OK, I know that every kid who ever played with one of these suckers knows they are the coolest science toys
I also think there might be one or two people out there who studied poetry and other girly stuff * who have no idea what the kewl kids are going on about when they talk about the VdG generator.
So, to bridge the gap of understanding here, I've tried to give two answers to every question you might ever have had about these little balls of fun.
Q1: What is a VdG generator?
A1a: it is simple machine capable generating very high voltages at low current by employing the physical principles of electrostatics.
A1b: They are the totally coolest way of making your hair stand on end and making really massive voltages. And you can build one yourself. And they are totally legal!
Q2 What sort of voltage are we talking about here?
A2a: In a simple home-built machine, 10 000 volts is easy to achieve. Larger scale machines on display in museums can easily get into millions of volts.
A2b: I reckon on about 30 000 volts per inch (10 000 volts/centimetre), but that's enough to blast holes in pieces of paper and make sparks two inches long and make a really loud crack!
Q3 Wow, that sounds like a lot of volts. Is it dangerous?
A3a: No, provided you follow the safety guidelines given at the bottom of this node.
A3b: I put my hand on the machine and charged it up to a million volts and I was like Emperor Palpatine, zapping everyone with sparks and force fields. Feel the power of the Force, Skywalker!. Zap!
Q4 Isn't that a bit childish? Sparks, blasting holes in bits of paper and all?
A4b:Are you kidding? You just try it. Then see what it does to your hair when you touch the ball.
Q5 I'm going to regret this, but what does it do to your hair?
A5a: The charge passes from the charged ball up your arm and onto your body. Each individual hair becomes charged to the same voltage and, because like charges repel, each hair tries to move as far away from its neighbours as possible. Your hair starts to stand on end in a very frizzy hairstyle that would make your trichological consultant faint.
A5b: it gives you this totally crazy look like a mad professor or something. You can laugh at all your friends, then get zapped by their mad Force-wielding spark zapping skillz.
Q6 OK, I can see what you like about it, tell me about this sparking business.
A6a: it's nothing to do with the Force. But it is a very effective demonstration of the way an electric field distorts as the surface geometry changes. A spherical surface allows the electric field to be symmetrical and smooth, which in turn means you can build up a large potential difference and a lot of charge without ionising the air. Once you concentrate the field around a pointed geometry (like a finger—or a lightning rod) the electric field strength exceeds the insulating capability of the air, and the electricity will discharge to earth. If the only route to earth is through another person, then the electricity will jump from your fingertips to that person, creating an ionised path between the two bodies. The path is illuminated by a plasma, a bit like lightning, as the charge balances itself out.
A6b:Whoa. I just lay my hand on the dome, charge the thing up and point with my other hand. Zap! If you turn the lights down, it looks just like in the movies. It tingles a bit, but it's not painful. Well, not unless I aim at their earlobes or some other sensitive bit of skin. Heh.
Q7 All these sparks and high voltages, why on earth is it not dangerous?
A7a: Seems a bit odd, doesn't it. But the thing about voltage is it is just a potential difference. It's like being at the top of a hill. Simply being there is not dangerous at all. The danger lies in how quickly you might fall to the bottom. So if your hair is charged to a high voltage, then it just means you climbed a high hill.
The thing about a VdG generator is first that it has a very high internal resistance, and second that it can only store a limited amount of charge. The high resistance means that even at huge voltages, there is very little current (and its usually the current which does the damage). While the lack of charge means that there is not enough electricity to do much damage, anyway. So using the hill analogy, the high internal resistance means there is no way you can fall down the hill. It's like the hill is surrounded by very gentle slopes on all sides. The lack of charge means that the only thing that can get to the top of the hill is a feather, or an ant.
A7b: Aww, c'mon, get a life, just try it!
Oh, err, yeah you guys like history and stuff.
The VdG generator was invented in the Autumn of 1929 by Robert Jemison Van de Graaff, who was working at the time in the Palmer Physics Laboratory at Princeton University. He later became a professor at MIT. The original idea was to use the machine to generate very large voltages used in early research into atomic structures.
The largest such generator—at least the largest one to be insulated by normal atmospheric air—is at MIT, housed in the Thomson Theatre of Electricity, where it is demonstrated a couple of times a day to students and tour groups. The machine originally used two electrodes, each an aluminium sphere, 5 m in diameter. Each of these could be charged to 2.5 million volts, giving a total potential difference of 5 million volts. Later, however, the mechanism was removed from one of them, and the two spheres welded together to form a single electrode. Because of the two inherent safety factors in the design, even this monster, will not kill if you get hit by a spark. This one, however, will hurt, quite a lot. Apparently most of the demonstrators have been hit by a spark at one time or another, and the biggest challenge after receiving such a belt is to avoid saying rude words in front of the audience of schoolchildren.
And here, from http://hypertextbook.com/eworld/vdg.shtml are some safety tips for everyone who wants to use a VdG generator
Operation and Safety
- Do not allow students to use the Van de Graaff generator unsupervised.
- People with cardiac pacemakers should never operate the generator or come in contact with it.
- Obviously, we are dealing with high voltage here. Stay about three feet away from the collector while it is charged. Full intensity, white-hot sparks can jump as far as 15 inches, less intense, red-purple sparks can jump 20-30 inches. While the current is too low to injure you, a surprise spark is no fun. Keep the generator at a safe distance from the outlet where you plan to plug it in. If you're too close, you won't be able to turn it off safely.
- Always discharge the collector dome between experiments and when you are finished. Use the discharge wand for this. Connect the alligator clip to a gas main or similar grounded object. Hold the discharge wand by the handle. Do not touch the grounding strap when discharging the generator. The voltage is so high that the current can pass through the insulation into your hand.
- The motor produces a lot of heat that could damage the belt or the motor itself. Do not run the generator continuously for long periods of time. Turn it off when not in use.
- Leave the upper and lower combs alone. They are not supposed to touch the belt. Do not bend them or mash them.
- Keep the entire device clean and dry. Dust and moisture degrade the generator's performance.
- Handle the aluminum parts with care. The collector, housing, and discharge wand are easily dented.