current flow in vacuum - Page 1
Forum section: electronics / beginners

author: wachanna
author_rank: Newbie
timestamp: August 19, 2024, 01:51:47 pm
content: hello. what is the minimum voltage required to make a current flow in a vacuum?i heard or rather red online vacuum has no breakdown voltage, but for some reason i assume a high voltage in the kilovolt range is required to get current to flow through a vacuum gap between electrodes. does anyone have any real life experience making current flow through a vacuum? just to be clear, there is nothing inside the vacuum gap between electrodes in this hypothetical situation. just two electrodes in a sealed vacuum chamber. since there is supposedly no breakdown voltage im assuming it doesnt matter what the gap size it, right? or does it? of course i could do my own experiments to find out but it would be cool if someone with hands on experience would share concrete facts about this subject. thanks

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 02:19:38 pm
content: At sufficient field strength (voltage gradient in V/m), "field emission" into the vacuum can occur, where the emitted electrons proceed to the positive electrode (anode) that created the voltage gradient.This depends on the "work function", which is the energy required for an electron to leave the cathode material, either by thermal agitation (in a normal vacuum tube), photon absorption (photoelectric effect), or by applied field.Typically, field emission is important with voltage gradients on the order of several Volts per nanometer.The Wikipedia articlehttps://en.wikipedia.org/wiki/Field_electron_emissionincludes discussion of the Fowler-Nordheim equation that governs the emitted current.In high-voltage engineering, "whiskers" that grow out of the cathode due to the applied voltage will magnify the local field strength above the average voltage gradient between the electrodes and the resulting current can be substantial.In grad school 50 years ago (close to "real life"), we used very sharp tungsten points (100 or 200 nm radius) with about 1000 V applied between the point and an anode to make a point source of electrons.Also, when testing the electrodes for high voltage operation, we found that emission from newly grown whiskers fit the Fowler-Nordheim equation reasonably well: applying the voltage through a current-limiting resistor could "burn off" the whiskers by passing sufficient current through the small feature.

author: wachanna
author_rank: Newbie
timestamp: August 19, 2024, 04:53:36 pm
content: the pin point as cathode makes sense but even though a vacuum is supposedly a zero resistance conductor of sort, it seems if we put plates it would form a sort of capacitor, which according to how capacitors work, and the pin point, shouldnt electrons accumulate and have enough energy from repulsing forces to just up and flow across the gap via any voltage?

author: CaptDon
author_rank: Super Contributor
timestamp: August 19, 2024, 05:01:09 pm
content: IF the applied voltage is A.C. and you know the capacitance and frequency, then yes you can calculate Xc and current from applied voltage.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 05:08:29 pm
content: Vacuumis nota "zero resistance conductor" of any sort.If it were, then it could not support any finite value of voltage gradient: the resulting current would be huge.Electrons require a finite amount of energy to leave the conductor plate (cathode, with negative voltage with respect to the other plate, the anode) in a vacuum capacitor.This is due to the physics of electrons in a solid conductor, which is an interesting topic: the required energy is the "work function" that depends on the material.In vacuum-tube and photocell engineering, coatings are applied to a metal substrate to reduce this energy and give higher current for a given voltage gradient.A tabulation of work function for various conductors:https://public.wsu.edu/~pchemlab/documents/Work-functionvalues.pdf: the values range from approximately 2 eV to 6 eV, where e is the electronic charge.A complication is that when the electron does leave the surface, the "image charge" retards it somewhat, due to attraction, not repulsion, so the actual energy required is somewhat higher than the work function proper.Vacuum capacitors are commonly used for very high voltage in RF applications, one vendor is Jenningshttps://www.amstechnologies-webshop.com/media/pdf/8e/a1/a8/Vacuum-Variable-Capacitors-Jennings-Datasheets.pdf.Besides the high voltage rating (dominated by the insulators used to support the electrodes), they can also carry very high AC (RF) current in resonant circuits.This is "displacement current", as in any other capacitor with AC voltage across it, not electron current.Can you think of a practical example where current flows across a gap at any voltage?

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2024, 05:17:38 pm
content: Suppose you start with a sealed space containing air and some high voltage electrodes.Air is an insulator, so current won't flow unless the voltage is high enough to exceed the breakdown voltage. Once this happens you will get a spark, or arc. Depending on the electrical source, this could be a spark that extinguishes, or an arc that burns so hot it melts the electrodes.If you start to evacuate the space and lower the air pressure, the breakdown voltage drops. At some low pressure, it will be possible to get a continuous glow discharge in the air, as long as the power supply limits the current to a low value.At a low enough pressure, down to a complete vacuum, the space becomes an insulator again, and I think only an invisible discharge will happen if the voltage is high enough (light from sparks or gas discharges comes from glowing gas molecules). No gas molecules, no glowing. With the invisible discharge, electrons leave one electrode and jump across to the other one, like an electron beam in a cathode ray tube.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 05:22:14 pm
content: What IanB discusses above is often called a "glow discharge" or "Paschen" effect.If you graph the breakdown voltage as a function of pressure times distance for a simple geometry, you get the Paschen curve that he describes.Note the graphs in the upper right corner of the first page in Wikipedia:https://en.wikipedia.org/wiki/Paschen%27s_lawElectrons and ions actually travel through the gap due to the gas molecules present in a glow-discharge tube, which is quite different from the empty space in a vacuum.Two interesting features of the Paschen curve:The horizontal axis is the product of pressure times distance, so devices scale in an interesting manner.An important parameter for a given gas is the minimum in this curve: both (pressure x distance) and voltage at that value.

author: wachanna
author_rank: Newbie
timestamp: August 19, 2024, 05:33:02 pm
content: i looked at the work function paper you linked. under W/tungsten, which im assuming they are referring to pure tungsten, has for some reason more than one work function, but what im wondering is if the work function is only 5 eV, shouldnt say any voltage 10 volts or higher be good enough to free the electrons from the surface of the cathode so they can flow across the vacuum gap to the anode? and to answer your question, i dont know and wont in reality unless i exhaust my efforts and ideas via experimentation, but first i must understand current flow through a vacuum gap to some extent in theory prior to experimentation

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2024, 05:39:55 pm
content: Quote from: wachanna on August 19, 2024, 05:33:02 pmTo get some idea of what might be involved in useful experimentation, check out the video below. Even if you don't aspire to make a working vacuum tube, you can begin to see some of the workshop equipment and techniques that might be involved.(Sorry, the video doesn't embed, you will have to click through.)Hand Made Vacuum Tubes by Claude Paillard

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 05:48:29 pm
content: Quote from: wachanna on August 19, 2024, 05:33:02 pmRead the article I cited about field emission: the relevant variable is the voltage gradient at the surface of the conductor, not the voltage applied to the entire device.Do not confuse E field (voltage gradient) in V/m, electrostatic potential (voltage) in V, and energy in eV.What elementary textbooks on electricity and magnetism do you own, or can borrow from your public library?Under appropriate conditions, electrons can be emitted from a cathode by heating it so that the high-energy tail of the kinetic energy distribution of electrons (a small minority) in the metal is sufficient to overcome the work function and escape the bulk metal.In a vacuum diode or CRT, these electrons can be accelerated by a second electrode (anode) to achieve a useful kinetic energy before they hit the anode or further electrodes in the device.The work function is hard to measure, and depends critically on surface cleanliness (hard to achieve in a vacuum), and the image charge has some effect on the measurement.I remember a rule of thumb that a monolayer of gas molecules will form on a newly-cleaned surface in something like 1 second at 10-9Torr pressure (approximately 10-12atm).I definitely remember how difficult and expensive it was to achieve pressure less than 10-9Torr in a practical vacuum chamber at room temperature.Commercial vacuum tubes operated at pressure well above that, since the hot cathode kept its surface reasonably clean.

author: wachanna
author_rank: Newbie
timestamp: August 19, 2024, 06:27:22 pm
content: alright well thanks everyone for all your responses. lots to look into. but long articles and thick books made short, whats the minimum voltage able to be used to make current flow across a vacuum gap using common metals, such as copper, iron, zinc, tungsten ect as electrodes?

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 06:39:30 pm
content: That's the wrong question.Consider the vacuum diode: the voltage required for a given currentobviouslychanges with the distance from cathode to anode.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 19, 2024, 06:45:13 pm
content: Quote from: wachanna on August 19, 2024, 06:27:22 pmYou are oversimplifying this. Under the right circumstances, with either the right geometry (extremely small gaps with highly pointed electrodes) or heated electrodes only a few dozens of of volts are required. Any more specific answer requires the exact geometry, and application of the data in the referenced sources.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 06:52:48 pm
content: In my first reply to this question, I quoted a typical example of voltage gradient required for field emission from a cold cathode of a few V/nm.For a practical example with a simple capacitor geometry (parallel plane electrodes spaced by 1 mm), 1 V/nm would require 106V across that 1 mm gap, well past the voltage of a neon sign transformer.That's why practical field emission requires a small radius emitter, such as a needle point.In my experience (related in the first reply), we applied about 1 kV to a needle with end radius of about 100 nm, achieving the requiredE> 1 V/nm, and emission current of several microamperes.For an impracticable geometry such as an isolated sphere of radiusr, the gradient at the sphere surface for a voltage applied to that sphere with respect to an anode a reasonable distance away is dV/dx=V/r.An actual needle, a cylinder that tapers to a small-radius point, the field will be less than that for the sphere (an interesting calculation for those inclined to electrostatic modeling).

author: wachanna
author_rank: Newbie
timestamp: August 19, 2024, 06:56:17 pm
content: you guys are making it sound like its easier to pass current through air than a vacuum

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2024, 06:57:05 pm
content: Quote from: wachanna on August 19, 2024, 06:56:17 pmIt is.There are also many experiments you can do passing current through air, such as spark gaps, Jacob's ladders, Tesla coils, carbon arcs. The list is endless.Not only that, it is hard to make a vacuum. And boring. Nothing to see if you achieve it.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 07:00:34 pm
content: Absolutely: there is much more stuff (gas molecules) in air at reasonable pressure than in achievable (at great expense) high vacuum to conduct electrons (and ions) between electrodes.If you have a Crookes' tube (glass tube with electrodes at each end)https://en.wikipedia.org/wiki/Crookes_tubeconnected to a vacuum pump to vary the pressure between electrodes you can see all sorts of interesting features as you vary the pressure, without using horribly dangerous voltages.When I try to clarify subjects for novices, I find that stating the usual terms for existing equipment and known effects (e.g., field emission, glow discharge, Paschen Law, or Crookes' tube) can be useful since the terminology is not immediately obvious to the newcomer.

author: wachanna
author_rank: Newbie
timestamp: August 19, 2024, 07:20:42 pm
content: so, according to timfox, if im understanding him correctly, he is saying it takes approximately 1 million volts per mm vacuum gap in order to make current flow across. but, if you google search the voltage for a plasma ball, it says it only uses around 3 thousand volts, and the gap between the electrode and the wall of the ball varies but can be multiple inches. so, how come the plasma ball doesnt require a million volts per mm?

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2024, 07:21:47 pm
content: Please read the replies given to you in this and the other thread. Then read them again. And again. Please.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 07:35:38 pm
content: Quote from: wachanna on August 19, 2024, 07:20:42 pmPlasma is formed from gas molecules, which don't exist in vacuum.Cold emission into vacuum requires a vacuum.Your example belongs with the conditions for the Paschen Law for glow dischargein gas: did you read any of my replies about that?Are you being annoying on purpose?PS: if you apply voltage gradients of this magnitude to softer and weaker metals such as iron and copper in a hard vacuum, you will probably grow whiskers (monocrystalline pointy structures formed by the stress on the metal crystal structure) and get field emission (until the whisker burns off due to power dissipated in a small structure) before you get field emission from the overall flat electrodes.

author: wachanna
author_rank: Newbie
timestamp: August 19, 2024, 08:13:36 pm
content: im probably just going to find out from experimentation. i find it difficult to believe it requires a million volts per mm to get current to flow across a vacuum gap. the supposed electrons in the wires connected to the power supply already have a force upon them like water in a hose with a nozzle at the end. squeezing the nozzle handle opens the dielectric barrier to allow current to flow. since the vacuum gap has no dielectric breakdown voltage, the current should just flow from the power supply, through the wire, along the field through the vacuum gap, through the "anode" back to the power supply, because there is nothing blocking the flow to overcome. if a vacuum gap actually behaves like a dielectric, its because something is hindering the flow of electrons, which would mean a vacuum isnt actually empty. i know you dont need a continuous strip of conducting material in order to make charge carriers to flow through an electric field. electric field experiments which show a charged object flowing from one charged plate to the other through the electric field shows this. there is no plasma or material with a continuous flow of charges used as a conductor like electrons bound to a atom of copper wire which already has sufficient force upon it to remove it from its atom to flow down the wire, so, the force applied during induction should be sufficient to make electrons flow across a vacuum gap as the electrons already are given the force needed to remove them from their atoms and should be able to continue to flow along the electric field across the vacuum gap, if they cant its not from having insufficient "work function" it would be because properties of the vacuum are preventing it. if you think im annoying, its becuase i dont have gullible as my profile name, nor idiot, nor airhead. seems you are mistaking annoying with a proper functioning brain. kids also seem annoying when they seek things they desire that the people they ask dont want to give them it.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 19, 2024, 08:22:02 pm
content: Another form of electronc emission is thermionic emission. Heat up a cathode in vacuum and electrons will "boil" off. This is what was used in most vacuum tubes. Even a small potential (say on the grid electrode of a triode) is enough to push them around. Once they fly past the grid, they now see the large anode potential with accelerates them, adding energy. In this case, the maximum current is limited by the rate thermionic emission as well as the repulsion from other electrons pushing them back towards the cathode. Small changes in the grid potential can generate large changes in current flow at first, but eventually saturate.The vacuum itself doesn't have resistive losses, although the vacuum is different from metalic conduction. In metals, the charges of electrons are screened from each other and they propagate relatively freely. In vacuum, the electrostatic repulsion can be significant.

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2024, 08:23:38 pm
content: Quote from: wachanna on August 19, 2024, 08:13:36 pmWhat is annoying is when someone comes along and tries to teach the teachers that they are wrong. Because obviously the intuition of someone who has nothing to go on but their imagination is going to be more likely than the work of careful experimenters over the past 200 years. What do you think scientists were doing for 200 years? Scratching their heads and being confused until someone was able to tell them where they were going wrong?

author: coppercone2
author_rank: Super Contributor
timestamp: August 19, 2024, 08:51:12 pm
content: Whats interesting to think about that kinda makes it different then a regular circuit is what happens with the electrons if you were to some how hide the electrode the electrons are flying towards really fast. They leave the circuit ? I.e. if there is a very long vacuum tube with a spring loaded electrode that moves very fast into a hole. Start up a emission and then retract the electrode.I feel like a plasma cutter does this kind of. Its supposed to have the work grounded. But you can try to cut something without connecting the ground, it just cuts like complete crap.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 08:53:51 pm
content: Such imaginative persons tend to ignore capitalization, punctuation, and division into sentences because they don't want to be understood.I, on the other end, have actually applied voltage to achieve 106V/mm field at the surface of a sharp tungsten point to get microamperes of electron current.The current increased roughly exponentially with applied voltage.I did not need to imagine it, but assembling the required current meter and high voltage supplies was a major part of my experimental work, most of which was fixing vacuum leaks.(Perhaps things have changed in 50 years, but I doubt it.)Simple experiment: take a 1B3GThttps://frank.pocnet.net/sheets/093/1/1B3GT.pdfor 1X2Bhttps://frank.pocnet.net/sheets/049/1/1X2B.pdfvacuum diode and clean the outside of the glass envelope thoroughly to reduce leakage.The anode (plate) of each device connects to a small cap on one end of the tube and the cathode/filament goes to pins on the other end (see datasheets cited for connections).Connect the cathode/filament pins to each other for the other end of the test device.Measure the current through the cold diode as a function of voltage.(These tubes are readily available through surplus vendors.)These tubes were originally used in the high-voltage supplies for CRT televisions.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 08:57:21 pm
content: Quote from: coppercone2 on August 19, 2024, 08:51:12 pmIn the electron gun of a typical CRO CRT, the electrons are accelerated to an anode, but pass through a hole in the middle of the first anode to continue to further electrodes at higher positive voltage (with respect to the cathode) and the phosphor screen.Some of the electron emission current will hit the first anode, the rest should pass to the phosphor screen in a good device.Typical electrostatic lenses used to focus the beam use coaxial cylinders or discs (with holes) at different potentials: there is a large literature about "electron optics".

author: coppercone2
author_rank: Super Contributor
timestamp: August 19, 2024, 09:10:24 pm
content: I assume the CRT is leaky or something so it does not build up too much of a positive charge, but it must build up some charge thats why stuff sticks to it so good. IIRC the old big screen CRT tv would get balloons stuck to it and stuff

author: ejeffrey
author_rank: Super Contributor
timestamp: August 19, 2024, 09:12:42 pm
content: Quote from: TimFox on August 19, 2024, 08:53:51 pmThat's not entirely accurate, sometimes you have to fix water leaks too.

author: coppercone2
author_rank: Super Contributor
timestamp: August 19, 2024, 09:16:16 pm
content: does a non grounded plasma cutter shoot out electrons that go outside the nozzle ? I mean if you just point it at a dielectric and leave the ground clip disconnected. It makes a huge difference in cutting performance if you disconnect the ground. I thought maybe some electrons escape through the hole

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 09:17:59 pm
content: Quote from: ejeffrey on August 19, 2024, 09:12:42 pmI didn't have much trouble with water leaks (and they were easy to locate).Vacuum leaks, leakage currents, and high-voltage breakdown (in air and power supplies) were the worst problems.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 09:20:39 pm
content: Quote from: coppercone2 on August 19, 2024, 09:10:24 pmTypically, the phosphor screen (usually on a metallized surface) was grounded through a wire from the metallization, but the phosphor itself was an insulator that could hold a charge.Often, the interior of a CRO CRT envelope was coated with AquadagTM, a colloidal suspension of graphite.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 09:31:05 pm
content: Quote from: ejeffrey on August 19, 2024, 08:22:02 pmIn a simple thermionic vacuum diode, if you heat the cathode it will emit electrons that just hang around as a "space charge" cloud (with no voltage applied to the anode).When you add a positive voltage to the anode, there are two important limiting conditions for the anode current as a function of cathode temperature and anode-cathode voltage:1. "Saturated emission": with a very high anode-cathode voltage, the electrons are drawn away from the cathode fast enough that no space-charge cloud persists. The current is almost independent of voltage (if high enough), but a very strong function of cathode temperature.2. "Space-charge limited": normal operation for a vacuum tube. The Child-Langmuir law gives a current proportional toV3/2, roughly independent of cathode temperature (if high enough). For typical amplifiers, this operation is preferred since it is not so sensitive to cathode temperature and depends mainly on the voltages applied to the device.Adding the grid to control the current, the E-field at the cathode is a function of both the plate-cathode and grid-cathode voltages. Since the grid is much closer to the cathode than is the anode, it has a stronger control of the cathode field (the relative factor is called "mu") than the anode voltage. The field at the cathode surfaceEkis proportional to (Vak/(mu) +Vgk).In this ideal case, ifVgk= -Vak/(mu), then the field at the cathode goes to zero and the tube "cuts off".

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2024, 09:39:03 pm
content: Quote from: coppercone2 on August 19, 2024, 09:16:16 pmA plasma cutter is like an arc welder. There is an electric circuit between the torch tip and the work and back again. If you don't ground the work to complete the circuit, the cutter won't work very well.The difference between a plasma cutter and a regular arc welder is the addition of a high velocity gas jet inside the arc, which blows aside the molten metal created by the arc and leaves a hole.

author: Stray Electron
author_rank: Super Contributor
timestamp: August 19, 2024, 09:40:00 pm
content: Quote from: wachanna on August 19, 2024, 06:56:17 pmIt is easier to pass current through apartial vacuumthan it is to pass it through a gas at STP (Standard Temperature and Pressure) but it's not a purely linear function. (Which is why I recommended that you look at some of the technical data sheets and/or the CRC Handbook.) And it is much harder to pass current through an absolute vacuum. But it's very nearly impossible to create anything close to an absolute vacuum so a lot of the arguments here aren't really relevant even though they are theoretically correct.

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2024, 09:42:55 pm
content: Quote from: Stray Electron on August 19, 2024, 09:40:00 pmThey are close enough correct that you won't readily get any current to flow through a thermionic diode unless you turn on the heater.

author: coppercone2
author_rank: Super Contributor
timestamp: August 19, 2024, 09:53:35 pm
content: Quote from: IanB on August 19, 2024, 09:39:03 pmbut does it shoot some electrons. it looks like maybe some might miss and go through the opening

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2024, 10:06:43 pm
content: Quote from: coppercone2 on August 19, 2024, 09:53:35 pmWhy electrons? A plasma arc consists of ionized gas molecules. You can't really have free electrons in air, there are too many gas molecules in the way.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 10:06:56 pm
content: Quote from: IanB on August 19, 2024, 09:42:55 pmIt is literally impossible to create an absolute vacuum.However, compare the average density of molecules/cm3and the corresponding average distance between molecules to the important dimensions of your contraption, such as electrode spacing.In 1950s science fiction, interstellar space was usually assumed to be 1 molecule/cm3.At a really, really good laboratory vacuum of 10-10Torr, the molecular density at room temperature is about 3.3 x 106molecules/cm3: feel free to check my math.This is maybe 0.07 mm average spacing between molecules, so there are lots of molecules in a normal lab system.

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2024, 10:14:32 pm
content: I have not done the experiment. Given a typical thermionic diode, can you strike an arc and get a stable plasma discharge between the electrodes supported by the residual gas pressure inside the envelope?

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2024, 10:25:54 pm
content: I haven't done that exact experiment on purpose, but I have seen (visible) gas discharges inside glass vacuum tubes operated at high voltage.When electrons hit the internal electrodes, they can knock off adsorbed gas molecules: typically, grid current due to gas ions drops during minutes of operation as gas is absorbed in the getter.Note that vacuum-tube residual pressure values can be much higher than "ultra-high vacuum" laboratory systems (including large-scale particle colliders).

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 20, 2024, 01:26:42 am
content: The 1B3 experiment is going to be interesting. The newest of these available are probably going on 40 years old. Some are undoubtedly gassy, and I would bet on quite a bit of variability in the "vacuum" in the remainder. I would also bet that OP if he runs the experiment won't understand or implement your instructions for carefully cleaning the outside of the tube.

author: radiolistener
author_rank: Super Contributor
timestamp: August 20, 2024, 05:52:23 am
content: The formula of breakdown voltage in a partial vacuum can be found here:https://en.wikipedia.org/wiki/Breakdown_voltage#Gases_and_vacuumRegarding ideal vacuum, I think it's hard to talk about ideal vacuum if there is some conductors at temperature > 0 K and electric field between them, because such vacuum will consists a lot of electrons detached from a conductor due to its speed + applied electric field.And if you increase electric field (applied voltage between conductor plates in the vacuum) it leads to more electrons emission and at some point energy of flying electron will be high enough to get sputtering, so the vacuum will be contaminated not only with electrons, but also with ions.So when you put two conductor plates into a vacuum and apply some voltage or just heating these plates, the vacuum transforms into some gas or plasma and it will not be a vacuum anymore.

author: MathWizard
author_rank: Super Contributor
timestamp: August 20, 2024, 06:09:50 am
content: I want to do a pile of electromagnetism this winter. I have some good textbooks on it. I need to find some simulator program for some of that, so you can check your equations, and hopefully have some graphs or something to visualize.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 20, 2024, 06:23:07 am
content: Out of sequence now in this otherwise interesting and informative thread.There will be many that appreciate the thread content but a side issue is learning to deal with narcissistic personalities.Quote from: IanB on August 19, 2024, 08:23:38 pm

author: Xena E
author_rank: Frequent Contributor
timestamp: August 20, 2024, 06:55:41 am
content: Quote from: TimFox on August 19, 2024, 10:25:54 pmYes.The vacuum tube only had to be fit for purpose, and it was quickly learned that in use any attempt at perfection was lost on first use.Therefore the inclusion of the getter was a pragmatic solution to the problem of outgassing of the internal surfaces and structures within the envelope.I don't have any citation of it, but was once told that very high powered transmitter tubes were in some circumstances continuously pumped down during use.Regards,X.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2024, 11:24:37 am
content: Quote from: wachanna on August 19, 2024, 08:13:36 pmDo you want mayonnaise with that?Quote from: IanB on August 19, 2024, 08:23:38 pmI'm surprised so many people have the patience to read his word salads.I justpolitely suggestthey improve their posting style, then walk away, if they continue on the same line.Yes, it seems counterintuitive that it's more difficult for current to flow across a small gap containing a vacuum than a much larger air gap, but one has to have patience to learn and take the time to understand why. I don't have much else to say, other than those who've responded have done a great job of explaining it.

author: TimFox
author_rank: Super Contributor
timestamp: August 20, 2024, 02:52:42 pm
content: Quote from: Xena E on August 20, 2024, 06:55:41 amSome high-power klystron tubes came with pumps (sputter-ion or VacionTM) attached to maintain the vacuum.