Talk:Thermionic emission

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Strange wording[edit]

The fourth sentence seems strange to me:

After emission, a charge will initially be left behind in the emitting region that is equal in sign and opposite in magnitude to the total charge of the emitted carriers (whether positive or negative)

How can something be "opposite in magnitude"? Mr snarf (talk) 12:46, 17 March 2009 (UTC)

Awkward sentence[edit]

The second to last sentence in the first paragraph reads a little awkward to me, but I don't know how to correct it (partly because I don't know what the original author was trying to express):

"The magnitude of the charge flow increases dramatically with increasing temperature and for vacuum emission from metals tends to only become significant for temperatures over 1000 K."

I just wanted to point that out and leave it up to someone with better understanding of the subject to rephrase it. --71.108.165.243 (talk) 03:03, 11 December 2008 (UTC)

Eneco[edit]

The Application section of this article describes a company named "Eneco" that has purportedly based its business on the Thermionic Emission effect, but it does not provide any references. With the absence of references, I am inclined to believe that this is merely promotional copy and spam. I'll give it a few days for someone to indicate why such a link would be relevant. Otherwise, I hope no one minds if I delete it in a few days. — Ke6jjj 02:46, 13 February 2007 (UTC)

I deleted this section as vanity, but it was re-added. The "applications" of thermionic emission are numerous and already touched upon in the article. Schottky barriers, vacuum diodes, even thermionic converters (functionally the same thing as Eneco's device) are mentioned. I see no reason to single out Eneco's product for its percevied novelty. Even its novelty is in question, I've heard of other companies trying to commercialize semiconductor-based thermionic converters, and there are other effects that can be effectively used for direct heat-to-electrical energy conversion in semiconductors (Peltier effect, pyroelectric effect, etc). Please keep in mind that Wikipedia is not here to promote the products of any company, especially in an article on hard science. -- mattb @ 2007-03-07T01:31Z

Guthrie and Thermionic Emission[edit]

As stated in Richardsons Noble lecture and in the original paper (Phil. Mag. 46, 257 (1873)) Guthrie discovered that positive charge is lost, while negative charge is retained.

2006.08.21


  • Below comments by Moogega*

I am a student who is trying to understand thermionic emission. I have only done a small amount of research, but it suggests that although the Nobel lecture states that the positive charge is lost, isn't the negative charge lost?

  • end comments by Moogega*
What is a "Richardsons Noble lecture? " Please include a quote here showing exactly what Guthrie said. Preece and Fleming in the 1880s and Fleming in the 1896 paper cited below said the effect was discovered by Edison and did not appear aware of any prior discovery. Eddy Kurentz 14:46, 4 January 2007 (UTC)
Owen W. Richardson, “Thermionic phenomena and the laws which govern them.” Nobel Lecture, December 12, 1929. at http://nobelprize.org/nobel_prizes/physics/laureates/1928/richardson-lecture.pdf

“In 1873 Richardson showed that a red hot ball in air could retain a negative but not a positive charge.” Other sources have that the other way: "Professor Guthrie made one of the first discoveries in 1873. He was investigating effects associated with charged objects and he showed that a red-hot iron sphere that was negatively charged would become discharged. He also found that the same did not happen if the sphere was positively charged." History of the vacuum tube at http://www.radio-electronics.com/info/radio_history/valve/hov.php I would tend to believe the Nobel Prize site quoting Richardson. But Guthrie had a metal sphere in the air, where ionized air could be the charge carrier rather than electrons, and this did not show current flow in a vacuum tube. When Preece presented the Edison Effect to the British electrical scuentists in 1885, there is not record than anyone thought Guthries work had anything to do with it. The 1904 Fleming invention of the vacuum tube diode used lamps like the original ones built by Edison in 1884, which he had built by the Ediswan company in Britain. Thus the Edison effect was the leadin to the invention of the vacuum tube diode. Eddy Kurentz 16:39, 4 January 2007 (UTC)

The present Wikipedia article says" While doing work on charged objects, Professor Guthrie discovered that a red-hot iron sphere with a positive charge would lose its charge (discharging ions into vacuum). " This is contrary to the Richardson Nobel Prize lecture cited above, which says Guthrie studied a heated charged sphere in air. If his work was in airm then he was not studyiing thermionic emission, since [1] McGraw Hill Access Science defines thermionic emission as "The emission of electrons into vacuum by a heated electronic conductor. " Examination of Guthrie's publication and also examination of what reliable secondary sources (physics textbooks, refereed journal articles) would improve the article. The present Wikipedia article does not refer to vacuum in the definition in the introduction, which appears to deviate from the mainstream definition. Eddy Kurentz 17:12, 4 January 2007 (UTC)

Edison Effect vs. Thermionic emission[edit]

The term 'Edison Effect' seems very wrong to me; the term thermionic emission is the 'correct' term, I believe - certainly, it is the 'scientific' one.. Even the article says that it wasn't first discovered by him. I don't want to step on anyone's toes, but shouldn't the redirect be the other way around? -- James D. Forrester 02:45 Apr 24, 2003 (UTC)

I'm familiar with thermionic emission, and I've never heard it called the "Edison effect" before. Google searches:

The small number of hits on the intersection indicates that "Edison effect" usually refers to something else. Indeed, browsing seems to show that it usually refers to how brilliant Edison was in some way or another. I'll move the page. -- Tim Starling 03:07 Apr 24, 2003 (UTC)

Does the Edison effect really refer to thermionic emission in general, or to the conduction of electrons from one electrode to another, or to the property that it only flows in one direction, and can be made into a diode? Omegatron

Yeah, thermionic emission is a general term for electrons (or sometimes ions) being emitted from a heated object in a vacuum, and the Edison effect refers to the diode effect, from what I gather. This article is kind of about both at the same time. Maybe it should be split up? At least the two should be differentiated within the article. Omegatron 22:23, Feb 26, 2004 (UTC)

Reference? -- Tim Starling 23:06, Feb 26, 2004 (UTC)
the internet? i don't know. thermionic emission obviously refers to the emission of electrons from a heated element. the phrase "thermionic emission" has no connotation of conduction, one way conduction, or anything. it is just the emission of electrons, from the name itself. electrons are emitted from heated metal whether there is electricity going through it or there are other electrodes present or not. edison effect, on the other hand:

http://www-istp.gsfc.nasa.gov/Education/welect.html - edison effect is the property that current can be conducted from one electrode to the other

http://www.amershamhealth.com/medcyclopaedia/Volume%20I/EDISON%20EFFECT.asp - "a current generated by an electric field in a vacuum tube."

http://www.charlesedisonfund.org/TomEdisonBio/ch10.html - "the previously unknown phenomenon by which an independent wire or plate, when placed between the legs of the filament in an electric bulb, serves as a valve to control the flow of current."

http://www.acmi.net.au/AIC/EDISON_EFFECT_HOME.html - "But, when Edison connected the foil to the positive terminal of the filament, a small current was indicated by the galvanometer. This later came to be known as The Edison Effect."

http://teachingtreasures.com.au/sciencenames/thomas_alva_edison.htm - "He discovered that electricity would flow from the bulb's glowing filament onto a metal plate inside the bulb. This is known as the 'Edison Effect'"


those are just the first sites that came up when i searched for it. sure, internet searches are not conclusive, but it seems pretty obvious that edison effect has more to do with conduction through a vacuum by the emitted particles than the particle emission itself. i think they are sufficiently different concepts that they should be separate articles. Omegatron 15:16, Feb 27, 2004 (UTC)

Fair enough, go on then. -- Tim Starling 01:52, Feb 29, 2004 (UTC)
http://www.plexoft.com/SBF/T02.html says either effect can be called the Edison effect. Hmm... - Omegatron 17:54, Mar 3, 2004 (UTC)
GRR. this site http://www.uh.edu/engines/epi1323.htm says: "The [Edison] effect is this: in a vacuum, electrons flow from a heated element -- like an incandescent lamp filament -- to a cooler metal plate." to a cooler plate?? i thought it only depended on charge. a cooler plate would just not emit electrons, right? - Omegatron
Hmm... that's different. If you don't apply a field, the electrons will diffuse from the hotter plate to the colder plate, because the hotter plate emits more electrons than the colder one. It's a diffusion process, as opposed to the drift process observed when you apply a field. If there's both a temperature difference and an applied field, the currents from diffusion and drift would be added together.
Perhaps it would be better if we could find some more descriptive term for these processes, rather than naming them after the person who discovered them second. -- Tim Starling 03:18, Mar 6, 2004 (UTC)
So a negative hot sphere in a vacuum would become neutrally charged while its neighboring cold sphere would become negatively charged? Of course, then the hot sphere would radiate heat to the cold sphere until they were both at the same temperature, and the electrons would then diffuse back until the spheres were at the same charge, depending on the speed of each transfer... But what about the walls of the vacuum chamber? Where do the electrons go that aren't being emitted in the direction of another electrode? Is there a specific temperature required to get the electrons to start emitting (related to the work function, i think)? There are a lot of questions involved with this stuff that I can't find answers for online.
The term Edison effect is pretty established, so it at least has to be mentioned, though it is certainly a silly name. - Omegatron
It is pretty well established as you said Omegatron ... especially if you know the history of science. JDR 16:11, 2 November 2005 (UTC)

In a copy of the 1924 "History & Operation of the Vacuum Tube" Professor J.H. Morecroft of Columbia University writes in a chapter about "Electron Evaporation" that it was known as the Edison Effect for 20 years but not used or understood until Flemmings work. The term "Edison Effect" wasn't used to describe anything other than an observation of what was to become understood as "electron evaporation" and later "thermionic emission". It seems to me that the edison effect story/section should just be included in the history part along with a note that thermionic emission was originally called electron evaporation in the day of its discoverers. -- equant 02:50 Friday, July 15, 2005 UTC

Thankfully equat found a good reference ... but reguardless of the small number of hits on the intersection of "thermionic effect" and "edison effect", the "Edison effect" refers to this phenonomena and a good historical story is in wikipedia .... JDR 16:11, 2 November 2005 (UTC)
The internet sucks as a way to research the state of knowledge at a given period in the 19th century. Eddy Kurentz 14:48, 4 January 2007 (UTC)
The 1885 paper by Preece apparently coined the term "Edison effect" to describe 1 way (negative to positive) conduction in a vacuum from a hot filament to a cold plate. It would be good to run down the original paper as a source. I have read elsewhere that Fleming studied the original Edison effect bulbs Edison provided to Preece before Fleming invented the vacuum tube diode as a detector of wireless signals. Eddy Kurentz 19:30, 3 January 2007 (UTC)
Found[2] which is March 27, 1896 by Fleming's "A further examination of the Edison Effect in glow lamps. Fleming studied the diode characteristics of Edison Effect lamps, and says "..attention was first drawn by Mr. Edison in 1884." He does not mention the other two persons cited in the present Wikipedia article. He describes the unidirectional thermionic emission as the Edison Effect. He cites William Preece "On a peculiar behaviour of Glow-Lamps when raised to high incandescence," Proceedings of the Royal Society, 1885, p. 219. Also[3] Findlaw gives a 1943 U.S. Supreme Court ruling that Edison's 1884 patent anticipated Fleming's 1905 diode patent, with similar devices operating on similar principles. (Marconi Wireless T. Co. of America vs U.S., 320 U.S. 1 (1943) in Fleming's claim 1:" The combination of a vacuous vessel, two conductors adjacent to but not touching each other in the vessel, means for heating one of the conductors, and a circuit outside the vessel connecting the two conductors.' " This compares to Edison's U.S. patent 307,031 of October 21, 1884:

"I have discovered that if a conducting substance is interposed anywhere in the vacuous space within the globe of an incandescent electric lamp, and said conducting substance is connected outside of the lamp with one terminal, preferably the positive one, of the incandescent conductor, a portion of the current will, when the lamp is in opera- [320 U.S. 1, 54] tion, pass through the shunt-circuit thus formed, which shunt includes a portion of the vacuous space within the lamp. This current I have found to be proportional to the degree of incandescence of the conductor or candle- power of the lamp." This should be included in the article. Eddy Kurentz 00:43, 4 January 2007 (UTC)

When Fleming invented the diode as a rectifier of Ac in 1904, “The Edison effect was well known and 20 years old. Fleming did not modify it in any striking way, but simply used it as a rectifier” At http://andycowley.com/valves/old/history/genesis.html This is from '"THE GENESIS OF THE THERMIONIC VALVE" A lecture given to the Institution of Electrical Engineers in 1954 on the fiftieth anniversary of the invention of the thermionic valve.By Professor G. W. O. HOWE, D.Sc., LL.D., Member. Prof. Howe is Emeritus Professor of Electrical Engineering, University of Glasgow. Lecture delivered before The Institution 16th November, 1954.' This article also says Fleming visited Edison in 1884, and "About the same time as Fleming's visit to Edison and his introduction to the Edison effect, much research was being carried out by Elster and Geitel in Germany on a phenomenon which had been known for many years, namely that when a metal is heated to incandescence the air in its neighbourhood becomes a conductor. They found that a metal plate in the neighbourhood of an incandescent filament acquired a negative charge, and that the effect is increased if the filament and plate are in an evacuated enclosure. In 1884 Hittorf, in Germany, noticed that if the cathode of a vacuum tube was heated to incandescence a small e.m.f. passed considerable current. So far as I know, however, no practical application of these findings was suggested. " This gives Elster, Geitel and Hittorf no priority over Edison, who had built the first Edison effect tube in 1880, and who had applied for a patent in 1883. The article at present goes too far in calling Edison a rediscoverer rather than a discoverer. Eddy Kurentz 18:45, 4 January 2007 (UTC)
For what it's worth, the term "Edison Effect" is very familiar to any electrial engineer until after the disappearance of vacuum tubes. It was always mentioned in the dicussion of the historical developmentof the vacuum tube. It is accurate to explicitly say that "Edison Effect" is archaic? Is it officially depricated? DonPMitchell (talk) 19:52, 12 March 2010 (UTC)

Guthrie vs. Guthrie[edit]

I thought I had mentioned this here already. There are two Guthries who may have discovered the effect, but I don't know which did it. Both were alive and in the area at the time, and worked on similar things.

  • Frederick Guthrie - "In 1873 an English physicist and chemist Frederic Guthrie griped a bar with a glowing metal ball on its end and put it near a live electroscope. In that moment he could see, that the split gilding on its end have drooped: the electroscope was discharged! This accidental invention was the first step, at the other end there were the vacuum tubes and the photocells."
  • Peter Guthrie Tait - "In 1866 he started a joint project with the physicist Balfour Stewart on heating a disk which was rapidly rotating in a vacuum. This was a topic Tait came back to on several occasions throughout his career."

Probably Frederick, but it isn't clear online. - Omegatron 19:03, May 27, 2004 (UTC)

It is Frederick Guthrie. I work on this topic and have his original paper: Phil. Mag. 46, 257 (1873). 2006.08.21

Editorializing[edit]

I found this paragraph a little awkward to read the first time:

Edison built a bulb with the inside surface covered with tin foil. When he connected the foil to the lamp filament through a galvanometer, with the foil dropped to a negative voltage with respect to the filament, nothing happened (because the cold foil electrode was not emitting electrons). But, when he raised the foil to a positive voltage with respect to the filament, a small current flow was indicated on the galvanometer, as electrons being emitted from the hot filament were attracted to the plate, and completed the circuit. This one-way flow of current was called the Edison effect (although the term is occasionally used to refer to thermionic emission itself).

May I propose:

Edison built a bulb with the inside surface covered with metal foil. He connected the foil to the lamp filament through a galvanometer. When the foil had a more negative charge than the filament, no current flowed between the foil and the filament because the cool foil emmitted few electons. However, when the foil had a more positive charge than the filament, the many electons emmitted from the hot filament were attracted to the foil causing current to flow

Opinions?

The above was posted by User: Limbo socrates who didn't sign with his ~~~~.

Be bold. :-) -- PFHLai 22:21, 2005 Feb 13 (UTC)

Thx :) Limbo socrates 22:37, 16 Feb 2005 (UTC)

I like the proposed new text, but I wouldn't omit the last sentence of the original paragraph...
This one-way flow of current was called the Edison effect (although the term is occasionally used to refer to thermionic emission itself).
Equant 20:48, 7 Mar 2005 (UTC)

Dimensional analysis[edit]

For the two formulae in the section "Field-enhanced thermionic emission" to be consistent with each other, the exponent in the expression for the current density should be multiplied by the electron charge. However, then the dimension of the workfunctions here would be V and not eV as in the rest of the article. Which unit do you guys prefer?

This is correct, the formula for delta_W should be multiplied by the electron charge to give it units of eV and be consistent with the exponential part of the equation for W. The exponent should be dimensionless, and as kT has units of eV, therefore so should W and delta_W. There is a scientific article (Y.-Q. Peng, F.-P. Lu / Applied Surface Science IN PRESS (2005)) which quotes these formulae and confirms that this is true.

Richardson's Constant[edit]

At present, the expression for Richardson's constant reads :A = {4 \pi m k^2 e \over h^3} = 1.20173 \times 10^6 {A \over m^2 K^2}

At first glance, this is confusing, since the fraction on the righthand side refers to the dimensional features of the constant and is not part of the expression. In particular, A and m in the fraction should not be confused with A and m elsewhere in the expression. I suggest changing it to something like the following......

A = {4 \pi m k^2 e \over h^3} = 1.20173 \times 10^6  A mˑ² Kˑ²

Any thoughts? JXM 00:21, 19 April 2006 (UTC)

Temperature of vacuum tube cathodes[edit]

This article could use a simple statement of the temperatures used in vacuum tube cathodes, the sort of thing nontechnical readers might want to know (at least I do). --ChetvornoTALK 22:03, 10 October 2008 (UTC)

e & e[edit]

This article seems to use two meanings for the symbol 'e' interchangably. This is particularly ambiguous in the final section on schottky emission, where to my understanding, the top equation(J(F,T,W)=) is using 'e' to mean the base of the natural logarithm, and the bottom equation(delta W =) is using 'e' to mean either the charge of an electron or the absolute value of the charge of an electron. It is very unclear. 75.82.133.73 (talk) 02:51, 29 October 2009 (UTC)

Also, I believe the delta W equation produces a result in joules, but the work function for a material is normally in electron volts. This certainly isn't clear from the description. 75.82.133.73 (talk) 03:00, 29 October 2009 (UTC)

On second thought maybe it really does mean 'e' as in the base of the natural logarithm in both cases.75.82.133.73 (talk) 05:00, 6 November 2009 (UTC)

Should be referenced[edit]

The nomenclature preferred by the editor writing this paragraph is that the equation with only A0 in should be called the "elementary Richardson-type equation"...

Strictly, this sounds like POV and maybe even OR. Could we get something to explicate the multiple symbols used in the equation? --Wtshymanski (talk) 00:26, 12 February 2011 (UTC)

Why lose positive charge?[edit]

The article currently says

Guthrie discovered that a red-hot iron sphere with a positive charge would lose its charge (by somehow discharging it into air). He also found that this did not happen if the sphere had a negative charge.[1]

This makes no sense to me, and I don't think I'm the only one confused by this. (This part has been the bone of some contention, and the reference has been added here.) The only charged particles an iron body can lose are (A) electrons or (B) iron ions. Naturally, it is much easier to lose the former. Why would it be the opposite way?

As has already been mentioned above, "ionized air could be the charge carrier rather than electrons", but that's no explanation for the above. It still is harder to ionize air than iron{{cn}}, and even if some air gets ionized, that shouldn't stop the iron from emitting electrons. So at the very least something should happen both ways. Can someone solve this riddle? — Sebastian 14:04, 21 October 2012 (UTC)

It also seems to contradict the Edison Effect discussion in the History section (where electrons are clearly the ejected particles), but it is what the ref says. So either electrons are only ejected (from negative metal) if there is a sufficient potential difference, or else metal cations really are easy to eject from the surface of a positive surface, or else electrons in the air are attracted and stick to the positive surface but only when hot? The ref clearly distinguishes metal ions from electrons and low heat from high heat, noting that at higher temperatures, electrons are ejected as well. That's an important distinction that is not clear (and therefore appears inconsistent) in our article. DMacks (talk) 17:02, 22 October 2012 (UTC)
The ref also mentions that the positive emission doesn't last. It says that there's no limit to the decay of the positive emission in vacuum, i.e. it reaches zero. Perhaps it's just a side-effect of outgassing, not of the metal itself? Ssscienccce (talk) 07:23, 27 October 2012 (UTC)

Heated foil vs. heated filament[edit]

I'm confused about the following:

But when the foil was raised to a positive potential relative to the filament, negative charge could flow from the filament through the vacuum to the foil. It would only flow if the foil was heated sufficiently (by its own external power source).

In general it's the cathode - here the filament - that must be heated, so in my opinion an explanation would be in order as to why the anode - here the foil - needs to be heat too.

Seipher (talk) 01:44, 16 June 2014 (UTC)

I changed it to require the filament to be heated sufficiently. The filament was probably inefficient so heating it to a dull red might not register on Edison's setup. Glrx (talk) 21:33, 17 June 2014 (UTC)