MadSci Network: Physics

Re: Building a Wimshurst Machine

Date: Sun May 17 02:37:43 1998
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Physics
ID: 894909416.Ph

Hi Pete!
I remember being confused when I first encountered this device. There are a couple of keys to understanding it. One: all metals, even uncharged metals, contain movable "fluid" charge, they are filled with cancelled electricity. Two: in a Wimshurst machine, it takes voltage to make voltage, but there ALWAYS is some voltage (e-fields) in the environment which will start the process.

The nature of metals

Everyday metals are composed half of electrons and half of protons (we'll ignore the neutrons here.) Every atom in the metal contributes at least one electron to the common cloud or "sea" of electrons which soaks the metal. The electrons don't stay with individual atoms, they wander around somewhat like the molecules in a liquid. This is different than insulators like plastic. In plastic, both the positive and negative charges are locked into the solid matrix and cannot move much. Imagine the metal foil to be composed of a sponge of solid positive charge which is soaked with an equal quantity of liquid negative charge, and the equal and opposite charges cancel each other out. However, it's not hard to UN-cancel them.

Inductive Charging

If we bring a positively-charged object near the surface of metal foil, it will attract the metal's electron-sea. The side of the foil nearest the charged object will become negative, and the other side will become positive. The nearness of the charged object has "induced" a separated charge in the foil. But this is just temporary, and will vanish when the charged object is removed. However, if a grounded wire is briefly touched to the far surface of the foil, the positive charge on that side of the foil will attract electrons from the ground, and the positive charge on the surface will be cancelled. If the wire is then disconnected, and then the positive object is removed, the metal foil will still have a negative charge. Got it? Charging by induction: Bring a charged object near metal, touch the far side of the metal briefly to ground, then remove the charged object. Metal now has a charge opposite to that of the charged object, and the charged object has lost nothing during the process.

Wimshurst machine bootstraps itself

In the Wimshurst machine, the foils serve as the "charged object" for part of the cycle of rotation, and as the inductively-charged foil for another part. But you'll notice that the process of inductive charging requires that a charged object exist, and if the machine starts out with no charge at all, how can it ever start up? This is the old problem of the razor blade balanced on edge. How can it ever fall over, if one direction is as good as the other? Razor blades fall because they cannot be perfectly balanced, and Wimshurst machines always start up because they are never entirely neutral. Of course you won't know which polarity your Wimshurst machine will take when you first start cranking. The "razor blade" can fall in either direction. For this reason, some Wimshurst machines have a bit of fur in a particular place, and contact with the fur gives the same small initial charge, so the machine will (usually) start up with the same polarity every time.

Once the Wimshurst machine has started, there are four identical functions taking place, two on each disk, and two produce positive charge while the others produce negative.

A region of positive foils on one disk is brought near a region of neutral foils on the other. Obviously you want the two disks to be thin and as close to each other as possible, almost touching. The postive foils "induce" separated charges in the nearby neutral foils on the other disk. This pushes the positive to the surface of the neutral foils and attracts negative to the side of them towards their disk. These foils pass under the contact brush of a diagonal arm, which discharges the positive surface charge. The formerly neutral foils are now overall negative. They pass away from the positive foils and approach neutral foils on the other side. Now these negative foils can act as the "charged object", and they induce a positive charge in the opposite foils when those foils touch the brush of a diagonal arm. Finally, the negative foils slide between the two arms of the negative collector brush. When the collector touches them, most of the charge moves to the outside of the collector brush. The foils pass away from the U-shaped collector, neutral again. While all this is happening, three other identical processes are taking place on the other three halves of the disks (one is totally identical, the two others have reversed polarity.)

You may have noticed that the diagonal arms of the Wimshurst machine are used as ground, even though they aren't grounded. Actually, since one end of a diagonal arm takes charge from a positive foil while its other end takes charge from a negative one, the diagonal arms stay uncharged. They could have been connected to ground, but since the total incoming charge is neutral (equal + and - ), there is no need to ground them.

For more info on similar topics, track down explanations of the "Electrophorus" electrostatic generator. A Wimshurst machine acts as an automated version of four separate electrophori, two positive and two negative.

Plastic disks

I've recently heard that vinyl records make for a poor Wimshurst machine. They are supposed to be slightly conductive. I haven't checked this out myself, it might be true but it might be a myth. Or perhaps some vinyl records have an anti-static (conductive) coating to keep down clinging dust. If so, then probably some records work well while only the ones with the coating will have problems. Something you could try: spray them with corona dope. Corona dope is a paint which is formulated to have low conductivity and extremely high breakdown voltage. One source of the paint (bottles, not spray cans) is GC Electronics #10-5002, available by mail order from Allied Electronics. I have an old can of Dem-Kote Class F Red Insulating Enamel which seems to work for the same purpose. Who knows where I got it though.

The pulleys are probably OK as long as they are more than an inch away from the foils. Watch out for the drive belts though. They might be conductive in humid conditions, and you don't want them passing very close to the charged foils on the surface of the disks.

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