This question is prompted by (comments at) another one. There, I was surprised to find that despite traditional claims to the contrary, Boltzmann himself did once write his formula $S=k\log W$:


That’s in his book (1898, §61, p. 172), with a pointer to (1896, §8, p. 60) where he says the same thing in words, and emphasizes that $RM$ is the same constant for all gases. In fact, if we note that his $R$ is the specific gas constant (equal to $P\ /\ \rho T$ by the gas law, p. 53) and his $M$ the molecular mass $\rho V\ /\ N$, we see on multiplying that his $RM$ is indeed our $PV\ /\ NT=k$. So all seems well.

But now: there is another common story, which we get to hear again as 2018 is the year the kelvin should be redefined by freezing $k$. E.g. White and Fischer (2015) (emphasis mine):

Although Boltzmann published his famous definition of entropy in 1877, the constant of proportionality in Boltzmann’s definition, was not identified as Boltzmann’s constant until 1900 when Planck published his analysis of blackbody radiation (1900a, 1900b), where he identified the constant as $k$ and named it after Boltzmann.

And that, again, seems not true. Of course Planck does introduce $k$ — in (1900b, p. 241), after $h$, as “a second constant of nature” such that an entropy is $k\log\mathfrak R_0$. But he does not name it after Boltzmann there — nor, unless I missed it, in any of the obvious or oft-quoted places. Not in the follow-up articles (1901a, 1901b). Not in the Boltzmann Festschrift (1904: no mention of $k$). Not in his books (1906, 1910, 1913, 1930). Not in his Nobel lecture (1920):

This constant is frequently termed Boltzmann’s constant, although to the best of my knowledge Boltzmann himself never introduced it

Not in his autobiography (1948) where $k$ is “the so-called absolute gas constant”. And most remarkably — even if meant with self-deprecating irony — not in his recollections (1943):

I (...) was not even taken seriously, in some places. But I did not let such doubts deter me from trusting my constant $k$.

The literature also seems to have settled slower than legend has it — see chronology in the CW answer below. (A name Planck did propose in (1900b, p. 245) is “Boltzmann-Drude constant” for $\alpha=3k/2$, but few besides Abraham (1905, pp. 284, 362) seem to have adopted it — e.g. Perrin (1909) calls $\alpha$ “la constante d’énergie moleculaire”.)

So: If not in 1900, when did $k$ get its name? Was there ever a debate (e.g. after the unveiling of Boltzmann’s famous tombstone)? Was there a concerted decision? Were the above-quoted pages of Boltzmann’s book ever invoked? Or was it no-one’s doing, just resolution by attrition? Finally, if Planck did not name the constant after Boltzmann, how did we end up with the tale that he did?


Community-wiki timeline of relevant literature:

By year written, sources where $k$ is named after...


Author |Dated |Published |Denoted|Called Boltzmann 1877 (e) –– S ein constanter Factor 1 Boltzmann• 1895.09 1896 (e) RM S für alle Gase gleiche Constante RM 2 Boltzmann• 1898.08 1898 (e) RM S –– 3 Drude 1900.02 1900 2α/3 E –– (school using α = 3k/2 instead) Planck 1900.12 1900b (e) k S eine zweite Naturconstante Planck 1901.01 1901a (e) k S eine universelle Constante Planck 1901.01 1901b (e) k S die Strahlungsconstante k Planck 1901.05 1901c k U eine universelle positive Constante Jeans 1901.06 1902 λ R the gas constant Planck 1901d k S die Proportionalitätsconstante k Einstein 1902.06 1902 2ϰ E eine universelle Constante Lorentz 1903.04 1903 k U een universeele constante Planck 1903.07 1904 –– –– (Festschrift) Einstein 1904.03 1904 2ϰ E eine absolute Konstante Jeans• 1904.11 1904 R E an absolute constant Abraham• 1905.03 1905 k U die universelle Konstante k Einstein 1905.03 1905 C S eine universelle Konstante 4 Rayleigh 1905.05 1905 k U a constant Jeans 1905.06 1905a R R –– Jeans 1905.07 1905b k S –– Jeans 1905.10 1905c R R the constant of the theory of gases Langevin 1905a r R constante de l’équation des gaz Langevin 1905b r R constante de l’équation des gaz Einstein 1906.03 1906a R/N R –– von Laue 1906.04 1906 k U Konstante des Verteilungsgesetzes Planck• 1906.04 1906 k S universelle Integrationskonstante Winkelmann• 1906 k S eine Konstante Ehrenfest 1906.07 1906a k die Boltzmannsche Konstante 5 Einstein 1906.08 1906b k S eine (universelle) Konstante Ehrenfest 1906.10 1906b –– –– (obituary) Einstein 1906.12 1907 R/N R –– 6 von Laue 1907.03 1907a k U Konstante des Planckschen Gesetzes von Laue 1907.10 1907b k S eine universelle Konstante Reiche 1908.01 1908 k U eine gewisse allgemeine Konstante Planck 1908.06 1908 k S –– von Laue 1908.09 1908 k S eine universelle Konstante Kowalski 1908.10 1908 k U Konstante des Planck’schen Gesetzes Lorentz• 1909.01 1909 k U a universal physical constant Einstein 1909.01 1909a R/N R –– Reinganum 1909.04 1909 k E die Schwingungsenergie.. beträgt.. kT Planck 1909.04 1910a (e) k S die Konstante k Wien• 1909.05 1909 k S –– Jahn 1909.05 1909 k die Plancksche Konstante 7 Lorentz 1909.06 1909 2α/3 R α -- een werkelijk universeele constante Einstein 1909.09 1909b k S eine universelle Konstante Larmor 1909.11 1909 k S a universal constant Kowalski 1909.12 1910a K die universelle Boltzmannsche Konstante Planck 1910.01 1910b k U eine Konstante Haas 1910.03 1910 k U 2/3 der Boltzmann-Drude’schen Konstanten Kowalski 1910.05 1910b k la constante universelle de Boltzmann Lindemann 1910.06 1910 k U –– Debye 1910.10 1910a k S universelle Konstante, welche.. Einstein 1910.10 1910 R/N R –– Debye 1910.10 1910b k S –– Einstein 1910.11 1911 k U Konst. der Planckschen Strahlungsformel Planck 1911.02 1911a k S –– Planck 1911.04 1911b (g) k S la constante k Sommerfeld 1911.07 2004 k Planck’s.. universelle Constante k Ehrenfest 1911.07 1911 k –– Planck 1911.07 1911c k S der universelle Faktor k 9 Debye 1911.08 1911 k S constante de la loi de Boltzmann Ornstein 1911.09 1911 (f) R/N R –– Sommerfeld 1911.09 1911 k E –– Perrin 1911.11 1912 r R la constante universelle R/N Langevin 1911.11 1912 k le coefficient de Boltzmann Debye 1911.12 1912a k S universelle Konstante, mit welcher.. K.Onnes-Keesom• 1911.12 1912 kP Planck’sche Konstante Lorentz 1912 k E une constante universelle Planck 1912.01 1912 k S –– Einstein 1912.01 1912 ϰ –– (ambiguous!) Bohr 1912.02 1912 k R gas-constant referred to a molecule Kroò 1912.02 1912 k die universelle Plancksche Konstante Warburg 1912.02 1912 k Plancksche Strahlungskonstante Debye 1912.03 1912b k constante universelle de Boltzmann Tetrode 1912.03 1912 k S –– Frank 1912.04 1912 k U –– Keesom 1912.04 1912 kP –– Debye 1912.07 1912c k Boltzmannsche Konstante Drude-Boll• 1912 k S une constante universelle Schrödinger 1912.10 1912 k die Boltzmann-Planck-Konstante 8 Schrödinger 1912.10 1912 k die universelle Boltzmannkonstante Westphal 1912.10 1912 k S die Konstante der Entropiegleichung Nernst• 1912.11 1913 (e) –– –– (school using β = h/k instead) Planck• 1912.11 1913 (e) k S universelle Integrationskonstante Lorentz• 1912.11 1916 k E constante qui détermine l’énergie Born-von Kármán 1912.11 1913 k U –– Hasenöhrl 1912.12 1913 k S –– Reiche 1912.12 1913 k U –– Jellinek• 1913.01 1913 k S eine universelle Konstante Einstein-Stern 1913.01 1913 k U –– Tetrode 1913.01 1913 k U –– Sommerfeld 1913.02 1913 k E –– Born-Bolza & al 1913.02 1913 k die Plancksche universelle Konstante Einstein 1913.03 1913 R/N R –– Born 1913 k U –– Planck 1913.04 1913 k U –– Planck 1913.04 1914 k E absolute Gaskonstante Debye 1913.04 1914 k die Boltzmannsche Konstante Smoluchowski 1913.04 1914 H/N R –– Sommerfeld 1913.04 1914 k die Boltzmann-Plancksche Konstante Lorentz 1913.04 1914a k E –– Stern 1913.05 1913 k R –– Ehrenfest 1913.05 1913 k U –– Ratnowsky 1913.05 1913 k die Boltzmann-Plancksche Konstante Millikan 1913.06 1913 (g) k the Boltzmann entropy constant Bolza 1913.07 1913 k die Plancksche universelle Konstante Born-Courant 1913.07 1913 k die Boltzmannsche Konstante Debye 1913.07 1913a k die Boltzmannsche Konstante Ortvay 1913.07 1913 k die Boltzmannsche Konstante Debye 1913.08 1913b k die Boltzmannsche Konstante Kroò 1913.09 1913 k die Plancksche Konstante Lorentz 1913.09 1914b R/N R –– Einstein 1915 R/N R –– von Laue 1913.10 1913 ϰ die Boltzmannsche Konstante Fokker 1913.10 1913 k S de gasconstante voor één molecuul von Laue 1913.10 1921 k la constante de Boltzmann Sommerfeld 1913.10 1921a k U constante du rayonnement Lorentz 1913.10 1921 k la constante k de M. Planck Einstein 1913.11 1913 k S –– Schrödinger 1913.11 1913 k Boltzmann-konstante Schrödinger 1913.11 1914 k Boltzmannkonstante Pockels 1913 k die Boltzmannsche Konstante Stern 1914.02 1914 k R –– Bragg 1914.03 1914 k die Boltzmannsche Konstante Richardson• 1914.05 1914a k S a universal constant Griffiths 1914.06 1914 k Boltzmann’s constant Lorentz 1914.09 1917 k Planck’s well known coefficient Richardson 1914b R R the gas constant for one molecule Lindemann 1914.12 1915 R/n R –– Czukor 1915.02 1915 k die Boltzmannsche Konstante Tetrode 1915.02 1915 (e) k de constante van Planck Richardson 1915.04 1915 k Boltzmann’s constant Richardson 1915 k Boltzmann’s constant Guillaume 1915 k R la constante d’énergie moléculaire von Laue 1915.05 1915a k S –– von Laue• 1915.07 1915b k S –– Born 1915.07 1915 k die Boltzmannsche Konstante Rubens 1915.07 1915 k die Boltzmannsche Konstante Planck 1915.11 1915 k E –– Mandersloot 1915.11 1916 K Boltzmannsche Konstante Kroò 1915.11 1915 k U –– Planck 1915.11 1915 k –– Jeans• 1916.01 1916 R S the universal gas-constant Stern 1916.02 1916 k R –– Gerlach 1916.04 1916 k U die universelle Konstante k Wolfke 1916.04 1916 k die Boltzmannsche Konstante Stern 1916.08 1916 k die Boltzmannsche Konstante Einstein 1916.08 1916 ϰ S –– Debye 1916 k die Boltzmann’sche Konstante Einstein 1916 k die bekannte Boltzmann’sche Konstante Hertz 1916 k die Boltzmann-Plancksche Konstante

$=$ textbook or reference book. (e|f|g) $=$ English | French | German translation.
Letters S|U|R|E tell the formula each article not naming $k$ uses to first introduce it: $$ S=k\log W\qquad\quad U=\frac{h\nu}{e^{\frac{h\nu}{kT}}-1}\qquad\quad R=Nk\qquad\quad E=\tfrac32kT. $$

Notes (in progress, very much open to debate / correction):

1 First known occurrence of the principle. (Quoted by Planck 1900b.)
2 First known occurrence of the constant. (Ever quoted?)
3 First known occurrence of the formula. (Ever quoted?)
4 Earli(est?) to call the principle Boltzmann’s.
5 Earli(est?) to call the constant Boltzmann’s.
6 Earli(est?) to call the formula Boltzmann’s.
7 Earli(est?) to call the constant Planck’s.
8 Earli(est?) to call the constant Boltzmann-Planck’s.
9 Calls the constant not Boltzmann’s.

  • 3
    $\begingroup$ I have found an earlier (earliest?) reference to Boltzmann's constant: Paul Ehrenfest, Zur Planckschen Strahlungstheorie, Physikalische Zeitschrift 7: 528-532 (1906): In paragraph 3 he writes Boltzmannschen Konstante k when referring to the average kinetic energy per molecule $\frac{3}{2}kT$. $\endgroup$ Jan 17 '18 at 18:11
  • $\begingroup$ Great — you may very well have found the original proponent of “Boltzmann’s constant”. Interestingly, Ehrenfest seems to have rarely repeated this, e.g. it’s not in his Boltzmann obituary 3 months later. Nor did anyone else (that I could find) until much later: Debye (1912), Born, von Laue (1913); experimentalists even slower. I added some — note especially who said what at the Wolfskehl & Solvay conferences (April & October 1913). I wonder when Sommerfeld switched, and whether his Atombau und Spektrallinien was the first textbook to do so. Can anyone find a copy of the first edition (1919)? $\endgroup$ Jan 20 '18 at 7:19
  • $\begingroup$ It is of interest to note that Mehra and Rechenberg (volume 5, page 98) refer to this paper of Ehrenfest as "the official point of view of Boltzmann's institute in 1906". Note also that it was submitted by Ehrenfest in July, before Boltzmann's death in September, so the name "Boltzmann's constant" was not coined in response to that tragic event. $\endgroup$ Jan 20 '18 at 12:18
  • $\begingroup$ Speaking of representing Vienna: Hasenöhrl did that at the Solvay meetings of both 1911.11 and 1913.10. One might hope that the campaign described in his booklet left possibly interesting traces in letters by some of the protagonists... but I don’t know where to look for it. (So far only found these by Debye, Ehrenfest.) $\endgroup$ Jan 27 '18 at 13:39
  • $\begingroup$ Am I right to question the translation of Einstein 1912.01? To me Einführung der Planckschen Konstanten sounds plural, i.e. he’s introducing h = 𝜀/𝜈 and 𝜘 = R/N and calling both Planck’s. (That surprised me, as I was already picturing him as likely behind his friend Kowalski’s switch between 1908 and 1909.) Then again, it seems it could also be singular? $\endgroup$ Jan 29 '18 at 23:10

Q: When did the name “Boltzmann constant” prevail, and how?

In Herzfeld's review article from 1920 for the Encyklopädie der Mathematischen Wissenschaften the symbol $k$ is called Boltzmann's constant without further discussion, so this must have been common usage at that time. This implies that there is no connection with Boltzmann's tombstone, which is from 1933 (as described here).

Tetrode in his 1912 article still refers to Planck's constant (and makes a point that his $\ln$ is Planck's $\log$), so the formula $S=k\log W$ was still a novelty.

I would conclude that the transition from Planck's constant to Boltzmann's constant happened in the 1912-1920 time frame.

Concerning the role of Lorentz in the naming of the constant, I note that in a 1917 paper entitled Some remarks on the theory of monatomic gases, Lorentz refers to $k$ as "Planck's well-known coefficient" (online, page 47). I did not find a later reference from Lorentz, however, in the Ph.D. thesis of his student Van de Sande Bakhuyzen, from 1921, the constant $k$ is called "Boltzmann's constant" (online, page 39).$^*$

So this is consistent with the idea that the name Boltzmann constant emerged within a few years before 1920.

One might ask what prompted the switch from Planck constant to Boltzmann constant. This screen shot from Van de Sande Bakhuyzen's Ph.D. thesis suggests the obvious answer: once quantum mechanics and statistical mechanics came together, a single formula would contain both $h$ and $k$ and different names were needed.

$^*$ I also searched for references to Boltzmann's constant in other Ph.D. theses of Lorentz's students (online here):

  • Fokker (1913): calls $k$ the single-molecule gas constant (page 20)
  • Bruins (1918): uses the symbol $s$ instead of $k$ and calls it the gas constant of a molecule (page 92)
  • van Slingelandt (1919): on page 39 writes $\eta =\frac{R}{N}\log W$ and calls it the "principle of Boltzmann", but without introducing a separate notation for $R/N$.
  • van de Sande Bakhuyzen (1921) finally calls $k$ the "constant of Boltzmann" (page 39).

So 1920 seems to be the "phase transition" year, at least for Lorentz's students.

  • $\begingroup$ Ha, I had no idea the tombstone formula was this late. There is another monument (1912) with remarks by his successor Hasenöhrl (1913), apparently not found online. From Leiden, can you say more on ter Haar’s claim that Lorentz kept calling both constants Planck’s? What I found (e.g. Solvay 1913, footnote p. 283) is not so clear cut, but his collected works may have more... $\endgroup$ Jan 11 '18 at 15:46
  • 2
    $\begingroup$ I found a reference to Planck's coefficient in a 1917 paper by Lorentz, and a reference to Boltzmann's constant in the 1923 Ph.D. thesis of his last student, so the switch must have happened in that time frame. $\endgroup$ Jan 11 '18 at 17:27
  • $\begingroup$ Ah, that Hasenöhrl booklet of 1913 is actually online (linked by German Wikipedia). He writes on p.15: “When the idea arose to build Boltzmann a monument in our university’s arcade courtyard, a distinguished expert proposed to decorate it with the formula $S=k.\log W$. That would go against our university’s customs and may be superfluous, as the discovery itself is a monumentum aere perennius.” Also reproduced is an early 1911 “call to build” signed by 100+ physicists including Einstein, Lorentz, Nernst, Planck, Rutherford, Sommerfeld, Zeeman .../... $\endgroup$ Jan 17 '18 at 3:38
  • $\begingroup$ ... at least some of whom must have read, and perhaps been (one way or the other) influenced by, the idea Hasenöhrl is floating here. This and your other references put me on track to more, that I will next try to post in some sort of chronology. Do you know what (if anything) Lorentz’s earlier students were calling $k$? $\endgroup$ Jan 17 '18 at 3:43
  • $\begingroup$ I listed the Tetrode paper as not naming the constant, because unless I missed it, he’s only crediting Planck for its numerical value. Also, van Slingelandt actually introduces k — pp. 5, 8, 13, as “the constant which determines the average kinetic energy of a molecule at temperature T, which energy equals 3/2 kT.” (So far as I can tell, he doesn’t call it a name, nor mentions that R/N of page 39 is the same thing.) $\endgroup$ Jan 25 '18 at 1:16

Community-wiki timeline (continued, 1917–):

After 1920, a vast majority of papers call $k$ Boltzmann’s constant. So (with few exceptions) mention is only made of sources calling it another name, plus some textbooks and reference books.

Author |Dated |Published |Denoted|Called Millikan 1917.01 1917 k S the entropy constant Schrödinger 1917 R/N R –– Gerlach 1917.08 1918 k S die Konstante der Entropie Flamm 1917.09 1917 k S die Entropiekonstante Born 1917.10 1917 k die Maxwell-Boltzmannsche Konstante Sommerfeld 1918 k S die Konstante k des Boltzmannschen Prinzips Wien 1918 k S die Entropiekonstante von Laue 1918.07 1918 k E –– Schrödinger 1918.07 1919 k R –– Bruins 1918.07 1918 s R de gasconstante van een molecuul von Laue 1918.08 1918 k der Boltzmannsche k Stern 1918.11 1919 k die Boltzmannsche Konstante Reiche 1918.12 1919 k die Boltzmannsche Konstante von Laue 1918.12 1919 k die Boltzmannsche Konstante Lorentz• 1919a k E 2/3 der translatie-energie per graad Lorentz• 1919b (e) k constante van Planck Nernst-Wulf 1919.04 1919 k R Gaskonstante pro Molekül Lindemann 1919.05 1919a k Boltzmann’s constant Lindemann 1919.07 1919b k Boltzmann’s constant Birge 1919.07 1919 k the Boltzmann entropy constant Sommerfeld• 1919.09 1919 ? ? Lindemann 1919.09 1920 k Boltzmann’s constant van Slingelandt 1919.12 1919 k E constante, die kinetische energie bepaalt Stern 1919.12 1920 k die Boltzmannsche Konstante Silberstein 1920.02 1920 k U constant in Planck’ formula Lyon-Wolfram 1920.03 1920 𝔎 die Boltzmannsche Konstante Stern 1920.04 1920 k die Boltzmannsche Konstante Ladenburg 1920.05 1920 k die Boltzmannsche Entropiekonstante Planck 1920.06 1920 (e) k U die erste Strahlungskonstante 9 Sommerfeld• 1920.09 1921b (e) k die Boltzmannsche Konstante Reiche• 1920.10 1921 (e) k die Boltzmannsche Konstante Schaefer• 1920.10 1921 k R –– Herzfeld• 1920.12 1920 k Boltzmannsche Konstante Haas• 1921.03 1921 k die Boltzmannsche Konstante Gerlach 1921 k die Plancksche Entropiekonstante V.d.S.Bakhuyzen 1921.12 1921 k constante van Boltzmann Gerlach-Stern 1922.04 1922 k E –– Hertz 1922 k die Boltzmann-Plancksche Konstante Eucken 1922 k die Boltzmannsche Konstante Henning 1922 k die Boltzmannsche Konstante Born• 1922.09 1923 k die Boltzmannsche Konstante Lorentz• 1923 (e) k constante van Planck Darwin• 1923 k E the atomic gas constant Coblentz• 1923 k Boltzmann gas constant Berliner & al• 1924.04 1924 k Boltzmannsche Konstante Einstein 1924.07 1924 (e) ϰ S –– Schrödinger 1924.11 1924 k Boltzmannkonstante Einstein 1925.01 1925a ϰ –– Einstein 1925.01 1925b ϰ –– Smekal• 1925.06 1926 k Boltzmann-Plancksche Konstante Smekal• 1925.06 1926 k Boltzmannsche Konstante Schrödinger 1925.12 1926 (e) k –– Geiger-Scheel• 1926 k Boltzmannsche Konstante Henning-Jaeger• 1926 k Boltzmannsche Konstante Bennewitz• 1926 k Boltzmannsche Konstante Smekal• 1926 k die sog. Boltzmann-Plancksche Konstante Byk• 1926 k die molekulare Gaskonstante Jäger• 1926 k Boltzmannsche Konstante Henning• 1926 k Boltzmannsche Konstante Henning• 1926 k Boltzmann-Plancksche Konstante Grüneisen• 1926 k Plancksche Strahlungskonstante van der Waals• 1926 k –– Schrödinger• 1926 k –– Simon• 1926 k Boltzmannsche Konstante Gerlach• 1926 R/N –– Pauli• 1926 k die Boltzmannsche Konstante Ladenburg• 1926 k U die universelle Entropiekonstante 9 Franck-Jordan• 1926 k Boltzmannsche Konstante Guillaume & al• 1926 k R molecular gas constant Tolman• 1926.08 1927 k R the Boltzmann constant Uhlenbeck 1927.07 1927 k constante van Boltzmann Planck• 1930.03 1930 (e) k S der universelle Faktor k 9 Berliner & al• 1931.11 1932 k Boltzmann-Plancksche Konstante (tombstone) 1933.07 1933 k S –– Planck 1943 k S meine Konstante k 9 van Dantzig 1939.06 1939 k Boltzmann's number Planck 1945.03 1948 (e) k S die sog. absolute Gaskonstante 9 Meissner 1951 k the Boltzmann-Planck constant 9 Sommerfeld• 1952 (e) k die Boltzmannsche Konstante 9,10

$=$ textbook or reference book. (e) $=$ English translation.

9 Calls the constant not Boltzmann’s.
10 Calls the formula not Boltzmann’s.

  • 1
    $\begingroup$ Split off from the other CW answer, as it was reaching the 30k character limit. $\endgroup$ Jan 29 '18 at 23:09

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