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Adjustable Spark Gap

I decided to create an adjustable spark gap that I could adjust while it was arcing. I wanted to be able to fine tune the gap, have a spring loaded handle to draw sparks manually or discharge a capacitor and avoid having sharp points where corona could form.

Tesla described several types of spark gaps he created to try to obtain very disruptive discharges (1983 Light and Other High Frequency Phenomena, by Nikola Tesla - "On Apparatus and Method of Conversion").

Here's a couple examples from his lecture:

The first image shows the use of an electromagnet to quench the gap and the second image shows the use of hot air flowing over the gap. These are described in detail in the lecture.

So anyway I designed my spark gap with the with plans to test some of these means to blow out the arc, and this is what I came up with:

Next I ran some tests with the spark gap. First using discharges from my high-voltage power supply:

And then again with capacitor discharges:

I have previously done this experiment by sliding the magnet under wires that were just taped down, and the effects were much more visible (see Magnetic Quench project) . I'm guessing the reason it didn't work as well here is because:

  • The magnets were further away from the discharge due to the thickness of the discharge rods and the glass instead of packaging tape
  • The spark was probably set further apart with the wires. I was afraid to really push it too far here because I didn't want to pop my capacitor (it's only rated for 2kv).
  • The glass occluded the sound and rubbed the bars adding extra variables to the experiment.
So I didn't get great results from these tests with the capacitor discharge, and up until now I have mostly just described what I've done, but not what I'm trying to do or why I'm doing it. So before I go on, let me first take a step back and explain what I'm trying to do and why.

Over the years I've developed an interest in the experiments and inventions of Nikola Tesla. It's less clear what exactly Tesla was doing in his later years, but it's fun to imagine with his expertise what contraptions he concocted. It's pretty widely known that Tesla was capable of transmitting wireless power and there are countless patents which often include puzzling claims. So basically my motivation is to learn how Tesla was using electricity and what phenomenon he was able to produce with his methods. Tesla published a lecture in 1893, called "On Light and Other High Frequency Phenomenon". This lecture is a great resource to learn about his methods because he devotes a large section of the lecture to describe how he obtained the currents he was using, "On the Apparatus and Method of Conversion".

Since this was written over a century ago, and due to Tesla's skill with words, it can be a bit like reading poetry, which makes understanding his message sometimes rather difficult. I've decided to rephrase this section of the lecture in my own words (paraphrase), and yet trying to maintain the integrity of his meaning.

Here is the digested version of his lecture

I've left the comments open on my digested version, so please feel free to compare it with the original and let me know if you think I've misrepresented anything.

The experiments I've done thus far have been guided by ideas I've found reading this lecture. So, when I went back and examined the lecture comparing it to my results, I came across this (paraphrased):

It seems often the case that it's difficult to keep discharges oscillating, as they tend to be hard to start, and once started are difficult to stop. So it's useful to use an interrupter with the discharge. Tesla used a magnetic field and an air blast for this purpose. The magnet is most useful when used with large direct currents, but if alternating currents are used, it's best if they are of high current and low frequency.

This struck me as odd at first; the discharges I get from my high voltage power supply using my flyback transformer as such a high voltage they start very easily over an air gap, and the streamers are so thin and long they seem to have no problems stopping. Yet my discharges were barely effected by the magnetic quench, so I figured perhaps I need to reconsider the source of my high voltage. I have seen countless videos on you-tube of people using Microwave Oven Transformers (MOTs) and drawing arcs which look much like a flame. These arcs require a fairly small air gap to start but can be stretched very far, and I'm pretty sure this is because the current heats the air up so much that it remains conductive as a plasma.

So, I made my own MOT power supply, and had another go at my magnetic quench experiments:

So, the magnetic field had a much more significant effect when working with a high current discharge. With the standard AC, we have this wave form at around 60hz.

And these are the discharges with and without the magnetic field:

When the half-wave rectified doubler circuit was used, I was actually kind of surprised it worked so well because the discharges without the magnetic field could not produce long arcs and didn't make all that loud of a sound. However that was because the current was pulsed off and on and really there was in fact a lot of current, and the magnetic field turned out to produce a significant effect. This is what the wave form is like.


And these are the discharges with and without the magnetic field: