I built this one in about a week, using scrap parts from a sewing machine and materials ordered from http://www.grainger.com/Grainger/wwg/start.shtml
I have seen the larger machines in operation at my local science museum and this one does just as well. I have made hair stand on end and even pie pans float up and off the machine.
For an explanation of operation go to Wikipedia:
ALSO, note that this machine can generate enough static to stop a pacemaker and any other digital device, keep people with heart problems away. Also if you dont wont to blow up you sensitive electronic devices make sure you either use the machine on a separate circuit or unplug everything you want to keep. I have also stopped watches and killed cell phones with it. The current is low around 27micro amps but that's a thousand times whats needed to kill say your flat screen TV or pc/laptop.
Well if your know anything about Van de Graaffs you know that the bigger and smoother the sphere or collector the bigger the charge. Any bump or lip on the sphere and all the charge will run to it and bleed off.
The recharge rate or time it takes to build up a charge high enough to jump from the sphere is determined by belt speed and width.
I used an art program to generate some diagrams and several pics of the completed unit, its a simple mechanism and you could upscale it for a larger machine easily topping 1,000,000 volts. all you need to do it increase the sphere size and belt width.
This one is an upscale version that I used to test equipment at work and I used the little one to determine the best belt material.
I tried plastic tape, rubber, fabrics, and the hands down best was a plastic coated fabric used in hotel shower curtains. Its easy to cut and glue and lasts for a long time. The charge it carries was easily 50% better than everything I tried.
Also to get the most of your machine you will need to get your hands on some Teflon and some nylon to make the rollers out of. I ordered mine from Granger (links in parts list) I used the Teflon for the top and nylon for the bottom or drive roller. You can also get the Teflon from stacking little half inch thick disks cut with a 2" diameter hole saw from cutting boards. You could also make the top and bottom roller out of the same material and coat the top roller with Teflon tape.
Also if you want to boost your storage and spark a bit build this:
Leyden jar of DOOM!
I will try to list as many tips threw this process as I can. Its been several years since I built the machine and its worked well so far.
Next up a parts list......