Forget making balloons stick to the ceiling... We're gonna make Sparks!
But we needed to make them safely, by hand - such that this could be demonstrated in a grade school Science Fair.
The Hand-de-Graaff Generator
My solution involved three main components:
1 - The Charge-Rod : using PVC tubing, wool cloth, friction, and two conductor wires to collect the charges (+ & - )
2 - Storage : a modern Leyden Jar / Capacitor (two sizes for comparison)
3 - The Spark-Gap : (adjustable for taking measurements)
Click on Images for Larger Versions
Electricity can be dangerous, so let me just say this:
If there is not enough information on this page for you to build your own, then you probably need to learn more about electricity before you try...
The parts are all pretty simple... which means it is easy for something to go wrong, and then it won't work.
First - you can get tiny sparks from just the Charge-Rod and the Spark-Gap alone, but only with a tiny gap - less then one millimeter, and the sparks are very hard to see (try a really dark room). On the other hand you can hear them if you are quiet enough.
But we want to make big Sparks!
That takes some way to store up the electricity before letting it loose...
Are you ready for the Nitty-Gritty Principles and Parts involved?
Friction between the right materials causes electrons from one material to be transferred to the other (pushed or pulled I don't really know...) This effect was one of the earliest discoveries of electricity and finding the best pairs of materials was a quest for early scientists. It is known as Triboelectricity. I picked PVC and Wool cloth which worked great.
I got my inspiration from this page: http://www.sparkbangbuzz.com/els/stat-gen-el.htm
So, how I got it to work:
My invention is a shuttle-sleeve the holds two collector wires and the wool cloth. One wire is spread out just above the surface of the inner-rod, which moves past it as you slide the sleeve up and down the rod. This wire is then connected to one end of our path: notice the three-way junctions: gap-generator-capacitor on one side and gap-generator-capacitor on the other. We call this wiring method "in parallel." The other wire is secured to the sleeve as well but the only exposed metal is about 40mm (1 1/2") also with the strands spread out, this is held between the hand and the cloth.
As a side-discovery... As most of us know about friction - it creates heat. It seems that the 'generator' doesn't work as well once the Charge-Rod warms up. So you want to let it cool down when you can't get the big sparks anymore - this may save you some of the frustration we had during our experiment...
I think I should mention the outer sleeve has a large cut-away such that you are firmly holding the wool against the inner rod. In fact, the way I did it the cloth wraps around the inner tube and comes out the opening in the outer sleeve thus surrounding the charge-rod (the inner tube) and also being exposed to your hand and the second pick-up/contact wire.
I don't know if it matters for the design, but I wrapped the 'handle' of the Charge-Rod with electrical tape, figuring it might keep the your hands from altering the electron flow, and it looks cooler too!
Next we need to conduct the 'free' electrons where we want, so we needed... yep, conductors! In other words a couple of wires. We are not actually creating a complete circuit, rather we are creating a path of influence. Since we have two notable openings in our circuit-path - the spark-gap, and the film (plastic wall) of the capacitor(s). By the way, you get much better results with stranded wire rather than solid wire.
The Spark-Gap uses the air between the conductors (the 'Gap') as a resistor, it 'resists' electricity traveling across it. Which means it still CAN cross it if it has more 'Force' than the 'resistance' - Electricity can jump the gap but the larger the gap- the more resistance and thus the stronger the 'charge' needs to be to make the jump. By the way, this one was made from parts for the tank of a toilet - one that was made to adjust to get the water-level right. (Yes, I bought it new so it wasn't gross.) I just turned it side-ways and ran a wire down the middle - don't forget to leave slack for adjustments. That way by turning the sleeve until it "clicked" we got a consistent change in gap-width--about 1.5mm each click.
The key to big sparks is storing up a large charge... The Capacitors -- My modern Leyden Jars, one was just a film canister for one and the other a plastic food tub. Both have aluminum tape wrapped around the inside and the outside, but the metal parts never touch. Then we push a wire through a hole in the lid so it touches the inside foil and Violá - a Leyden Jar!
The principle here is maybe the most interesting, like a magnet, the force of extra electrons on one side influence the electrons on the other side - even though they don't touch. The more 'extra' electrons on one side the stronger the influence - which we know as "charge."
So, put it all together, friction on the Charge-Rod causes an imbalance of electrons, these move along our influence path - getting to the Capacitor, which has a lot of surface area to hold a lot of electrons, the more surface area the greater the charge. Once the charge is strong enough to push through the resistance of the air in the Spark-Gap, the electrons 'arc' across the gap restoring balance... until we charge it up again.
If you slide it up and down quickly you can get a pretty good spark in five to ten strokes - of course a lot of factors will change that - including humidity and temperature.
One last interesting tidbit - the electric arc heats the air, and really hot air actually conducts electricity nicely, hmmm kind of like a transistor... but that's for another day's project.
As part of our experiment we tried only the small capacitor, then only the large capacitor and then both together (in series).
One last note: I had a grounded wire (attached to metal plumbing) that, in theory would improve the capacitor build up. I can't say that it did. But, maybe someone else will try it and get better results. Again, electricity is dangerous, so be careful what you connect, and be careful how big a capacitor you use. Much larger than a cottage cheese tub can actually hold a dangerous and possibly lethal charge!
Well good luck, I hope this helped you learn something new and maybe even gave you what you need to build a really cool sciencce-fair project!