WARBURG, EMIL GABRIEL

(b. Altona, near Hamburg, Germany, 9 March 1846; d, Grunau, near Bayreuth, Germany, 28 July 1931), physics.

Warburg, who came from a wealthy family, grew up in Altona and attended the city’s humanistic gymnasium, the Christianeum, where he was almost as interested in languages as in mathematics. His musical education was not neglected, and he became a good pianist.

In 1863, aged seventeen, Warburg began to study science at the University of Heidelberg, which—through the presence of Kirchhoff and Bunsen on its faculty—offered outstanding instruction in physics and chemistry. Warburg was so impressed by Kirchhoff’s “magnificent” (vollendet schöner) lecture on experimental physics that he decided to change his major from chemistry to physics.

After four semesters at Heidelberg, Warburg transferred to the University of Berlin. He earned his doctorate and qualified as lecturer there, remaining until he received an offer of a professorship. During this period Gustav Magnus attracted many young physicists to his laboratory in Berlin, the only one in Germany besides Franz Neumann’s at Königsberg. Warburg soon became friendly with Magnus’ assistant, August Kundt; and they remained friends even after Kundt left in 1868 to take up a professorship at the Zurich Polytechnikum.

While at Berlin, Warburg wrote a number of works, most of them on oscillatory problems, including his Latin dissertation. “De systematis corporum vibrantium” (1867). In his Habilitationsschrift, “Über den Ausfluss des Quecksilbers aus gläsernen Capillarröhren” (1870), Warburg reported his discovery that no slipping occurs between glass and mercury. He often returned to problems of slipping. Warburg remained a Privatdozent for only two years: in 1872 he and Kundt were invited to the newly founded Kaiser Wilhelm University at Strasbourg. Kundt, who brought his assistant Wilhelm Roentgen with him from Würzburg, was named full professor and Warburg was made extraordinary professor.

At Strasbourg, Warburg and Kundt collaborated on two famous studies on the kinetic theory of gases. In 1875 they furnished conclusive experimental confirmation of a consequence that Max-well had derived from the theory: that the inner friction and the heat conduction of a gas are independent of the pressure, so long as the mean paths of the molecules are negligible with respect to the dimensions of the container. They extended their investigation to very rarefied gases and deduced from the theory the existence of a measurable slipping and of a jump in temperature at the container wall. They also demonstrated the existence of measurable slipping experimentally. Their second prediction, however, was not verified until around the turn of the century, in an experiment carried out at Warburg’s suggestion at Berlin by Marian Smoluchowski and Ernst Gehrcke. In their second joint study (1876) Kundt and Warburg showed that at constant pressure and volume, the specific heats of monatomic gases possess the value 5/3 predicted by the theory.

The explanation of the theoretical relations in these two papers was the work of Warburg, as is evident from a letter mentioned by James Franck. In it Kundt asks Warburg for information on a theoretical point and writes that, since Warburg has developed all the ideas about slipping and has calculated the heat conduction, he ought to help the “thoroughly ordinary experimental physicist” (ganz gemeinen Experimentalphysiker) out of a theoretical difficulty. Einstein considered Warburg and Kundt’s joint papers of very great significance for the kinetic gas theory. He wrote in 1922:

This was the first time that a new phenomenon was predicted on the basis of the molecular theory of heat–a phenomenon, moreover, the representation of which on the basis of the theory of continuity of matter was virtually excluded. If the energeticists at the end of the nineteenth century had sufficiently appreciated these arguments, they would have had great difficulty in calling into question the profound validity of the molecular theory.

The collaboration with Kundt ended in 1876, when Warburg obtained a professorship at the University of Freiburg im Breisgau, where he was the sole physicist on the faculty until 1895. At Freiburg he continued his investigation of the kinetic gas theory. It followed from the theory that the friction coefficient is independent of the pressure. He tested this prediction with carbonic acid at high densities and found that the basic notions of the theory were valid. Never losing his interest in this topic, he encouraged his students to work on it and published two comprehensive accounts of it himself. The first, Über die kinetische Theorie der Gase (1901), shows that Warburg had mastered the art of good scientific popularization. The second, Über Wärmeleitung und andere ausgleichende Vorgänge (1924), is essentially a summary of half a century of research on the subject.

Warburg also undertook research at Freiburg on many other topics. His investigation of elastic aftereffects led him in 1881 to one of his most beautiful results: the experimental discovery and theoretical interpretation of hysteresis in the cyclical magnetization of ferromagnetic materials. Warburg also devoted years of study to electrical conduction in solids, liquids, and gases; and his efforts yielded many discoveries. For example, he ascertained that conductivity in quartz is 100 times greater in the direction of the axis than in the direction perpendicular to it. Another interesting discovery was the electrolytic migration of magnesium and lithium ions through glass. The drifting of electrolytic impurities toward the electrodes acquired significance for electric purification. Warburg’s discovery of the cathode fall enabled him to gain important insights in his study of gas discharges. He recognized the significance of the cathode fall for breakdown voltage and measured this characteristic quantity for many gases.

Warburg’s works on gas discharges quickly attracted the attention of other scientists. In his unpublished autobiography Philipp Lenard recounts that Heinrich Hertz considered Warburg a leading expert on electric discharges in rarefied gases. Hertz’s opinion, which reached the influential Friedrich Theodor Althoff in the Ministry of Education, through Lenard, undoubtedly contributed to Warburg’s being invited to Berlin.

Two events remain to be mentioned from Warburg’s period at Freiburg: the dedication of the new physics institute in 1891 and the publication of his Lehrbuch der Experimentalphysik (1893). This textbook, precise and tersely written, was not easy to read but nevertheless had great success. At age eighty-three, Warburg prepared the twenty-first and twenty-second editions.

In 1895 Warburg succeeded Kundt as professor of experimental physics at the University of Berlin. He thereby obtained the “most eminent chair of physics in Germany” and became a very close associate of Max Planck. He continued to work on his research projects, enlisting the aid of many of his students. While pursuing studies on gas discharges, from 1897 he undertook others on spark discharges and point discharges and on the resulting ozone formation. According to James Franck, Warburg’s research on point discharges constituted the basis for the experiments that J. Franck and Gustav Ludwig Hertz conducted on electron collisions.

Warburg’s ten years as director of the Berlin physics institute were the most brilliant of his teaching career. The many students who came there constituted what was called the “Warburg school” of experimental physics. The intensive program of research that he and his students conducted is reflected in the 220 publications that originated in the institute during his tenure. Moreover, Franck calculated that around 1930 approximately one-fifth of the professors of experimental physics at German universities and colleges had studied under Warburg. Among the latter was his son Otto Heinrich (1883-1970), who became director of the Kaiser Wilhelm (now Max Planck) Institute for Cell Physiology in 1930 and received the Nobel Prize for physiology or medicine in 1931.

Warburg’s teaching activities included a weekly colloquium for professors and students held in the institute’s library. Friedrich Kohlrausch, then president of the Physikalisch-Technische Reichsanstalt, was an active participant. Besides his teaching, Warburg rendered important service to physics through his efforts within professional scientific organizations. He was elected chairman of the Berlin Physical Society in 1897; and in 1899 he led this body into the German Physical Society, heading the latter as well until 1905.

In 1905 Warburg left the University of Berlin to succeed Kohlrausch at the Reichsanstalt. Under Warburg’s direction the organization of the institute was streamlined and duplication of effort was eliminated. At the same time, however, several new institutes were created within it, including the radioactivity laboratory (1912), in which Hans Geiger developed his Spitzenzähler (or point counter). In addition funds were allotted for visiting researchers, who included Einstein and de Haas when they discovered the gyromagnetic effect named for them (1914-1915).

The pace of Warburg’s research did not diminish with his move to the Reichsanstalt. He pursued his investigation of point discharges and concurrently (from 1906) undertook photochemical studies that occupied him until shortly before his death. He was one of the founders of quantitative photochemistry and confirmed the fundamental law of the quantum nature of light absorption formulated by Einstein. Further, assisted by several co-workers, Warburg devoted himself to a task especially suited to the facilities at the Reichsanstalt: making precise measurements designed to test Planck’s radiation law.

Following his retirement in 1922, Warburg continued his photochemical studies as an independent researcher. Although more than eighty at this time, he wrote three articles: on silent discharge in gases, spark discharge, and photochemistry. He died a few months after his eighty-fifth birthday.

Until well into old age, Warburg followed advances in physics with great attention and impartiality. In 1913, shortly after the discovery of the Stark effect, he was the first to examine its relationship to the equally new Bohr theory. The experiment he devised, although premature and therefore a failure, reflects his openness and quickness of mind.

Of Warburg’s approximately 150 publications, only one is of a polemical nature; even then he did not begin the dispute, and it did not concern any scientific matter. All his other writings display a sober objectivity and critical detachment from his own results.

Unlike his contemporary Wilhelm Roentgen, for example, Warburg never achieved the brilliant success that makes a scientist known far beyond the circle of his colleagues. Nevertheless, he produced a wealth of important results that are now part of basic physical knowledge; and he was able to teach many students the procedures of intensive scientific research.

BIBLIOGRAPHY

I. Original Works. Warburg’s writings include Lehrbuch der Experimementalphysik (Tübingen, 1893; 22nd ed., 1929); Über die kinetische Theorie der Gase (Berlin, 1901); Helmholtz als Physiker (Karlsruhe, 1922); “Funkenentladung,” in H. Geiger and K. Scheel, eds., Handbuch der physik, XIV (Berlin, 1927), 354-390; “Über die stille Entladung bei Gasen,” ibid., 149-170; and “Photochemie,” ibid., XVIII (1928), 619-657. A list of his works can be compiled from Poggendorff, III, 1415-1416; IV 1598; V, 1334-1335; and VI, 2806.

II. Secondary Lliterature. See Albert Einstein, “Emil Warburg als Forscher,” in Naturwissenschaften, 10 (1922), 823-828, with a list of publications to 1921; James Franck, “Emil Warburg zum Gedächtnis,” ibid., 19 (1931), 993-997; Philipp Lenard, “Autobiographie” (unpublished); H. Moser, ed., Forschung und Prüfung. 75 Jahre Physikalisch-technische Bundesanstalt/Reichsanstalt (Brunswick, 1962), esp. 8-18; C. Müller, “Emil Warburg 80 Jahre,” in Elektrotechnische Zeitschrift, 47 (1926), 317; J. Stark, ed., Forschung und Prüfung, 50 Jahre Physikalisch-technische Bundesanstalt/Reichstanstalt (Leipzig, 1937), esp. 16-19, 60-63; and Eduard Zentgraf, ed., Aus der Geschichte der Naturwissenschaften an der Universität Frieburg im Breisgau (Frieburg im Breisgau, 1957), esp. 18-20.

Hans Ramser