Tesla Air Quenched Spark Gap Replication

In a previous post I performed a detailed literature review on the various types of spark gaps Tesla employed in his experiments. My motivation for that research was that I am trying to replicate Tesla’s stout copper bars, or  ‘hairpin’, circuit, and I wanted to figure out which type of spark gap he was most likely using for that particular experiment. My conclusion, based on the research I performed, was that he most likely used an air quenched spark gap. In this article I show you my replication of Tesla’s air quenched spark gap design from his 1893 lecture with a modern twist, including a parts list and my laser cutter ready files.

To start, let me repeat Tesla’s description of his air quenched spark gap.

“Another form of discharger, which may be employed with advantage in some cases, is illustrated in Fig. [1].  In this form the discharge rods d d1 pass through perforations in a wooden box B, which is thickly coated with mica on the inside, as indicated by the heavy lines.  The perforations are provided with mica tubes m m1 of some thickness, which are preferably not in contact with the rods d d1. The box has a cover c which is a little larger and descends on the outside of the box. The spark gap is warmed by a small lamp l contained in the box. A plate p above the lamp allows the draught to pass only through the chimney a of the lamp, the air entering through holes o o in or near the bottom of the box and following the path indicated by the arrows. When the discharger is in operation, the door of the box is closed so that the light of the arc is not visible outside. It is desirable to exclude the light as perfectly as possible, as it interferes with some experiments.” ¹

Based on these specifications, this is what I created:

Instead of a wooden box covered with mica for insulation, I chose to use acrylic, which is non-conducting by default. The particular type of acrylic I used is non-transparent, since Tesla mentioned “it is desirable to exclude the light as perfectly as possible.” The entire structure is built on top of a 12V PC fan. By hooking the fan up to a function generator, I can use pulse width modulation (PWM) to precisely control the airflow through the spark gap.

In figure 1 you see how the air flows through the chimney to the top of the box, and then out along the sides of the cover, “which is a little larger” than the box. I made sure the cover has 1mm room on all sides and also took away 1mm at the top of the sides of the box, so the air has small openings to flow through. I also added a window of welding glass to the cover, which allows me to look at the spark inside the box without damaging my eyes.

In my review of Tesla’s spark gaps, we found out Tesla preferred to use aluminum electrodes (d d1) and brass holders (b b1) for this type of spark gap, so that’s what I used. The electrodes in figure 1 also look rounded, so I got to work with a file and some sandpaper to round the edges, which worked well, since aluminum is fairly soft. Rounded electrodes were also recommended by Richard Quick, who said that the aerodynamic properties of rounded electrodes resulted in better quenching of his air gap.

Now let’s have a closer look at the inside!

The chimney is created from a 32mm grey PVC tube. However, I received a tip from Simon over at Tesla Scientific, that grey and black PVC is often colored using carbon, which conducts electricity. So it might actually be safer to use white PVC instead. The same might go for the acrylic too, but I have not found conclusive evidence of this yet.

Tesla says his spark gap is “warmed by a small lamp”, so I added a 12V 10W incandescent Soffitte lamp for this purpose directly under the chimney. This lamp is powered from a bench power supply, which allows control over the incandescence of the lamp and therefore the temperature inside the spark gap. I left a small hole in the side of the box through which I can poke a thermometer to keep track of the temperature inside.

And that’s it!

Notes on operation

This is not just a “fire and forget” spark gap, it requires some fine tuning. Let’s have a look!

Warming the gap

This spark gap is warmed by the heat coming from the lamp, but why would we want to do that in the first place? Tesla explains..

“This form of discharger is simple and very effective when properly manipulated. The air being warmed to a certain temperature, has its insulating power impaired; it becomes dielectrically weak, as it were, and the consequence is that the arc can be established at much greater distance. The arc should, of course, be sufficiently insulating to allow the discharge to pass through the gap disruptively. The arc formed under such conditions, when long, may be made extremely sensitive, and the weal draught through the lamp chimney is quite sufficient to produce rapid interruptions. The adjustment is made by regulating the temperature and velocity of the draught.” ²

So by warming the air, the arc can be made longer, making it more easy to quench by the draught of air coming from the chimney. So how warm do we need to make it in there?

Unfortunately, Tesla is vague about this. All he says is “warmed to a certain temperature”. However, he mentions using only a “small lamp”, so how much heat can such a lamp generate? To test this, I inserted the temperature probe into my spark gap and let the 10W lamp shine at full brightness for about 20mins, after which the temperature inside the box had risen from 20ºC to 50ºC. Pretty impressive!

But what is the actual effect of this temperature increase on the arc length? The dielectric strength of air at 25ºC and normal atmospheric pressure is 3130V per mm ³, but the amount of voltage required to bridge a certain distance decreases as temperature increases, as can be gleamed from the correction factors in figure 6 below.

The spark my circuit will pass through the gap will be 80.000V, and I mentioned before that the temperature in my spark gap started at 20ºC, at which the correction factor read from the chart is about 1.02 compared to the default of 25ºC. At this temperature, the maximum spark gap length I could use is therefore

80.000 / (3130 * 1.02) = 25mm

50ºC is unfortunately not on the chart, but if we extrapolate the linear decrease in dielectric strength for the sake of this discussion, the correction factor would be approximately 0.898, resulting in a maximum spark gap length of

80.000 / (3130 * 0.898) = 28,5mm

In other words, by increasing the temperature inside the spark gap from 20ºC to 50ºC, the arc can be made 3,5mm longer, which is a 14% increase. I’m not sure if this counts in Tesla’s book as “a much greater distance”, but it is definitely a significant effect. In case you want to extend your arc over even greater distances, you could switch the lamp for a heating element, which allows for quicker heating and higher temperatures, although Tesla talks about “warm” air and not necessarily “hot” air, so the 50ºC might be sufficient.

Controlling air flow

When it comes to operating the fan, only a slow rotation should be sufficient, since Tesla mentions a “weal draught” is all it takes to quench the arc, as long as the medium is properly warmed up. Making the fan rotate too quickly will both cool the air down, as well as increase the air pressure inside the box, both of which result in a higher breakdown voltage, which is exactly the opposite of what we’re trying to achieve.

As mentioned before, the fan is controlled through Pulse Width Modulation (PWM). These are the steps:

1. Hook up the fan to a function generator
2. Set the function generator to a 12V square wave
3. Set offset to 100% (else you’re only getting half the voltage)
4. Now play around with the duty cycle to control the fan speed

I found that if I went lower than a 30% duty cycle, the fan would stop spinning. So your range is between 30% and 100%.

Noise

Spark gaps are known to be loud, but placing the electrodes inside a box like we are doing here greatly increases the noise coming from the gap, since the box acts like an echo chamber. I am considering several approaches to reduce the noise levels, but the hard part is that the materials chosen should not conduct electricity or easily catch on fire. To be continued…

Parts & tools

In my search to replicate Tesla’s devices and experiments, I usually ended up being frustrated after seeing an amazing replication without a word on which parts were used and where to buy them. That’s why I will share a detailed parts list here for the spark gap I described in this article. I purchased many parts in my home country, The Netherlands, so some links will be to Dutch websites, but at least you get an idea of what part you should be looking for at your local hardware store.

Parts

• 600mm x 300mm x 3mm non-transparent black acrylic (your laser cutter company can arrange this for you)
• 2x 10mm diameter aluminum rods
• 2x brass 8-14mm connector (hard to find)
• 12V 120mm computer fan
• 10W 12V incandescent miniature Soffitte bulb (avoid the LED version)
• Soffitte lamp holder
• Welding glass
• 32mm white PVC tube
• Acrifix 0192 acrylic glue
• High-voltage connection cables
• 4x M4 40mm bolts + nuts
• 2x M10 20mm bolts + nuts

Tools

• Function generator to power the fan
• Bench power supply to power the lamp
• Oscilloscope to measure frequency, waveforms, and performance of the spark gap (optional)
• Digital kitchen thermometer to measure air temperature inside the spark gap (optional)

The final piece of the puzzle is of course the actual laser cutter ready design of the spark gap enclosure, which I also want to share with you. All you have to do is leave your name and email address below, and I’ll send you the vector drawing (.ai) directly to your inbox!

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1. Tesla, N. (1893). On light and other high frequency phenomena. Retrieved from https://www.tfcbooks.com/tesla/1893-02-24.htm
2. Tesla, N. (1893). On light and other high frequency phenomena. Retrieved from https://www.tfcbooks.com/tesla/1893-02-24.htm
3. De Podesta, M. (2002). Understanding the Properties of Matter. New York, NY: Taylor & Francis.