RUBENS, HEINRICH (HENRI LEOPOLD)

(b. Wiesbaden, Germany, 30 March 1865; d. Berlin, Germany, 17 July 1922)

physics.

Rubens was the son of Barend Eliazer Rubens, a jeweler who had left Amsterdam to settle in Frankfurt am Main in 1859, and Bertha Kohn, who came from Speyer. On 17 March 1884 he matriculated from the Frankfurt Realgymnasium “Wöhlerschule,” then entered the Darmstadt Technische Hochschule to study the new subject of “electro-technics” for one semester. He next attended the Polytechnic Institute in Berlin-Charlottenburg for a year. Having realized that his interest and talent “were preferentially directed to the theoretical study of mathematics and natural sciences,”¹ Rubens entered the University of Berlin to study physics in the winter of 1885. He next attended the University of Strasbourg, where he studied chiefly with the physicist August Kundt and the electrical engineer Franz Stenger. After four terms Rubens followed Kundt to the University of Berlin, from which he received the Ph.D. on 4 March 1889.

In 1890 Rubens became an assistant at the Königliches Physikalisches Institut of the University of Berlin; while he was doing research there he received the venia legendi on 17 February 1892. On 1 October 1896 he was given the title “professor” at the Charlottenburg Technische Hochschule und appointed director of its physical laboratory. He was made full professor in 1900, and in 1904 he was named director of the Physikalische Sammlung. During this period he continued to carry out experimental work, chiefly at the Physikalisch-Technische Reichsanstalt. In 1906 Rubens succeeded Drude as professor of experimental physics at the University of Berlin and at the same time became director of its Königliches Physikalisches Institut. In 1908 he was elected to the Königlich Preussische Akademie der Wissenschaften zu Berlin.

Rubens’ work was considerably influenced by that of Kundt, whose investigation of the optical properties of metals stimulated him to do research on the reflection of light from metals as a function of rays of great wavelength. From this early direction— and from the technology then at hand (including the principle of Svanberg’s resistance thermometer)— Rubens’ lifework, the exploration of the far infrared region, was determined. He had, moreover, favorable circumstances in which to work, since the University of Berlin was at that time becoming a center for European physical research. At the same time, he continued his theoretical work, as evidenced in his lectures on the “Theorie der Elasticität fester Körper mit besonderer Berücksichtigung einiger Probleme der Akustik and Optik” (winter 1892/1893 and 1893/1894) and “Theorie der Bewegung von Flüssigkeiten” (summer 1894 and 1895). “Die experimentellen Grundlagen der Maxwell’schen Theorie,” a course that he announced for the winter terms of 1894 and 1895, grew out of his study of Maxwell’s theory of electromagnetism and out of his correspondence with Hertz about it. In the summer of 1893 Rubens also gave a lecture entitled “Ausgewählte Capitel aus der Geschichte der Physik,” one of several on the history of physics given at the University of Berlin.²

Rubens’ interest in electricity and electrically generated waves is apparent in his first researches. In his doctoral thesis he set forth the result of an experiment in which he employed Carl Baur’s tinfoil resistance thermometer to demonstrate that metallic reflection increases in the range from visible to infrared light. In collaboration with his former teacher Carl A. Paalzow, Rubens then repeated the experiment with a bolometer dynamometer that they had constructed to measure the intensities of electrical currents through the heat effect. Having confirmed the elastic theory of light, Rubens next turned to proving the existence of electromagnetic waves and, with R. Ritter, extended his earlier experiments by measuring bolometrically the polarization and reflection of electric waves. In 1890, in partial collaboration with M. Arons, Rubens measured bolometrically the velocity of the propagation of electrically produced waves in isolating liquids and solid bodies and thereby tested the validity of Maxwell’s law

Following a suggestion of Hertz, Rubens also used the bolometer to investigate standing electric waves in wires.

In 1892 Rubens extended the work of his former teacher Henri Du Bois, using both the bolometer and fine metal gratings to show the reversal of polarization of optical rays of sufficiently great wavelength—a first step toward establishing the electromagnetic theory in the far infrared region. By confirming and using such dispersion formulas as those of Eduard Ketteler (1887) and H. von Helmholtz (1893), Rubens was able by 1894 to measure wavelengths up to 8.95 microns, a value that Paschen had also just attained.

Rubens used a variety of substances in his refraction experiments, including rock salt and sylvite. With the American physicist E. F. Nichols,³ who collaborated with him at Berlin from 1894 until 1896, Rubens began a series of experiments based on his observation that the refractive index of quartz cannot be measured for wavelengths greater than 4.20 microns because of the absorption properties of this substance. Their investigations of reflection, transmission, and dispersion indicated that in this region quartz became opaque, that it “[passed] over completely from a nonmetallic to a metallic body.” Drawing upon this data—and upon the phenomenon of selective absorption observed by G. Magnus (1870), together with the absorption bands assumed by the Ketteler-Helmholtz dispersion equation—Rubens and Nichols pointed out that by reflecting waves several times in the bands of the “same substance as the source advantageously emits … unassisted by either prism or grating, homogeneous rays of great wavelength may be obtained.”

Using a sensitive bolometer of platinum, coated with an electrolytically deposited layer of platinum black, and the “Panzergalvanometer” (a galvanometer shielded against electromagnetic disturbances by an armor of cast steel, recently invented by Du Bois and Rubens), Rubens and Nichols then succeeded in detecting what Rubens in 1897 named Reststrahlen (or residual rays). The first substances that they tested were quartz, for which the value of the residual ray is about 0.0088 millimeter, and fluorite (about 0.0244 millimeter). In 1898 Rubens and Emil Aschkinass obtained the residual rays of rock salt (0.0512 millimeter), using Rubens’ extremely sensitive new thermopile of constantan and iron. In accordance with his close attachment to the idea of Hertzian waves, Rubens in 1896 produced a resonance of the fluorite rays on a grating of five-micron silver strips, finding it to be “in complete analogy” to the resonance of electric waves that Antonio Garbasso had demonstrated in 1893.

As early as 1898 Hermann Beckmann had, under Rubens’ supervision, measured the variations with temperature of the residual rays reflected by fluorite to determine the exponential constant in Wien’s law of energy distribution. He discovered that this constant was nearly twice as large as was consistent with the theory, a result that Rubens endeavored to save by a calculus of error.⁴ On 7 October 1900, however, Rubens mentioned to Planck that his results concerning residual rays of great wavelength were actually consistent with a law that Lord Rayleigh had suggested in June of that year; inspired by this, Planck discovered his own radiation law on the same day. In their later work, neither Rubens nor his collaborator Kurlbaum ceased to stress the approximate character of Rayleigh’s early law as well as the limitations of other formulas (including Planck’s) on the distribution of radiation energy.

Rubens subsequently returned to the problem of relating properties of infrared waves to the electromagnetic theory of light. With Ernst Hagen, of the Physikalisch-Technische Reichsanstalt, he used different metals to determine experimentally the absorption, reflection, and emission of waves up to 25.5 microns. In 1903 they discovered that the coefficient of penetration of such waves, 100-R, is inversely proportional to the root of the electrical conductance k, a relationship for which Rubens’ friend Planck immediately provided a theoretical basis. In 1910 Rubens and Hagen determined the region in which, according to Maxwell’s theory, the heat radiation of metals perceptibly changes to a function of temperature to be between two and five microns. This result constituted the first confirmation of the validity of the electromagnetic theory as applied to the infrared.

From 1910 on, Rubens and his collaborators were concerned with bridging the gap between “optical” and “electric” waves. They found residual rays of greater length by using potassium bromide (88.3 microns) or, in 1914, thallium iodide (151.8 microns). Working from another direction, Rubens and Otto von Baeyer attempted in 1910 to close the gap by using small “electrical” waves of about two millimeters; their results were not unequivocal because of the inconstancy of the waves produced by the very small vibrators that they employed. The earlier method of isolating waves by virtue of the higher refractive index of quartz (first used in 1898) proved to be more successful; in 1910 Rubens and R. W. Wood obtained from quartz waves of 108 microns, measured interferometrically by means of the radiomicrometer. After having replaced the older zirconium-oxide burner by the Welsbach mantle (1905), which radiates selectively, Rubens and Baeyer in 1911 discovered the waves of 210 microns and 324 microns emitted by the quartz mercury lamp, a device that is still the only source of optical waves of great length.

The far-infrared rays that Rubens had obtained allowed him and Du Bois to prove conclusively the inversion of polarization of optical rays of great wavelength. Moreover, in 1914 Rubens and Schwarzschild demonstrated that solar rays of sufficiently great wavelength are absorbed by the atmosphere of the earth. Rubens’ discoveries found further application in the atomic theories of Erwin Madelung and Frederick A. Lindemann in 1910, and, most importantly, in Niels Bjerrum’s 1912 theory of a rotationalvibrational spectrum, a theory that was in turn applied in later work by Rubens and Georg Hettner. (Bohr’s theory of equating the differences between atomic energies with the light quanta—rather than using the energies themselves, as Bjerrum did—was not then utilized by workers concerned with the far infrared.) In 1916 Planck saved the classical electrodynamics of Rubens and Hettner’s “Bjerrum-quanta,” while Einstein and Nernst had previously used Rubens’ values to explain the temperature variation of specific heats. But in 1921 Planck’s quantum hypothesis was confirmed by Rubens and by Gerhardt Michel, in response to questions raised by Nernst and by T. Wulf, who had in 1919 recognized certain differences between experimental data and the theoretical values embodied in the hypothesis. In these and other experiments Rubens proved his skill in the investigation of the far infrared and its applications.⁵

Rubens was also engaged in other areas of physics, including wireless telegraphy, radioactivity, acoustic waves, photoelectric effect, and the concept of potential. That lie was open-minded toward the development of physical theories may be seen in the historical introductions to some of his papers.

NOTES

1. According to his own handwritten curriculum vitae, dated 19 Feb. 1892.

2. See H. Kangro, Vorgeschichte des Planckschen Strahlungsgesetzes (Wiesbaden, 1970), 126–127.

3. Rubens collaborated with Americans several times, for example, with Benjamin W. Snow of Henry, Illinois, and Augustus Trowbridge of New York.

4. See H. Kangro (1970), 176.

5. Rubens’ monograph on this subject, although under contract with a publisher, was never printed.

BIBLIOGRAPHY

I. Original Works. A nearly complete list of the papers that Rubens published in journals, compiled by G. Hettner, is in Naturwissenschaften, 10 (1922), 1038–1040; this may be supplemented by Poggendorff, IV and V. A bibliography of Rubens’ papers on infrared radiation, compiled by E. D. Palik, is in K. D. Möller and W. G. Rothschild, Far-Infrared Spectroscopy (New York–London–Toronto, 1971), 680-689; the bibliography is nearly complete for this aspect of Rubens’ work and lists reprints and translations of the original articles in addition to those in the above bibliographies, but it contains a number of errors (note that paper no. 9 of Du Bois and Rubens is correctly entitled “Einige neuere Galvanometerformen,” in Elektrotechnische Zeitschrift, 15 [1894], 321–323; that no. 10 does not exist; that no. 50, “Recherches sur le spectre infrarouge …,” in Revue générale des sciences pures et appliquées, 11 [1900], 7–13, is signed only by Rubens; and that no. 76, “Spectre d’émission des machons Auer,” in Radium, 2 [1905], 397, is only an announcement of four lines by Léon Bloch).

Among the works of Rubens upon which this article is based, the most important are a diss. (2 March 1889), Die selective Reflexion der Metalle (Berlin, n.d. [1889]), which includes a “Vita” in Latin; “Über die Fortpflanzungsgeschwindigkeit electrischer Wellen in isolirenden Flüssigkeiten,” in Annalen der Physik, 278 (1891), 581–592, written with L. Arons, which contains a new approach to Maxwell’s relation; “Einige neuere Galvanometerformen,” in Elektrotechnische Zeitschrift, 15 (1894), written with H. Du Bois, and containing an account of important new experimental devices; and “Über eine neue Thermosäule,” in Zeitschrift für Instrumentenkunde, 18 (1898), 65–69.

Rubens’ first paper on residual rays is “Über Wärmestrahlen von grosser Wellenlänge,” in Naturwissenschaftliche Rundschau, 11 (1896), 545–549, written with E. F. Nichols. On the irregularities caused by selective molecular absorption, see “Beobachtungen über Absorption and Emission von Wasserdampf and Kohlensäure im ultraroten Spectrum,” in Annalen der Physik, 300 (1898), 602–605, written with E. Aschkinass. “Isolirung langwelliger Wärmestrahlen durch Quarzprismen,” ibid., 303 (1899), 459–466, also written with Aschkinass, is concerned with chromatic aberration. “Über die Emission langwelliger Wärmestrahlen durch den schwarzen Körper bei verschiedenen Temperaturen,” in Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften (1900), 929–941, written with F. Kurlbaum, sets out the decisive measurements that led to Planck’s new radiation formula. “Über Beziehungen zwischen dem Reflexionsvermögen der Metalle and ihrem elektrischen Leitvermögen,” ibid. (1903), 269–277. written with E. Hagen, gives proofs of the relationships of the properties of metals to the electromagnetic theory of light; see also “Änderung des Emissionsvermögens der Metalle mit der Temperatur,” in Verhandlungen der Deutschen Physikalischen Gesellschaft, 10 (1908), 710–712, written with Hagen; and “Über die Änderung des Emissionsvermögens der Metalle mit der Temperatur im kurzwelligen Teil des Ultrarots,” ibid., 12 (1910), 929–941, written with Hagen.

Rubens’ plan for a scientific life was presented in his inaugural lecture before the Berlin Academy and is in Sitzungsberichte der Preussischen Akademie der Wissenschaften zu Berlin (1908), 714–717. His remarks on the history of the Royal Physical Institute, founded in Berlin in 1862 by G. Magnus, are in Geschichte der Königlichen Friedrich-Wilhelm-Universität zu Berlin, III (Halle, 1910), 278–296.

Final proof of the polarization of rays in the far infrared, according to Maxwell’s theory, is in “Polarisation langwelliger Wärmestrahlung durch Hertzsche Drahtgitter,” in Verhandlungen der Deutschen Physikalischen Gesellschaft, 13 (1911), 431–444, written with H. Du Bois. Rubens’ first application of Bjerrum’s quanta is in “Das langwellige Wasserdampfspektrum and seine Deutung durch die Quantentheorie,” in Sitzungsberichte der Preussischen Akademie der Wissenschaften zu Berlin (1916), 167–183, written with G. Hettner; a document on contemporary knowledge of atomism, published shortly before Bohr’s work, is Die Entwicklung der Atomistik (Berlin, 1913). An excellent survey of heat radiation is “Wärmestrahlung,” in P. Hinneberg, ed., Die Kultur der Gegenwart, ihre Entwicklung and ihre Ziele, 1 , pt. 3, sec. 3 (Leipzig–Berlin, 1915), 187–208. Rubens’ only historical survey of his own work in infrared spectroscopy is “Das ultrarote Spektrum and seine Bedeutung für die Bestätigung der elektromagnetischen Lichttheorie,” in Sitzungsberichte der Preussischen Akademie der Wissenschaften zu Berlin (1917), 47–63. For a final proof of Planck’s radiation theory against justified criticism of systematical experimental deviations, see “Beitrag zur Prüfung der Planckschen Strahlungsformel,” ibid. (1917), 590–610, written with G. Michel.

MSS (including letters) are listed in the T. S. Kuhn, J. L. Heilbron, P. Forman, L. Allen, eds., Sources for History of Quantum Physics (Philadelphia, 1967), 80, which contains some errors, and to which the following items may be added: in the Staatsbibliothek Preussischer Kulturbesitz, Berlin, Germany (not in Marburg) are two curricula vitae, dated 20 June 1890 and 19 Feb. 1892; a visiting card, dated 22 Nov. 1903; three letters to an unknown person, dated 2 June 1903, 17 June 1903, and 24 June 1903; a letter to Max Iklé, dated 31 Aug. 1908; a postal card to Hans Geitel, dated 8 Nov. 1907; a letter to Wilhelm Waldeyer, dated 17 July 1916; a letter to Eilhard Wiedemann, dated 25 Nov. 1916; a visiting card to Karl Flügge, dated 9 Dec. 1917; and a card to L. Darmstaedter, dated 27 Oct. 1909. In the Nachlass of Felix von Luschan are two postcards, dated 6 Apr. 1918 and 1 Mar. 1919, and one letter, dated 19 Dec. 1917, all directed to Felix von Luschan. In the Nachlass of Johannes Stark are 10 letters to J. Stark, dated 15 Apr. (without year), 17 Nov. 1908, 4 Feb. 1909, 15 Oct. 1909, 3 Nov. 1909, 18 Jan. 1910 (xerographic copy), 16 May 1911, 13 Nov. 1912, 17 Feb. 1917 (xerographic copy), and 29 Mar. 1917; two letter-cards to J. Stark, dated 27 Nov. 1909 and 3 Nov. 1913; five postcards to J. Stark, dated 15 Feb. 1913, 18 Apr. 1913, 29 May 1913, 15 July 1913, and 14 Nov. 1913; two visiting cards to J. Stark, dated 15 May 1907 and 8 Dec. 1911; the copies of two letters of J. Stark, probably to Rubens, one dated 14 Nov. 1909, the other n.d. (not legible); one protocol of a discussion between Stark, Rubens, and Einstein (n.p., n.d.); and one letter of Marie Rubens to Stark, dated 12 Nov. 1909. The library of Erlangen mentioned in the Sources for History of Quantum Physics (1967) is in Germany (BRD); the 4 letters to H. Hertz in the library of the Deutsches Museum, Munich, are dated 3 June 1890, 13 Nov. 1890, 29 Oct. 1910, and 9 Feb. (without year); the letter to Leo Graetz is dated 21 July 1904.

A letter of Rubens to Arnold Sommerfeld, dated 18 June 1917, is also in the library of the American Philosophical Society, Philadelphia; and at the publishers F. Vieweg and Son, Braunschweig, Germany, are two letters of Rubens to the publishers, dated 28 June 1904 and 15 May 1906.

II. Secondary Literature. A memorial collection, “Dem Andenken an Heinrich Rubens,” is in Naturwissenschaften, 10 (1922), 1015–1040. It includes articles by W. Westphal (on Rubens’ personality), E. Regener, G. Hertz, and O. von Baeyer (on two-millimeter electrical waves and the radiation of the mercury lamp), J. Franck and R. Pohl, and G. Hettner (on the history of Rubens’ role in the immediate origin of Planck’s radiation law), in addition to a bibliography of Rubens’ papers in journals, compiled by Hettner.

A historical sketch of Rubens’ scientific achievements is J. Franck, in Verhandlungen der Deutschen physikalischen Gesellschaft, 3rd ser., 3 (1922), 76–91. See also obituaries by J. Franck and R. Pohl, in Physikalische Zeitschrift, 23 (1922), 377–382; and “R. W. L.,” and Joseph Larmor, in Nature, 110 (1922), 740–742. M. Planck, “Gedächtnisrede,” in Sitzungsberichte der Preussischen Akademie der Wissenschaften zu Berlin, Phil.-hist. Klasse (1923), cviii–cxiii, emphasizes Rubens’ work for the Academy. M. von Laue, “Heinrich Rubens,” in Deutsches Biographisches Jahrbuch, IV (1929), 228–230, is somewhat superficial and contains some errors.

On Rubens’ early research and the relationship between his residual-ray experiments and Planck’s quantum physics, see H. Kangro, Vorgeschichte des Planckschen Strahlungsgesetzes, Messungen und Theorien der spektralen Energieverteilung bis zur Begründung der Quantenhypothese (Wiesbaden, 1970; English ed. in press), 49–60, 126–128, 160–164, 173–175, 200–208; see also H. Kangro, “Ultrarotstrahlung bis zur Grenze elektrisch erzeugter Wellen, das Lebenswerk von Heinrich Rubens, I. Experimenteller Beweis der elektromagnetischen Lichttheorie fÜr das Ultrarot,” in Annals of Science, 26 (1970), 235–259, and “II. Experimente zur überbrückung der Spektrumslücke zwischen optischen and elektrischen Wellen, Verknüpfung mit der Quantentheorie,” ibid., 27 (1971), 165–170, which together give an account of Rubens’ life and of his work on infrared radiation from about 1900.

Two portraits of Rubens are that in Naturwissenschaften, 10 (1922), facing page 1015, and that in Physikalische Zeitschrift, 23 (1922), 377.

Hans Kangro