The Wimshurst machine is a device for generating static electricity and was first described by James Wimshurst in 1883. My first contact with this device was at age 15 when I found one broken and forgotten in a physics class cupboard. I replaced various missing parts and brought the device back to life. It took some time for it to work well because it had been in a damp cupboard for a long time and so had to dry out to the point where I could get a decent spark. My brother and I had fun giving each other electric shocks.
It was fun to learn about this machine, especially as they often appeared in Frankenstein movies which were exciting to me as a kid.
In around 1999 I decided to build a really large one myself. This project was pretty complex, and in the end it was not very successful. I did manage to get some pretty beefy sparks out of it, but a number of design issues limited the available charging current. I thought about making some changes, but in the end it sat in storage for a while and I eventually gave it away.
I built a machine with counter-rotating wheels which were made of quarter inch thick acrylic and about four feet in diameter. These were spaced about 1.5 inches apart and driven by a treadmill motor. They rotated in double ball-bearing mounts in a wooden frame. I made 36 copper foil segments on each wheel to collect the static electricity and glued six reinforcing struts per wheel to prevent flexing.
The neutralizers were made of brass tubing and had brushes on the ends made from very springy piano wire to maintain a reliable contact. The charge collectors were made from a horseshoe of copper pipe. These connected to the capacitors via thick wire insulated with 3/8 inch diameter polyethylene tubing.
The two capacitors were made from cast acrylic tubing, five feet high with 4.5 inch diameter, 1/4 inch wall, wrapped inside and out with aluminum flashing, for a capacitance of 500pF each and a dielectric strength of around 400kV. The capacitors were the best part of the design and worked very well.
The electrodes were brass rods with brass doorknobs attached to make a spark gap. The drive consisted of a DC treadmill motor which was run from 24 Volts and the rotation transmitted via a pulley ystem. An important part of the drive was a bicycle freewheeler which allowed to disks to continue to rotate after the power was turned off, so that it would not just abruptly try to stop the motion.
Everything was constructed from hardware store items! It took about two months to build and cost around $1000.
Although I got some pretty huge sparks out of it, the charging current was too low. Firstly the setup was too wobbly and unstable to really spin up the wheels to a high rate of rotation. Secondly there was too much charge leakage off relatively small diameter parts. I started replacing the charge collectors with fatter pipes without pickup spikes, and this seemed promising in order to remove the corona losses. Also the foil segments were rather small in area, so they didn’t carry much charge. I designed another set of much larger segments to use, but I lost interest in the project before replacing them all.
The final result was sparks which were around 8 inches long. Interestingly the sparks would still appear if the capacitors were disconnected from the charge collectors – presumably there was still a lot of charge picked up by the disconnected wire ends. In general it was an interesting project. To make something like this really function though, you have to iterate the design numerous times, and this gets to be a bit like hard work!