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Wimshurst Machines

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Joined: 11 Jan 2008
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Location: uk

PostPosted: Fri Jan 11, 2008 9:26 pm    Post subject: Wimshurst Machines Reply with quote

I would like to know how a Wimshurst machine works? For those that have not come across this machine, it consists of two discs made from an insulating material such as glass. On each glass disc there are metal foil sectors which hold the charge. The discs rotate in opposite directions when a crank is turned by hand. Even a small difference in charge on one of the foil sectors induces an opposite charge on the opposite disc. The other key component in the machine is called a neutralising bar which sits over each disc. When the charged sector move under the neutralising sector it induces an opposite charge on the sector on the other side of the disc. On the other side disc there is a second neutralising bar but rotated in the plane of the disc by about ninty-degrees.
The operation of the machine is to amplify any small imbalance in charge by positive feedback so that all the sectors become charged. When a charged sector is opposite to another sector it induces a charge on the sector of the disc but surely it loses the charge as the sectors go away from each other? The Wimshurst machine is an electrostatic machine but does the fact that the static charge is moving on the disc generate a magnetic field which induces charge on the other sectors as it moves past?
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Joined: 09 Mar 2008
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Location: Fremont, CA

PostPosted: Tue Mar 11, 2008 4:26 am    Post subject: Wimshurst Machine Reply with quote

One of the common errors made in explaining this device is ignoring the role of space charge. The metal foil sectors are actually picking up space charge from the air nearby as it is attracted to the static charge on the nonconducting part of the discs. This happens at the top and bottom of the discs. As the discs rotate, the foil then carries the charge into the horseshoe-shaped collectors where there is near zero electric field in the plane of the horseshoe-shape. This is similar to the way charge is carried into the sphere of the van de Graff generator.

[Note: This text editor balks at the use of the letter that rhymes with "you," so I have substituted the word, "horseshoe." ]

So the charge on the foil segments is actually opposite the charge on the dieltectric disc.

The neutralizing brushes and the horseshoe-shaped collectors sit at 60 degree intervals on my machine but it will work with different angles. Some machines allow adjustment of the angles.

When charge is moving there is certainly a magnetic field established, but I haven't seen any magnetic effects or magnetic induction phenomena that equal the huge electric forces at play.

Another seldom explained phenomenon is the reversal of the electric polarization of the dielectric discs in the 1/6 sector after passing through the horseshoe-collector. Once the metal foil segments spray much of their charge into the horseshoe-collector, the strong space charge reorients the dielectric polarization, reversing it for the next 180 degrees of travel. The neutralizing bars bring the foil segments to a much lower voltage and they are ready to pick up opposite space charge in the next 120 degrees of travel.

Here are some things I recommend to do to better understand the Wimshurst Machine:

1. Observe its operation in a very dark room and make the spark gap so large that it will only spark once in a while. You will be able to see the corona several centimeters away from the discs, more or less depending on humidity.

2. Short out the spark gap and rotate the discs a few times. The whole machine will lose it's ability to generate a spark for a while. In fact, you might have to prime it by holding a plastic bag or a silk cloth against one disc for a couple rotations.

3. With a small jumper wire connect one end to the horseshoe-collector and hold the other alligator clip near the sector after the horseshoe-collector and the neutralizing brush. The wire will pick up a lot of space charge and will negate the reversing of the dielectric polarity, thus reducing the spark size.

4. Try running the discs backwards if your machine will allow it (mine didn't until I modified the crank shaft a bit.) The machine will not work at all backwards.

As a physics teacher, I would recommend the van de Graff generator as the best demo equipment and then the Wimshurst Machine as an additional "gee-whiz" device and mechanical model of electric storms. It is more complex and confusing; and it is dangerous with its significant capacitance storing charge. They are not too expensive but kids just love to turn it faster and faster until something breaks. Shocked
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