Follow-up Q15 (2026-10-08): a gas tube with one tin and one mercury electrode

Question (owner, recorded 2026-10-08). Tin and mercury electrodes: if a gas tube had one tin electrode and one mercury electrode, and the two combined (amalgamated tin), would the current change? Would it increase, or would the tube rectify? [Context: Le Bon's amalgamated tin discharged fastest in sunlight, only when negative (A12: photoelectric). Orchestrator's reading, to check: mercury-arc rectifiers already rectify; mercury vapor amalgamates tin in operation; the 253.7 nm mercury line (4.9 eV) exceeds the work function of tin and mercury (about 4.4-4.5 eV), so the tube's own glow could free photoelectrons from an amalgam cathode. Find prior work on amalgam cathodes in glow tubes, measured rectification ratios, and whether Le Bon or anyone tried it.]

Web time: about 3 minutes (Chrome: 7 Google searches; curl: Wikipedia raw text (2 pages), Internet Archive (Prince and Vogdes 1927), World Radio History (Güntherschulze 1928), UV Lamp Consulting (amalgam lamps), arXiv (LISA Pathfinder)). I stopped well short of the cap because two 1920s rectifier textbooks answer the mechanism directly; no further source on a tin-mercury tube turned up in the last two searches.

Terms. Work function (symbol phi, in electron-volts, eV): the least energy needed to pull one electron out of a metal surface. Photoelectric effect: a photon of energy E = hc/lambda (h Planck's constant, c the speed of light, lambda the wavelength) frees an electron only if E exceeds phi. Glow discharge: a gas discharge at low current (microamperes to a fraction of an ampere) and a few hundred volts, in which electrons come from the cold cathode when positive ions strike it; almost all the voltage drop sits in a thin layer in front of the cathode, the cathode fall. Normal cathode fall V_n: the nearly constant voltage of that layer while the glow does not yet cover the whole cathode. Arc: a discharge at amperes and only 10-30 V, fed from a small, very hot cathode spot. Rectify: pass current much more easily one way than the other. Arc-back (backfire): a rectifier conducting in the wrong direction.

What was searched

What was found

1. Why the mercury-arc tube rectifies (the orchestrator's first point holds, with a qualification). - Wikipedia, Mercury-arc valve: "The mercury emits electrons freely, whereas the carbon anodes emit very few electrons even when heated, so the current of electrons can only pass through the tube in one direction" [web https://en.wikipedia.org/wiki/Mercury-arc_valve]. It credits Jamin and Maneuvrier (Comptes rendus 94, 1615, 1882) with first observing the rectifying property of a mercury arc, and Cooper Hewitt (1902) with the rectifier [web, same page]. - Güntherschulze (1928) lists why mercury succeeded: the vapour condenses and runs back to the pool, so "no loss of material"; the arc drop is small; "the anode can be removed from the cathode sufficiently far to be no longer affected by its radiation"; and "the anode in a vacuum arc remains comparatively cold" [web https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/Technology-Early-Radio/Electronic-Rectifiers-and-Valves-Guntherschulze-1928.pdf, p.54, saved]. He adds: "Every metal-carbon arc can be used as a valve." - So the rectification is an arc effect: a self-sustaining cathode spot exists on the mercury and must not form on the anode. It is not a photoelectric effect.

2. Mercury on the anode destroys rectification (the "amalgamated in operation" point, turned round). - Prince and Vogdes (GE, 1927): "Cold anodes may be a cause of premature arc-back. In this case, the condensation of the mercury on the anode causes the formation of a cathode spot there." Also arc-backs "traced to globules of condensed mercury falling onto hot anodes" [web https://archive.org/details/principlesofmerc00prin, saved djvu text]. - A valve catalogue: "Wide spacing on upper section eliminates danger of arc-backs by preventing mercury condensation near anode" [web, Google snippet of a National Electronics industrial-tubes catalogue, https://bms.isjtr.ro (PDF), not opened]. - Güntherschulze on back-starting: favoured by "placing the rectifier anode and cathode in the same compartment so that the rays sent out from the cathode can strike the anode directly" and by "impurities at the anode, especially alkali and calcium salts" [web, Güntherschulze 1928 pp.162-163]. Copper seal caps were "plated electrolytically with a thin layer of iron to protect it from the amalgamating mercury vapour" [same, p.162]. So builders knew that mercury vapour amalgamates exposed metals and designed against it. - Bearing on the question: a tin anode that becomes amalgamated is a mercury-like surface; in arc mode that is exactly the condition that lets the tube conduct backwards. Amalgamation pushes the tube towards symmetry (less rectification), not towards more.

3. Glow-tube rectification: material and area asymmetry (the closest prior work to "amalgam cathodes in glow tubes"). - Güntherschulze describes a "photoelectric valve": "By using a photoelectric alkali electrode and a photoelectrically inactive electrode (such as iron) in one of the rare gases at a low pressure, a photoelectric valve can be obtained"; in the reverse direction it conducts only above the normal cathode fall of iron in a rare gas, "about 180 volts" [web, Güntherschulze 1928 pp.29-30]. - Cathode-fall valve action: a tube with "a platinum and an alkali metal electrode, is permeable in one direction at about 80 volts, in the other at about 180 volts. In the impermeable direction, therefore, between these two voltages, it acts as a perfect valve" [same, p.46]. The valve action is improved "by combining the valve action produced by the different cathode falls of different materials with that given by different surface areas" (large alkali electrode opposite a very small iron one) [same, p.51]. - His formula (18) makes the normal cathode fall rise with the cathode's work function, with a coefficient of 44.4 volts per volt of work function (the rest of the formula, garbled in the text layer, depends on the gas) [web, same, p.44; read from the text layer only, not the page image]. His Table VII (after Langmuir, 1920s values) gives Sn 3.8 V, Zn 3.4 V, Na 1.82 V, Li 2.35 V; mercury is not in the table. - Commercial glow rectifiers (Pintsch; Osram; Raytheon and Burndept named): helium-neon at about 10 mm; a large iron cylinder cathode, "For 110-volt mains this plate is coated on the inside with sodium-potassium alloy"; the anode a thin iron wire with only its tip exposed; up to 0.2 A; oscillogram shows "the negligible current in the impermeable direction" [same, pp.153-155]. No numerical forward/reverse ratio is given. - Alkali-mercury cathode: the "Argonal Rectifier" (Deutsche Telephonwerke) uses "a mercury cathode with a quantity of added alkali" and argon; normal cathode fall with an alkali cathode is 64 V, so the glow passes easily into an arc, and the tube runs down to 0.1-0.3 A; but "back-starting occurs at much smaller voltages than with the mercury rectifier", cured by narrow bent anode arms, up to 700 V [same, pp.192-194]. This is the one amalgam cathode found in a rectifier: the alloying element (alkali) was chosen to lower the work function by about 2.5 eV, far more than tin can.

4. Tin against mercury: almost no material asymmetry. - Work functions: Hg 4.475 eV, Sn 4.42 eV [web https://en.wikipedia.org/wiki/Work_function, table, saved raw text]. Difference 0.055 eV. - [calc] With Güntherschulze's 44.4 V per volt: 0.055 x 44.4 = about 2.4 V difference in normal cathode fall, against a cathode fall of order 100-500 V (530 V for iron in pure mercury vapour, Güntherschulze p.74). In glow mode a clean tin electrode facing a mercury pool would therefore be nearly symmetric in material; any rectification would come from unequal areas, unequal temperatures, or the arc mode. - [memory, medium] A dilute tin amalgam's surface is mostly mercury (mercury has the lower surface tension and dissolves only about half a per cent of tin at room temperature), so an amalgamated tin cathode should behave much like mercury. Not checked on the web.

5. The tube's own 253.7 nm light: real, but a small current. - [calc] 253.7 nm: E = 1239.84 eV nm / 253.7 nm = 4.887 eV. That exceeds Hg (4.475) by 0.41 eV and Sn (4.42) by 0.47 eV, so both surfaces can emit photoelectrons with at most about 0.4-0.5 eV. The orchestrator's arithmetic is right. - Yield near threshold is tiny and surface-sensitive. LISA Pathfinder, discharging gold test masses with 253.7 nm mercury lamps: "With 4.89 eV photons, differences in the surface contamination can produce values for the gold quantum yield ... that vary between 10^-6 and 10^-4"; measured in flight about 1-4 x 10^-5 electrons per absorbed photon [web https://arxiv.org/html/1807.02435v2]. Gold's work function is near those of tin and mercury. - [calc] If 1 mW of 253.7 nm light falls on the cathode: 1e-3 W / (4.887 x 1.602e-19 J) = 1.3e15 photons/s; at 1e-5 to 1e-4 electrons per photon, 1.3e10 to 1.3e11 electrons/s = 2 to 20 nA. A glow tube runs at milliamperes (Pintsch rectifier up to 200 mA), so the photocurrent is roughly 1e-5 of the running current or less. Its practical role is to seed breakdown (fewer delays in starting), not to raise the steady current. The 1 mW figure is an assumption for scale. - Güntherschulze's own point about photoemission at a cathode: such electrons "add an extra voltage" of 0-4 V rather than consume one, which is why the alkali (very low work function) photocathode valve works at all [web, p.29]. With 0.4 eV of excess energy from 253.7 nm, the gain for tin or mercury is a fraction of a volt. - Same LISA paper, a related point: two facing surfaces with different quantum yields under the same UV give a net one-way photocurrent ("The asymmetry of the discharge process was attributed to an imbalance in the quantum yields") [web, same]. This is photoelectric "rectification" in vacuum at nanoampere scale; it is the honest version of the effect the question imagines.

6. Le Bon's own numbers show the amalgam advantage shrinks as photon energy rises. - In sunlight (cut off near 295 nm, about 4.2 eV, just at threshold): "Amalgamated tin = 1000 ... Gold, steel, copper, nickel, mercury, lead, silver ... = 2 maximum" (tin alone in the "least sensitive" list) [read: saved research/pages/lebonmat.htm, Ch.V]. - Under the ultraviolet of electric sparks (well above threshold): "Aluminum = 1000 Amalgamated tin = 680 ... Tin = 270" [same]. So amalgamation multiplies the leak by about 500 near threshold and by about 2.5 well above it [calc from his tables]. Inside a mercury tube the 4.9 eV line is above threshold, so Le Bon's own data predict at most a factor of a few from amalgamation, and that on a current that is already tiny (finding 5). - Le Bon also reports a positive leak under spark light, 7-16 degrees/minute against 30-360 for the negative [same]. Ramsay and Spencer did not reproduce a measurable positive leak (A12). - No passage in the saved Evolution of Matter or Evolution of Forces puts amalgam electrodes in a discharge tube or tests rectification [read: saved copies, grep]. Le Bon's amalgam work was electroscope work in air.

7. Tin as an electrode in a discharge. - Dickson (Proc. R. Soc. A, 1967), Google snippet: "The cathode fall for tin in argon is constant below 30 A" [web https://royalsocietypublishing.org/doi/pdf/10.1098/rspa.1967.0172, snippet only, not opened]. Tin sustains an arc cathode spot, so a tin electrode would not block reverse arc conduction by itself. - [memory, high] Tin melts at 232 C, so a tin anode carrying amperes in an arc would melt; mercury-arc rectifier anodes were iron, steel or graphite (the electricalpowerengineering.nl snippet: "a steel anode (later made up of carbon) and a mercury cathode" [web, Google snippet]).

8. Amalgams in modern lamps (related, different purpose). Low-pressure "amalgam lamps" for 254 nm disinfection use an amalgam to hold the mercury vapour pressure near its optimum (0.8-1.5 Pa) at high wall temperature [web https://uvlampconsulting.com/wp-content/uploads/2017/08/the-analysis-of-modern-low-pressure-amalgam-characteristics-fred-van-lierop.pdf]. Amalgamation there controls the vapour, not the electrode emission. It also means an amalgam lowers the mercury vapour pressure over it [memory, high, Raoult-type behaviour].

Not found. No tube with one tin and one mercury electrode, no measured rectification ratio for such a tube, and no amalgamated-tin cathode in a glow tube. No experiment by Le Bon or by Ramsay and Spencer with amalgam electrodes in a discharge tube.

Reasoning: the answer

1. Would it rectify? Two regimes. - Arc (amperes, a mercury pool at the bottom): yes, as any mercury-arc rectifier does, provided the tin is the anode, kept cool, dry and away from the pool. The mercury pool is the cathode in the forward direction. This is Cooper Hewitt's tube with a poor choice of anode (tin melts at 232 C). - Glow (milliamperes, cold electrodes): only weakly from the materials, because tin and mercury differ by about 0.06 eV in work function, about 2 V in cathode fall [calc]. Rectification would come mainly from geometry (a large cathode and a small anode), as in the Pintsch and Raytheon glow rectifiers, or from a deliberately low-work-function cathode (sodium-potassium; alkali in mercury).
2. What does amalgamation do? In a working mercury tube, mercury vapour condenses on and amalgamates the tin. That makes the tin surface mercury-like. For the arc it is the documented cause of arc-back (Prince and Vogdes 1927): rectification gets worse, not better. For the glow it removes the small material difference that existed; the two electrodes become alike.
3. Would the current increase? Not appreciably. In a glow the running current is set mostly by the external resistance (the glow holds a nearly constant voltage); the cathode's ion-impact emission depends on its work function, which amalgamation changes very little for tin. The 253.7 nm light does free photoelectrons from tin, mercury or amalgam, but at 1e-5 to 1e-4 per photon this is nanoamperes per milliwatt of ultraviolet [calc], useful for starting the discharge, not for the running current. Le Bon's own spark-light table predicts only about a factor 2.5 from amalgamation well above threshold.
4. Competing readings kept. - (a) A clean-surface reading: in a sealed tube, ion bombardment keeps both cathode surfaces clean (sputtering), so the oxide-removal advantage that made amalgams strong in Le Bon's air experiments largely disappears. This is inference, not shown in the sources read. - (b) An enhancement reading: if amalgamation lowered the work function more than the bulk values suggest (surface alloying, impurities), the glow cathode fall would fall by about 44 V per volt (Güntherschulze's formula) and the amalgam side would conduct more easily, giving glow-mode rectification. The Argonal tube shows the effect is real for alkali in mercury (about 2.5 eV lower); for tin no measurement was found, and the bulk numbers suggest a few volts at most. - (c) Photoelectric asymmetry in vacuum: two surfaces with different quantum yields under the same ultraviolet give a one-way nanoampere current (LISA Pathfinder). A tin-mercury tube pumped to vacuum and lit by an external mercury lamp would show this. It is a real, measurable effect at nanoamperes.
5. Bearing on Moray. No tin or mercury is among Moray's reported detector ingredients (A12), and Moray's texts do not describe a tin-mercury tube. The prior art shows why gas valves rectify (a cathode that emits and an anode that does not) and why mercury in the wrong place undoes it. That bears on how the question reads the "Moray valve" as a one-way element, not on any Moray claim directly.

Open

• A measurement: forward and reverse current of a tin-mercury tube in glow mode (argon or neon at a few mm, or mercury vapour), clean tin against after amalgamation, with and without an external 254 nm lamp. Nothing in the sources gives this number.
• The work function of a dilute tin amalgam surface (whether it differs measurably from mercury); an older photoelectric handbook (e.g. Hughes and DuBridge, Photoelectric Phenomena, 1932) may list amalgam thresholds.
• The 1920s German literature on glow rectifiers (Güntherschulze's papers in Zeitschrift für Physik; Schottky's and Seeliger's work on cathode fall versus work function) for measured forward/reverse ratios.
• Jamin and Maneuvrier 1882 (Comptes rendus 94, 1615; Phil. Mag. 14, 154): what electrodes they used when they first saw a mercury arc rectify.