Science (Leyden Jar)
I originally heard about the Leyden Jar while reading safety instructions for a Van Der Graaff generator. The instructions were that it was a bad idea for anyone to touch a Van Der Graff generator that was attached to a Leyden Jar, due to the higher than normal voltages involved. Naturally this piqued my interest! A quick bit of research followed, in which I learnt that Leyden Jars were the earliest forms of capacitors and were used in the early telecom industry. If you’re looking for more in depth information on Leyden Jars then it can be found here and here, but if you just want to see how I got on with tinkering in this field, then read on.
Pictured below is my original design sketch, and the first version of Leyden Jar, which worked pretty well. It basically consists of a large bell jar (around 40 cm in height) which has no bottom. The inside and outside surfaces are coated with normal aluminium foil – exactly the same as you might use in the kitchen – held close to the surface of the jar with masking tape. A hollow sphere with a conductive metal surface sits in the neck of the jar, held in place with blu-tack, and connected to the inside layer of foil using thick copper wire (diameter approx. 2mm). The inside layer was also connected to the positive terminal of a high voltage generator by a long insulated lead, as shown in the sketch. The outside layer was connected to the earth terminal on the generator, as was the negative terminal. The generator I used only output 5kV (5000 volts), so I eventually replaced it with a Van Der Graaff generator, but more on that later.
When the generator is switched on, positive charge accumulates on the inside layer, and then the metal sphere. It is important to use a sphere, rather than a cube for example, to prevent charge leaking off at edges or corners. As it is, there will be some leakage along the edge of the foil. I should probably note this point that the ammeter and it’s connection to earth is not actually required, I just put it there to satisfy my curiosity on what sort of current there would be. As with most high voltage experiments, the current was very low, not exceeding about 70 micro Amps (or 0.07 milliAmps), well below the lethal current.
As positive charge builds on the inside layer, it induces an opposite (negative) charge on the outside layer. Once there is a sufficient potential difference, electrical breakdown will occur in the air between the surface of the sphere and the copper wire that is connected to the outer layer, cancelling out the opposing charges and reseting the system. This electrical breakdown manifests itself as a very impressive spark that is both bright and loud, and certainly better than the sparks you might get from a Van Der Graaff generator alone. They certainly made several unsuspecting viewers jump or flinch!
After this initial success, I resolved to make a larger version. A bigger Leyden Jar means a greater area of foil, a greater capacitance and therefore a bigger and more impressive spark! Click on the thumbnails below to see this second version, based around a large plastic bucket, from different angles. Notice that in the second image, the 5kV generator has been replaced with a Van Der Graaff generator. These are theoretically capable of producing voltages in the range of hundreds of thousands, although the one at my disposable was a bit past it’s prime. Never the less, I’m reasonably sure it was giving me more that 5kV. Believe it or not, I don’t actually have a voltmeter that reads past 1kV at my disposal, so I couldn’t check for sure. I’ll have to get one at some point!
Despite the different appearence, the design is identical, for all intents and purposes. In this version the metal sphere is suspended on a spiral of thick wire (still attached to the inside layer), which is held steady by non-conducting string. This was to stop it wobbling back and forth after each spark. This new version worked just as well, although the difference in spark ‘quality’ wasn’t amazingly better. To notice any significant difference a much larger surface area is probably required, so it I ever get hold of a huge plastic barrel, I’ll let you know. 
With all this talk of sparks, now’s the time to show some I think. Because they were so brief in duration, it was impossible to capture them with a normal digital camera taking individual pictures, so one was used that could take a series of pictures in very rapid succession. The two images below show the first version of the Leyden Jar in operation. Note that in the image on the left the spark has decided that the shortest route for it to take is over the surface of the jar and around the blu-tack! This required a slight readjustment of the wire to get it working as intended again. Normal operation can be observed in the second picture.
Now we have close ups of the second version in action. The second version included the slight modification of a rubber sleave around the wire that was attached to the outside, to make sure the spark went where intended. The image on the right is a ‘long exposure’, so what you are seeing many sparks from a period of about 30 seconds superimposed over each other in the same image.
The Leyden Jars were essentially a ‘one off’ project, unlike the continuing saga that is the Railgun, so unless I do manage to find the above mentioned plastic barrel, I doubt there will be any updates to this page. Having said that, if anyone e-mails me any questions about this project, then I’ll add a little Q&A section at the end. In the mean time, I hope you found this interesting! 
Last update 06.06.06. All photography on this page courtesy of Sven Latham and his Canon EOS 350D. Thanks Sven!
