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So, I know that vacuum tubes only work because they're, well, a vacuum, and gas-filled tubes have a hysteretic effect relating to gas ionization.

My question, however, is this: Would an open-air 'tube' be a viable device in any situation? Of course, a cold-cathode version of such a device is just a spark gap, but could a spark gap with one side heated (assume, for the moment, that the oxidation of the electrode isn't a problem. Obviously this would be a concern if one was to actually try to make one of these.) act as a crude latching rectifier? Would it be at all usable, assuming actual sealed tubes and semiconductors aren't an option? Could you add a grid, to make an open-air thyratron?

If not, well, I also know that gas-filled tubes are typically (always?) filled with low pressure gas. Would it be possible to make one work at atmospheric pressure in a gas that has the appropriate properties? This would be more practical than the above because an inert gas could be used.

This is a purely academic question, just a strange thought I had. Obviously, this is horribly impractical and unsafe; I'm more concerned as to whether such a device would do anything at all that could make it useful, even if it's much less useful than the obvious alternatives.

Hearth
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    They would work in space – JIm Dearden Apr 24 '17 at 18:02
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    There are vacuum tube equivalents that operate in a flame. Basically, the parts of a tube arranged in tge flame of a bunsen burner. – JRE Apr 24 '17 at 18:12
  • Does electric discharge machining count? Sparks are used to erode conductive material, but it is usually done in a liquid dielectric medium. – glen_geek Apr 24 '17 at 18:44
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    maybe OP wants to make a beta-radiation ray gun. – robert bristow-johnson Apr 24 '17 at 19:00
  • @robertbristow-johnson knowing engineers, it would be much more likely to successful if he opened the question with, "after much consideration, I decided to build something dangerous & fun: a beta-radiation ray gun, my approach so far…" (omnomnom, [betatrons](http://en.wikipedia.org/wiki/Betatron)) – Marcus Müller Apr 24 '17 at 20:19
  • I'm just curious, pretty much! Wondering about the physics of it. Pretty sure trying to actually make one would be futile at best and deadly at worst. – Hearth Apr 24 '17 at 22:21
  • There is an old short story about an astronaut trapped on moon with radio that had the tubes smashed by enemy spy. He was able to get in spacesuit and vent command module to space and have the radio work in moon vacuum. – KalleMP Apr 25 '17 at 05:21

4 Answers4

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The electron wouldn't get very far, the mean free path in air would be very short or around nm's before it would collide with an air particle. This would lower the current of the 'open air' electron tube significantly. You might be able to overcome this by shortening the distance between anode and cathode or by raising the voltage of the 'tube'.

The point of a vaccum tube is to enhance the mean free path of the free electrons, and you also need a heat source to 'boil off' electrons. Air would cool down the metal and lower the mean free path.

A gas filled tube works with a similar principle, but uses ions instead of electrons to create a current and they operate in the breakdown region of the gas.

If you wanted to create a breakdown device that is ionizing the are here is some more info: enter image description here

Air has a very high impedance so the current would be in the pA to fA range, so you would end up having to amplify the current anyway which would kind of defeat the purpose. The humidity in the air would also vary greatly, and would cause the 'open air' tube to be dependent on humidity and temperature.

Voltage Spike
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  • So that's why gas-filled tubes use low-pressure gas, I take it? Or is that for different reasons? To note, I am thinking of this hypothetical device as a type of gas-filled tube in which ionization plays a role, not a type of vacuum tube. – Hearth Apr 24 '17 at 22:23
  • Yeah, basically because of the mean free path, you need an electron source and then it needs to be accelerated and some means to vary the amount of electrons by varying the voltage between plates. The won't be any current if all the electrons you create from the source run into air molecules and ionize them. So you need to remove the air. You might be able to get something workable if you shorten the distance but there are probably issues with doing that also. – Voltage Spike Apr 24 '17 at 22:27
  • My understanding was that gas-filled tubes worked because the electrons run into gas molecules, ionize them, and then the resultant ions both carry current and ionize other molecules. – Hearth Apr 24 '17 at 22:29
  • Yes, but those types of tubes use nobel gasses which are easy to ionize. Regular air does not ionize so easily, they also have a much higher voltage than a normal vacuum tube as the ionization voltage needs to be much higher. – Voltage Spike Apr 24 '17 at 23:17
  • Are you sure of that? Noble gases are known for being hard to ionize. I think it might have more to do with pressure than anything else. – Hearth Apr 24 '17 at 23:39
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This guy built diodes and triodes that operate in a flame.

I think that's about as close to open air as you can get.

You could, of course, move to the moon and do your experiments in the great outdoors. The "open air" there is a pretty good vacuum.

Here's a picture of a flame triode:

enter image description here

JRE
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  • I have to say, I absolutely was not expecting actual working examples! That's fascinating. – Hearth Apr 24 '17 at 20:43
  • Now that's a plasma device! – Voltage Spike Apr 24 '17 at 22:28
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    Flame rectifier "tubes" are quite common actually -- applying a low current mains voltage between an electrode in a flame and ground then measuring the DC offset generated by the rectification is a common strategy for sensing flame in gas burner controls. (It's faster than a thermal sensor such as a thermocouple.) – ThreePhaseEel Apr 24 '17 at 23:48
  • I see the zigzag grid. Is the anode (B+) the middle bolt? – analogsystemsrf Apr 25 '17 at 07:14
  • @analogsystemrf: The middle bolt is one of the terminals, yes. – JRE Apr 25 '17 at 08:09
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Obviously, this is horribly impractical and unsafe; I'm more concerned as to whether such a device would do anything at all that could make it useful

Au contraire regarding the unsafe. They would actually be much safer if there was no vacuum inside them. Consider a rather large old style 36" TV. Scarily, the air pressing on the front such a CRT is almost equal to two of these (in their stock condition):-

Defender 110

and this is therefore what happens...

https://www.youtube.com/watch?v=dzdnQCklfc0 with the good stuff @ 3:00.

(I'm not getting into anti implosion devices as they're relitively new.) This might affect the community here more than other Joes in that we use CRTs in the form of analogue oscilloscopes. This must be one of the most common CRTs in the UK now, especially since the analogue TV switch off. I love my 30 MHz Hameg, but realise that it could go with a rather large bang if I get too angry with it. Shame it wouldn't work at all with air in it.

And for the audiophile community, cathode poisoning due to air admittance into valves would probably be reduced as there would not be a differential pressure across the envelope. Obviously I cannot substantial this as no one makes full atmospheric pressure valves due to suspected poor audio quality.

Paul Uszak
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    The guy making that video who thought safety goggles were all the protection he needed must be a complete moron. I wonder if his health insurance would have paid out for getting a few chunks of glass removed from his bare legs! Probably not worth removing any that ended up in his skull - nothing inside that to damage! – alephzero Apr 24 '17 at 23:54
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    @alephzero You just don't get it. It's all for the art. The art I say. – Paul Uszak Apr 25 '17 at 00:01
  • Air pressure tubes might be safer. But open air ones would not, because that means exposed terminals with very high voltages on them! – Hearth Apr 25 '17 at 11:21
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Perhaps you'd be delighted to learn that this very problem was faced by none other than Nikola Tesla while he was developing a device that would be able to transmit large amounts of power wirelessly through the use of a thin particle beam where the particles had all been accelerated to high velocities; think of it as an open-ended particle accelerator, if you will (the particles he had in mind primarily being heavier metal particles).

In this process he had to come up with a way of creating a vacuum tube that was open-ended, permitting the particles to be beamed out. What he came up with was actually quite smart: he essentially fixes an external tube around the tube that is meant to be at vacuum, and then drives in high-velocity gas (e.g. air or steam) to this external tube; since the internal and external tubes both taper to the same nozzle, this effectively lets this high-velocity gas to keep out any atmospheric particles that might enter into the internal vacuum tube, ensuring that it remains a vacuum. Genius, if you ask me.

You can find descriptions of the device and modified forms of it, including diagrams, as well as the entire context for the invention of it, in many places on the web; the treatise he wrote on it is called New Art of Projecting Concentrated Non-dispersive Energy through Natural Media.

Outis Nemo
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  • Since then, there have been more practical realizations; there are electron-beam welding machines capable of limited in-air operation. Actual "force fields" have been used, which is to say, some manner of ion pump to help push air away from the aperture. There are also beryllium or other very thin light window materials such as for LINACs for general irradiation applications (sterilization, cancer treatment). – Tim Williams Mar 05 '23 at 18:12
  • @TimWilliams: I don't quite see how any of those are more practical for Tesla's stated purpose; in the first one you mention you say that in-air operation is limited (not to mention how electrons would not be capable of transmitting the power intended, which is why he sought to use heavy metal particles), and in the third you can only transmit light, not particles, which is exactly what Tesla wanted to achieve. The second case you mention is more akin to Tesla's solution, but is an ion pump really more practical than a simple air pump in this case? I'm not so sure about that. – Outis Nemo Mar 06 '23 at 06:48
  • Oh. .....Ohhh. Nevermind then. – Tim Williams Mar 06 '23 at 16:08