Kaufmann, Walter (or Walther)

(b. Elberfeld [Wuppertal], Germany, 5 June 1871; d. Freiburg, Germany, 1 January 1947)

physics.

Kaufmann studied at Berlin and Munich, receiving the doctorate at Munich in 1894. In 1896 he was assistant in the Physics Institute at Berlin; three years later he accepted a similar position at Göttingen, later being promoted to privatdozent. Kaufmann became associate professor at Bonn in 1903 and full professor and director of the Physics Institute of Königsberg in 1908; he retired in 1935 as professor emeritus. He then moved to Freiburg, where until his death he served occasionally as visiting professor.

While at Berlin in 1896–1898 Kaufmann began research on the magnetic deflection of cathode rays, attempting a first approximation of the ratio of electron charge to mass (e/m). His most accurate determination of this ratio was 1.865 X 10⁷ cgs/gm.

During this period a controversy arose over whether electrons, believed to be the ultimate constituents of matter, could have “apparent” mass in addition to “real” (material) mass. Apparent mass would be the “electromagnetic mass” gained from the interaction of the moving charge with its own field. Kaufmann’s major works were concerned primarily with attempts to measure and characterize this electromagnetic mass of electrons.

During the Göttingen years, 1899–1902, Kaufmann conducted research on the magnetic and electric deflection of radium emanations—then known as Becquerel rays. From the Curies he obtained several radioactive particles of radium chloride and set about measuring the e/m ratio . Since these newly discovered rays had velocities approaching the speed of light, it was assumed that the maximum possible electromagnetic charge was imparted to them . On the basis of his initial e/m measurements in 1901, Kaufmann asserted that the apparent mass was appreciably larger than the real mass—by an estimated magnitude of at least three to one. His successful measurements apparently were made possible by his experimental apparatus, which attained a more complete vacuum than other experimenters could produce in their vacuum tubes.

About the same time a fellow professor at Göttingen, Max Abraham, had formulated a theory of electrons assuming the electromagnetic mass as the total mass of rigid, spherical electrons. Kaufmann adopted this hypothesis. By 1902 Kaufmann produced experimental evidence that the mass of electrons was entirely electromagnetic, that is, that electromagnetic mass constituted the total mass of electrons. More importantly, in these same investigations he presented evidence that the mass of electrons was dependent on their velocity, noting that this dependence was accurately calculated by Abraham’s theoretical formula. Thus, a sacrosanct Newtonian principle —that mass was invariant with velocity—was contradicted by Kaufmann’s experimental data! By March 1903 Kaufmann confidently declared that not only the Becquerel rays but also the cathode rays consisted of electrons having a mass entirely electromagnetic.

By May 1904 H. A. Lorentz had developed a theory of electrons as being contractabgle with velocity and in the direction of motion. This view of electrons later became associated with Einstein’s theory of relativity. In the same year Alfred Bucherer advanced a view intermediate between Abraham’s theory and that of Lorentz. He believed electrons were elastic and could be deformed or contracted in the direction of motion but would maintain constant volume.

During his years at Bonn, Kaufmann undertook a new series of measurements in an attempt to corroborate one of the three rival theories. Upon completion of this work, and after requesting a thorough review by Sommerfeld, he published his results in 1906. He found that both Abraham’s and Bucherer’s theories were within the limits of experimental error for his measurements, but that the Lorentz-Einstein theory was not. He concluded that Lorentz’s theory was thus refuted and that Einstein’s theory of relativity was faulty in this respect.

Near the end of 1906 the significance of Kaufmann’s measurements was challenged by Max Planck. Developing his own mathematical calculations, Planck reached the tentative conclusion that neither Lorentz’s not Abraham’s theory conformed closely to Kaufmann’s data. He contended that a different interpretation of Kaufmann’s measurements might conceivably place Lorentz’s theory in a more favorable position. In 1907 Einstein reviewed Kaufmann’s data, nothing that these data could conform to relativity theory. He objected to the theoretically limited scope of Abraham’s theory—it could not explain as many phenomena as could the theory of relativity. By 1908 Bucherer published experimental data, of greater accuracy than Kaufmann’s measurements; these new data supported the Lorentz-Einstein viewpoint.

After 1906 Kaufmann apparently abandoned further investigations in this area. He progressed academically, performing other types of research until his retirement.

As early as 1901 Kaufmann reviewed the history of electron theory in his address “Die Entwicklug des Elekrtronenbegriffs,” delivered at the seventy-third Naturforscher Versammlung at Hamburg. He noted the fruitless efforts in the past to reduce electrical phenomena to mechanical phenomena and advocated reversing the process by attempting to reduce mechanics to electrical principles. Acknowledging the contributions of Lorentz, J. J. Thomson, and W. Wein in this direction, Kaufmann reasoned that if atoms consisted of conglomerates of electrons, then their inertia resulted as a matter of course. In this sense, at so early a date, Kaufmann may be considered a pioneer of twentieth-century physics. The significance of Kaufmann’;s experimental evidence that electron mass varied with velocity, coupled with his belief that mass could be expressed as essentially electromagnetic phenomena, has rarely been recognized. He outlined a major pathway along which research in twentieth century physics would be directed.

BIBLIOGRAPHY

I. Original Works. Kaufmann’s major works are “Über die Deflexion der Kathodenstrahlen,” in Annalender physik und Chemie, 62 (1897), 588–595, written with Emil Aschkinass; “Die magnetische Ablenkbarkeit electrostatisch beeinflusster Kathodenstrahlen,” lbid., 65 (1898), 431–439; “Grundzüge einer elektrodynamischen Theorie der Gasenladungen” (pts. 1 and 2), in Nachrichten von der Gesellschaftg der Wissenschaften zu Göttingen, Math-phys. K1., 1 (1899), 243–259; “Die magnetische und electrische Ablenbarkeit der Bequerelstrahlen und die scheinbare Masse der Elektronen,” ibid., 2 (1901), 143–155, translated into English as “Magnetic and Electric Deflectability of the Becquerel Rays and the Apparent Mass of the Electron” (editors’ translation), in The world of the Atom (edited by Henry A. Boorse and Lloyd Motsz), I (New York, 1966), 502–512; “Die Entwicklung des Elektronenbegriffs,” in physikalische Zeitschrift, 3 (1901), 9–15, translated into English as “The Development of the Electron Idea,” in Electrician (8 November 1901), 95–97;“Die elektromagnetische Masse des Elektrons,” in Physikalische Zeitschrift, 4 (1902), 54–57; Kaufmann’s letter (microfilm) to Arnold Sommerfeld (4 Nov. 1905), in the Archive for the History of Quantum Physics of the American Philosophical Society Library, Philadelphia; and “Über die Konstitution des Elektrons,” in Annalen der Physik, 19 (1906), 487–553.

II. Secondary Literature. See the following, listed chronologically: Max Abraham,“Prinzipien der Dynamikdes Elecktrons,” in Annalen der Physik, 10 (1903), 105–179; Max Planck, “Die Kaufmannschen Messungen der Ablenbarkeit der βStrahlen in ihrer Bedeutung für die Dynamik der Elecktronen,” in Physikalische Zeitschrift,7 (1906), 753–761; Max Planck, “Nachtrag zu der Besprechung der Kaufmannschen Ablenkungsmessungen,” in Verhandlungen der Deutschen Physikalischen Gesellschaft, 9 (1907), 301–305; Albert Einstein, Über das Relativitätsprinzip und die aus demselben gezogenen Folgerungen,” in Jahrbuch der Radioaktivitgät und Elektronik, 3 (1907), 411–439; and John T. Campbell, “Walter Kaufmann and the Electromagnetic Mass of Electrons.” unpub, research paper (The Johns Hopkins University, 1967). Obituarices are W. Kossel, “Walter Kaufmann,” in Naturwissenschaften, 34 (1947), 33–34; and (author unknown) “Walther Kaufmann, “in Physikalische Blätter, 3 (1947), 17.

John T. Campbell