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FAQ - Paschen, gas, vacuum, electrostatics, break down, etc., etc.

Posted: Sat Dec 14, 2019 2:19 am
by Richard Hull
A recent discussion here related to breakdown, (ionization in gases), became filled with issues related to Paschen's law and the many kinds of break down conditions in gases at varying pressures and temperatures.
A real discussion on these issues could not be covered in a good year long course in college on this specific issue without much lab time with hands-on effort! I am not here to do that course. It is up to you and self-directed learning, hands-on experiment and open mindedness, to plot and plod your own path, if you so desire.

I have plodded this path to a degree that satisfies me. Each day I still learn something new about something that interests me. This FAQ will give a real physical embodiment to some of these issues in a rather marvelous way about a vacuum tube back filled with a gas at some pressure, X, with no obvious source of electrons, (heated filament), one electrode, a hollow cylinder, the other a flatten roll of screen wire, another electrode connected to a flat plate exposed to the world with a terminal capacity on top. The tube is subjected to an internally applied electrical field, yet subjected to a totally external electrical field at the same time! Too much internal voltage applied will cause the tube to breakdown to a current limited glow discharge, just like a fusor.

I will introduce you to a cold cathode, vacuum FET with its gate subject to turn on currents in the femptoamp range with no external electronic amplification or circuitry to give a visual indication of those currents.....In short, everything we would like to discuss is taking place in this novel and interesting device. I will demo this device in two days at our monthly HEAS meeting here in my lab.

The telling of the tale

I include a diagram of this tube below. Refer to it at all points as you read the following to anchor you understanding of what is be done, said and its implications.

I came across this vacuum tube at a hamfest for 25 cents. I think it must be out of some sort of smoke detector. There were three in the box. They were too cute to leave in the 25 cent box filled with other trinkets of like price. with typical hamfest mentality, I offered 50 cents for the three. It was gladly accepted.

Like all unknowns before me, I took Ben Franklin's advice...."Let the experiment be done!" All of my past experience was brought to bear during the examination of this device before experiment. I immediately had an idea of what to do and what this little guy was all about. I put a variable DC voltage across the two leads coming out of the bottom of the vacuum tube. I varied the voltage to see what happens while monitoring the current draw. As I have heard and seen hollow cathode devices, I put the negative lead on the little metal cylinder in the tube. I put the positive lead through a 47k current limiting resistor to the really odd mashed flat screen wire electrode.

As I advanced the voltage, no current was drawn, even at the micro amp level. This was expected. Suddenly, at about 240 volts a pinkish-purple, neon glow took place inside the hollow cathode and the tube drew 5-6 mA.... Cool beans!.... I carefully lowered the voltage until extinction occurred, realizing such lamps have a good bit of hysteresis in them. The glow extinguished at about 140 volts. Hmmm?

Then there was that tiny central fine wire attached to the flat, circular metal seal at the top of the tube. There was a little needle point taking a 45 degree jag suddenly near the screen wire electrode. I noticed this interesting aspect at the hamfest and was my sole reason for parting with the 25 cents.

To the external part of this metal end plate was a threaded rod that rose out of it to secure a very prominent aluminum cylinder, (cup, actually), with a small nut on top to retain it. I thought I knew what to do here, having a good deal of electrostatic experience with my old fusor II during my year long adventure with that wonderful demo device. This was coupled with my self-directed reading and experiments with fusor II.

Wiping the tube down with absolute alcohol, I placed it on my demo table ring stand in an elevated test tube gripper arm. Now, with some very delicate, failed 10 of 20 attempts at raising the voltage just shy of break down, I succeeded to my satisfaction of placing this system, (yes, system), in a delicate state of adjustment. I crossed the lab to secure my "Teflon wand", (a rod of about 3/4-inches in diameter and 14 inches long). As I walked the 15 feet back to the demo table, I vigorously rubbed the rod with a soft cloth. At a range of about 6 feet from the tube, I saw the tube suddenly light up!! Exactly as I suspected!! This was an electrostatically biased system. I continued to experiment. I found that with a lot more effort than it takes to say, I could stand 10 feet from the hair triggered electrical field established within the gas of the tube between the hollow cathode and screen wire plate and just wiggle the wand through a top arc of as little as three inches and the gas in the tube would ionize, (breakdown), due to the high variable electrostatic field collected by the terminal capacitance, (aluminum top electrode), transferred to the little needle point near the screen wire terminal. The current between the wand and terminal was surely under the pico amp range and probably in the fempto amp range.

Subsequent adjustments became so finely tuned, that just my moving in the cold winter, low humidity air in my lab would trigger the tube. This little tube and demo shows just how electrical the world and the air around us can become. A hidden sea of undetected and unsuspected electronic exchanges between all objects in motion.

In summary

Yes Paschen's law was at work here, but try and apply it precisely....You will never do it beyond some struggle with the math, a ton of assumptions, most of which would be wrong and guesses at the parameters.
Further more, and again through past experience and knowledge, the assumed pure neon back fill could have had some argon or helium in the mix and even some radioactive gas to "pre-excite" the gas.

So, if you think you have read up on Paschen's law and think you understand it and can apply it across the board spectrum of apps, You are wrong. There is much to learn. learn via experiment and acquire real knowledge of all the principles involved that screw things up in the seemingly simple calculations. Get you mind around the total qualitative experience via experiment for it will always teach volumes over images and reading alone. Complex issues abound here in this little tube. Oddball electrodes, bizarre and unknown gas fill at unknown pressures, electrostatic triggering, all foul the mix to somewhat turn this into an experimentally designed device for a specific purpose.

To put a bit of real life imagery, I attach some photos.

Richard Hull

Re: FAQ - Paschen, gas, vacuum, electrostatics, break down, etc., etc.

Posted: Sat Dec 14, 2019 8:55 am
by Jim Kovalchick
I wonder if the hysteresis you spoke of is really because once ignited into plasma the atmosphere has a completely different conductance than nonionized gas. Once conductive, the plasma is sustained by the current. This is why it extinguishes at a lower voltage than it flashed.

There are lots of ways to initiate crossing Paschen's curve. When my son was in high school, he did a series of experiments that showed that spontaneous static discharges from solids that had become charged by repeated collisions with other solids, varied along the Paschen curve with varied sub atmospheric gas pressure. The Paschen curve illustrates the potential of electron acceleration causing plasma formation no matter the source of the potential.

Re: FAQ - Paschen, gas, vacuum, electrostatics, break down, etc., etc.

Posted: Sat Dec 14, 2019 10:07 am
by Richard Hull
I am familiar with neon glow lamps from years back. 100% of them when used in DC circuits all exhibit this wonderful on-off differential which make them great data storage devices, oscillators, flip-flops, gates, and ring counters. GE put out the ultimate big engineering data manual with circuits for use of their NE series lamps back in the mid 60's. Page after page of theory, tech charts, diagrams and engineering data, but most important, scores of schematics of real circuits you can assemble with only a tiny number of passive components. These can do all manner of useful and cool stuff to amaze folks with technology of yesteryear in demos. I actually took apart an early Friden electronic calculator many years ago and it had banks of about 50 Ne-2 lamps to serve as memory before there were IC's!!!

Print this puppy out and you will be amazed and have a valuable reference at hand in future. ... dition.pdf

Yes, load your printer up with paper if you want a real copy.

I currently have a large fraction of these specialized lamps cataloged and boxed in my lab, as I have picked up the more odd ball numbers over the last 40 years at hamfests. I lust after the never seen little Ne-77 three element lamp. I think that you can only order a tiny fraction of these special lamps today. They are used strictly as replacement indicator lamps in this day and age.

Richard Hull

Re: FAQ - Paschen, gas, vacuum, electrostatics, break down, etc., etc.

Posted: Sat Dec 14, 2019 7:47 pm
by Cai Arcos

What an interesting device indeed! I have a little fascination with the measurement of small currents using vacuum tubes, and because of that have read a lot of papers regarding the so called "electrometer tubes", as well as others that discussed the use of common receiver tubes like the Type 38, 954 and 959 in such roles, but have never came across a gaseous low current measurement tube.

It seems to me like these were intended to be "electrostatic thyratrons" to be used in one-shot applications, just like the smoke alarm you mentioned. I would also caution to block all light around the tube: in the previously mentioned papers, one of the sources of unwanted grid currents that would change the bias occured due to ionization of the residual gas and photoemission of the metals. Taking into account that this is explicitly filled by gas, might be the hairy point of operation you mentioned can be changed by such effects.

Is there any serial numbers or have you come across similar apparatus like these in the past? In any case, thanks for the detailed write-up.

Cai Arcos

Re: FAQ - Paschen, gas, vacuum, electrostatics, break down, etc., etc.

Posted: Sat Dec 14, 2019 9:08 pm
by Richard Hull
No these are the first such items I have come across. These might be termed "electrostatic indicating vacuum latching FET thyratrons".

The 5886 vacuum tube is the ultimate electrometer tube used in all electrometers prior to the very damagable FET front ends of today.

Richard Hull

Re: FAQ - Paschen, gas, vacuum, electrostatics, break down, etc., etc.

Posted: Sat Dec 14, 2019 10:03 pm
by Maciek Szymanski
Indeed very interesting device. Ingenious tricks of the past will never stop to amuse me.

Of course Paschen’s law is an idealization of the gas breakdown limited to specific geometry and pressure-gap region, but I will be far from neglecting it. It’s quite useful engineering approximation. 240V is about the minimum breakdown voltage for Ne (that’s not surprise - the tube designer chas chosen right pressure-gap combination for the application). The pd for this voltage is 10mm*Torr. From the photo I will estimate the electrode gap to about 10mm. This gives the tube pressure of 1 Torr. A value in the typical order for the glow discharge tube.

So I’d say that the Paschen’s law is doing good job, as long as you know it’s limitations. For the other hand the statement of L. Loeb Form almost 100 years ago seems to be still true, as the glow discharge is no longer a fashionable research subject:

About this region which lies in general below the pressure of the minimum sparking potential, ranging from 10e-5 up to 10e-1 mm of pressure, very little is known. Leonard B. Loeb “Fundamental Processes of Electrical Discharge in Gases” John Wiley & Sons, New York 1939, p. 476.

Re: FAQ - Paschen, gas, vacuum, electrostatics, break down, etc., etc.

Posted: Sun Dec 15, 2019 1:19 am
by Frank Sanns
Here are some related fusor pictures from the distant past. Starting and stopping and in-between with many needlepoint passive ionizers makes for interesting study. viewtopic.php?f=18&t=7844

Re: FAQ - Paschen, gas, vacuum, electrostatics, break down, etc., etc.

Posted: Sun Dec 15, 2019 9:49 am
by Cai Arcos

In the article of Wikipedia regarding smoke detectors, it is stated that:
In 1939 Swiss physicist Ernst Meili devised an ionization chamber device capable of detecting combustible gases in mines.[8] He also invented a cold cathode tube that could amplify the small signal generated by the detection mechanism to a strength sufficient to activate an alarm.[8].
Searching further through various references and books, I found this patent, that seems to fit the device here presented:

However, the patent was presented in 1962, and no reference is made to the supposed original year of invenion 1939. In any case, it seems like we now know the typical application circuit for such a device.

Cai Arcos