Van de Graaff Tips and help
The Van de Graaff generator
The Van de Graaff is a very simple machine, it amazes me that they are so expensive
considering how uncomplicated they are, but to get the best out of them does seem
to cause people a lot of problems. I probably use the Van de Graaff more that anybody else
in this country and so have slowly learnt a bit about them.
Potentially they are dangerous, you can always spot the biology teaches because they will not go near them. The main risk is from charging something up and then taking the charge through a person, it is current that kills. So take steps to stop a charge building up on something. Making a chain of people and charging them up is not a good idea. People with weak hearts should not play with static and neither should computers, mobile phones computers or other bits of sensitive electrical equipment.
Beware of induced charges. By that I mean watch out for things near the machine getting charged, then when you get near the object it will bite you. I once did some demonstrations in a museum where I was given a table with one of those mirrors above so that people could see what I was doing. After a while I noticed a ticking noise, and looking up I noticed sparks jumping to the frame. The big sheet of aluminium was acting as a plate capacitor and might have been a bit of a problem. I moved the Van de Graaff to another table.
How does it work?
People always think that it is by friction, even the old name for static of
"frictional electricity" would make you think that but just because
it is obvious does not mean it is true. It is not the rubbing of the belt but merely
its contact with the pulleys, it is being in contact and then not in contact that
makes it go. This very strange phenomena is caused by triboloelectrification.
Simply put, when two materials are in contact one will take electricity off the
other, when they are separated the charge will move, if the materials are chosen
with care then the effect can be pretty big. For more details see my page on static.
As the belt goes round it touches first one pulley and then goes up to the other one. If the pulleys are made of different materials then charge can be moved from one to the other. During the great age of steam when leather belts were used to drive all sorts of big bits of kit there are reports of sparks flying around engine houses. One of the most bizarre static generators was invented by the engineer Whitworth and this actually used escaping steam to produce charge, but I digress.
In fact the model When it comes to Van de Graaffs size is important. Big ones are better than small ones and mine is the second biggest I've ever seen.I use was made by a company called WBN apparently in Scotland some time in the second half of the last century and might even be older than me. To digress once again the same model has even been on Dr Who - the episode where Torchwood had its base in Canary Wharf and was attacked by Cyber Men and Daleks - it was sitting on a bench as a bit of alien technology. So a famous bit of stuff eh?
Most machines I see in schools are tiny.
Not only are most Van de Graaffs in schools small, they are filthy. Having the
domes covered in dust and corrosion will not affect the charging but will let the
electricity that does build up to leak away from it too quickly. Dings and dents
will also not be good as charge just jumps off the points. So make it as smooth
and shiny as possible, metal polish like you would use on the bumpers of your
chrome plated E type Jaguar is the stuff to use. Don't worry you can touch it,
that will not make it not work but a mirror finish is the thing to aim for.
What does need to be clean are the pulleys and the belt. They must have no dirt or nasty finger prints. Any contamination will interfere with the contact forces and stop the charging. The best designs have the belt and pulleys enclosed in a tube, but lots of the commercial ones have the belt exposed to the air and mucky little fingers.
Normally the one pulley will be made of aluminium or at least coated with it, the other one a plastic - perspex seems popular. This affects the polarity of the machine, the top being positive and the bottom negative. If you swap the pulleys then the polarity will also swap. I once set up two machines so they were opposite in the hope of making really big sparks. It did work but not as well as I expected, probably due to the machine with the negative top experiencing much higher losses.
The belt. I am sure the manufacturers will charge a fortune but I find that exercise belts, you know the stretchy things that you can pull around to burn off those excess calories work a treat. "Dynaband" is the one that works best for me. You will have to cut it to size , scissors work and then stick the ends together, gel super glue is the stuff to use, make sure the ends are as neat as possible. Really good machines would have an adjuster to get the tension right, but on most it is a bit of trial and error to get it just so. If it is too loose then the belt will be attracted to itself when running and discharge, but at least your trousers won't fall down.
The belt and pulleys must be clean. Methylated spirits works for cleaning but when finished, make sure it is dry before running as an explosion would be bad. A technician once gave me a hard time because I touched the dome of his Van de Graaff, it's the belts that matter, mate.
The combs can be made of many different materials. What matters is that they are points and do not touch the belt. They pick up the charge, and transfer it to the domes. I have frequently seen machines where the combs touch the belt. No, they shouldn't. I once saw a machine that had just come back from being refurbished by the makers and it was set up like that.
On my toy if I take the combs off then it will still charge, a little spark jumps from the pulley to the metal work. But the combs help.
One machine I saw had the axle of the top pulley made of plastic in the mistaken belief by the makers that it should not conduct, rubbish, all it did do was bend and make the machine rather loud and nasty.
The best tip
A digital multimeter is vital. With it a Van de Graaff can be set up to optimal working with out the risk of getting a shock through the end of your nose as you play with the combs. Set the meter to its most sensitive amperage setting and connect it to the top and bottom combs. The meter will read the current produced and the machine can then have the combs moved until the best charging is found.