced the physicist’s stereotype, and set to work constructing a twenty-thousand-volt spark generator out of an ice-cream tub, two plastic taps, some metal cans and a few bits of wire. And I mustn’t forget the most important ingredient: a pint of water. I am happy to say that it worked a treat. It’s fun to mix electricity and water. Forget I said that. What I meant, of course, was, “Never mix electricity and water, kids!” ...unless you follow in the footsteps of William Thomson, a.k.a. the First Baron Kelvin, because this collection of bits and bobs comprises the near-miraculous invention known as Lord Kelvin’s Thunderstorm.

The finished article, looking like Heath Robinson’s offering to a Blue Peter junk-art competition, is a strange and tantalizing thing to watch. It has no battery or electrical supply and only the most basic constituent parts. Yet, as water pours from one vessel into another, the metal hoops and wires that surround them spontaneously spring into life, and begin to crackle and flicker with the blue light of high-voltage electrical discharges. As with any contraption, there was a fair amount of anguished troubleshooting before it agreed to play, so, when the first blue arcs finally illuminated my sweat-drenched forehead, I was tempted to run from my workshop shrieking, “It’s alive!”

The weird spectacle and sheer sense of achievement that the machine generated were well worth the effort. So, this blog entry takes the form of an instruction manual. You too can safely create high-voltage electricity from soggy junk.

You’ll see from the photo that I made a wooden stand to support the whole thing. Although wood is normally regarded as a good electrical insulator, surprisingly, at the very high voltages and low currents involved in this particular escapade, wood conducts electricity too well, and will short-circuit the electrics unless all of the metal parts are mounted on plastic supports. At the bottom of the contrivance are two tin cans (actually one of mine was a stainless steel cup) for catching drips, which I stood on plastic trays intended for house plants. Above them are two metal cylinders (which I made by cutting up a hairspray can - white in the photo) held by a plastic bar (hacked off a kitchen cutting board). Two pieces of wire connect the right-hand cylinder to the left-hand tin can and vice versa and a further two pieces almost connect the cylinders to each other, but are separated by a small gap across which the sparks jump. Above all this stands a tub of water, from which emerge two plastic taps (the only parts I had to buy: £5 including delivery from Ebay) designed for kegs of home-brew. That’s really all there is to it.
Now, switch on the taps just a fraction, so that thin streams of water fall though the metal cylinders without touching the sides. The continuous streams must break up into drops whilst within those cylinders. Then stand back and watch the sparks fly.

“But how does it work?” I hear you say.

I thought you would never ask. Well, there are only two pieces of physics that you need to know: opposite charges attract, and all matter contains positive and negative charges in equal amounts.

Now, the water, as you know, is made of negatively charged electrons and positively charged nuclei of hydrogen and oxygen. Let’s suppose that, as those countless trillions of charged particles drip out of the left-hand tap, just one electron too many ends up in the left-hand tin, without a positive hydrogen nucleus to balance its charge. So the left-hand tin now has a tiny overall negative charge, and the water left in the ice-cream tub has a tiny net positive charge. The extra electron in the tin can easily travel up the copper wire to the right-hand cylinder, attracted by the nearby positively charged tub of water. Now the positive charges in that tub of water feel the pull of that negatively charged cylinder, and get attracted towards the right-hand tap, while the negative charges are predominantly repelled towards the left-hand tap. So we get positively charged drips falling from the right-hand tap, and negative from the left. The right-hand tin collects those positive charges, and the left tin collects negative, making the cylinders more strongly charged, which in turn pull more strongly on the charges in the tub.

The tins keep on getting more and more highly charged until the voltage across the spark gap is high enough (around 20 000 volts in this case) to push an electrical current through thin air. Like Evel Knievel, the electrons jump the gap, crashing into air molecules on the way, making them shine with the blue light so familiar to Doctors Thomson and Frankenstein alike. Only by performing this dare-devil stunt can the electrons be re-united with the surfeit of positive nuclei in the right-hand tin. Neat, isn’t it?

This is similar to the way in which large amounts of opposite charge get separated by water droplets moving around a thundercloud (hence the device’s name), though the finer details of cloud electrification are still up for debate, and I’m fairly sure there are no hairspray cans involved.

At first, this spontaneous self-charging mechanism seems too simple to be true. You might even worry that it seems to violate the principle of conservation of energy, creating electrical power for free, but not so. Can you spot where the machine got its energy from? Whoever lifted the tub of water onto the wooden stand had to expend some energy to overcome gravity. That stored gravitational energy is used by the machine when the charged drips fall into a tin that has a like charge. Without gravity, those drips would be repelled from the tin.

In fact, as the voltage rises, the electrical repulsion competes noticeably against gravity. As a prelude to each spark, the splashing sound actually gets quieter as the drops hit the surface more slowly, and the smallest droplets don’t even make it into the tin, instead fleeing the high charge by spraying out sideways, drenching the operator in what should logically be called “Lord Kelvin’s Drizzle”. This repeated diminuendo culminating in a tiny crack and flash makes quite a striking rhythm. Not only are the electrical forces felt by the water; the copper electrodes of the spark gap (being long and bendy in my particular construction) become pulled slightly together due to their opposite polarities, then suddenly released when discharged by the spark, causing them noticeably to spring apart, in synchrony with the rhythmic son-et-lumière.

Even knowing in advance that the design was feasible, it took me several hours to get the thing working, and gave me a huge sense of achievement when I finally did. You have to admire its inventor’s remarkable cleverness, particularly considering that he and his Victorian contemporaries didn’t know of the existence of electrons or atoms.

If you’re sufficiently intrigued to build your own weird and wonderful spark generator, I recommend a look at Bill Beaty's electrostatics website http://amasci.com/emotor/kelvin.html for some helpful troubleshooting tips and other design ideas.

Having generated electrical arcs that lasted only a few microseconds, it was quite another task to photograph them for your delectation, dear reader. But I won’t bore you with the details. Suffice to say, I spent a disagreeable half hour hunched over a camera in the pitch dark, stoically enduring his lordship’s drizzle. But I can bear him no grudge. Not only did he invent a wonderfully entertaining curio, he also earned me the highest praise from my six-year-old son who, on witnessing my ramshackle handywork in action, declared it to be “cooo-el”. Thanks Lord K.

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