(b. Vienna, Austria, 24 April 1879; d. Vienna, 4 March 1952)
Ehrenhaft was raised in comfortable, cultured surroundings; his father, Leopold Ehrenhaft, was a physician; his mother, Louise Egger, the daughter of an industrialist in Hungary. Ehrenhaft studied at the Technische Hochschule in Vienna and, after earning his doctorate in 1903 at the University of Vienna, became an assistant of Viktor von Lang and, in 1905, Privatdozent. Ehrenhaft was first married, to Olga Steindler, around 1912. They had two children, Johann and Anna Marie. She died in 1933. He was married again in 1935 to Bettina Stein; she died late in 1939. There were no children from this marriage. His early published papers (1902, 1903) dealt with the preparation of metallic colloids and their optical properties, including the anomalous direction of polarization maxima for reflected light. His lifework was launched in these early studies.
The colloidal state, in which particles of dispersed matter are on the order of 10-4 to 10-7 cm, was then an exciting research frontier for theory as well as for application, not least because of its bearing on the burning question of the “reality” of atoms, With the invention (1902–1903) of the slit ultramicroscope by Henry Siedentopf and Richard Zsigmondy, this “world of neglected dimensions” (so called by Wolfgang Ostwald), long thought to be forever beyond sense perception, became accessible to direct observation. Soon after Albert Einstein’s (1905) and Marian Smoluchowski’s (1906) papers on the kinetic molecular theory interpretation of Brownian movement, Ehrenhaft published (1907) the results of a simple but imaginative experiment for which he was awarded the Lieben Prize (1901) of the Vienna Academy of Sciences. It was a semiquantitative measurement, on ultramicroscopic particles of silver and other substances, of the Brownian motion in air. As Ehrenhaft stressed, this test on gases provided a much more plausible support for atomism itself than had the earlier results on Brownian motion for liquids, including those of Franz Exner (1900) at Vienna, because the mean free path is larger in gases.
It was a natural step for Ehrenhaft, using essentially the same apparatus— soon aftyer a similar attempt by Maurice de Broglie (1908) and at about the same time as Robert A. Millikan— to measure the electric charges such individua(ticles can carry, and hence to calculate the charge of the electron, e (published in March 1909). An accurate value of e, of highest importance for all branches of physics, was widely desired. For Ehrenhaft, joining this search was a decisive point in his scientific and personal life. He had entered into the labyrinth of enormously complex ultramicroscopic phenomena, which fascinated him to the end of his career, and he had also stumbled into a scientific controversy with Millikan and others that was to last for decades.
In his work of 1909, Ehrenhaft followed the motion of individual charged ultramicroscopic metal particles in a horizontal electric field. He obtained the value e=4.6 × 10–10 ESU— far closer to Rutherford’s 4.65 × 10–10 (from radioactivity work) and Planck’s 4.65 × 1010 (from blackbody radiation), than to the value (mean, 4.03 × 10–10) reported in Millikan’s first attempt (1908). On the occasion of the British Association for the Advancement of Science meeting of August 1909, Rutherford referred prominently to Ehrenhaft’s experiments and noted that his value of e was one of the recent measurements, “which are far more reliable than the older estimates.” But in his next paper (1910), Millikan, using balanced drops of water and alcohol, dismissed Ehrenhaft’s results for e because they had been obtained by a method Millikan regarded as inferior, although Ehrenhaft’s results were numerically very close to his own. Ehrenhaft responded to the challenge with ferocious energy.
Ehrenhaft subjected Millikan’s 1910 treatment of data to a scathing critique and began a series of ever more intricate experimental attacks on the notion that the electric charge on bodies is invariably a multiple of a definite charge e. He claimed the discovery of the “subelectron,” a concept completely at variance with all current theories of electronic and atomic phenomena, and indeed a repudiation of his own earlier atomism.
Opponents would argue in vain that in his data reduction Ehrenhaft refused to use Stokes’s law in the modified form necessary for small particles; that he falsely assumed the density of the small, spongelike metal fragments, obtained in an electric arc, to be the same as that of the mother material in the electrode; or that using small, jagged particles rather than round drops would cause leakage effects. Ehrenhaft, however, charged that his adversaries were building the existence of their sought-for indivisible electrons into the theory they were using to calculate the charge from their data; that they were using relatively large droplets on which “subelectrons” tended to be clumped together; and that only he proceeded “from the direct facts,” avoiding hypotheses and relying as much as possible on the direct observation of natural phenomena.
In fact, experiments of the Millikan and Ehrenhaft type are difficult. Although in time virtually all researchers outside Ehrenhaft’s circle came to corroborate Millikan’s view of the atomic nature of the electric charge, there usually had to be some hypothesis-guided selection among the raw data. If Ehrenhaft had had access to Millikan’s laboratory notebooks, he would have had no difficulty “proving” his case by concentrating on the runs that Millikan had omitted from his final calculations as flawed. Conversely, Ehrenhaft’s greatest methodological flaw may have been that he accepted all observations, whether good or bad, having come to embrace a sensationist view of science that owed some allegiance to Ernst Mach’s Philosophy.
But at the time, the “quarrel about the electron” was engaging large and often distinguished audiences. This was furthered in Europe by Ehrenhaft’s energetic defense, in papers and at scientific meetings, coupled with ever-new experiments issuing from his institute. As a consequence of Ehrenhaft’s work, a cloud hung over Millikan’s claim for years. In the 1916 edition of The Theory of the Electron, Hendrik A. Lorentz concluded that “the question cannot be said to be wholly elucidated,” and material in the Nobel Prize archives in Sweden show that as late as 1920 Svante Arrhenius noted that while most physicists agreed with Millikan, the dispute with Ehrenhaft was not regarded as resolved, and that Millikan therefore should not be recommended for a Nobel Prize. On 19 November 1940, Albert Einstein wrote to William F. G. Swann: “Concerning his [Ehrenhaft’s] results about the elementary charge I do not believe in his numerical results, but I believe that nobody has a clear idea about the causes producing the apparent sub-electronic charges he found in careful investigations.” Like most such controversies, it never came to a definite falsification of Ehrenhaft’s point of view by some crucial experiment; rather, the debate faded into obscurity, although Ehrenhaft continued to publish on “subelectrons” into the 1940’s.
During the first decade of his publications, Ehrenhaft’s credibility as an experimenter was furthered in 1918 by his demonstration of an effect he called photophoresis (the effect of light on the motion of aerosol particles that both absorb and scatter light). Later he continued work on the interaction of ultramicroscopic particles and light, describing what he called transverse photophoresis, magnetophotophoresis, and the rotation of particles in low-pressure gases in light beams. Some of these effects have since been explained in terms of known effects (such as radiometric forces), while others are still not fully understood. Beginning in the mid 1930’s. Ehrenhaft claimed to find experimental evidence for the existence of magnetic monopoles, magnetic currents, and the decomposition of liquids by permanent magnets (magnetolysis). Like most of his proposals after about 1910, they combined the observations of surprising behavior of small particles near the limits of perception, his belief in the validity of direct observation in a regime where many effects interact, and his willingness to go far beyond known theories to explain his observations. While the existence of the raw phenomena was rarely challenged successfully, his interpretations of them became progressively more estranged from the main body of scientific understanding.
In 1912 Ehrenhaft had become associate professor, and in 1920 he was named professor of experimental physics and director of the Third Physical Institute (established for him at the University of Vienna). Those who knew him well regarded Ehrenhaft as deeply devoted to his scientific work, an effective lecturer to large classes, and a stubborn and often difficult fighter for his interpretations. Albert Einstein was one of the many scientists who liked to stay with Ehrenhaft when visiting Vienna. After Ehrenhaft’s forced emigration, first to England and then to the United States (1940), following the takeover of Austria by Austrian and German Nazis in 1938, he and Einstein kept in correspondence until the interchange was terminated in unresolvable disagreement over Ehrenhaft’s more and more extreme scientific theories.
While in the United States, Ehrenhaft found it very difficult to obtain research support. Thus, in 1946, as a U.S. citizen, he returned to the University of Vienna as U.S. Guest Professor and director of the combined First and Third Physical Institutes, holding these offices until his death.
1. Original Works. A full bibliography of Ehrenhaft’s publications is in Poggendorff, V VI, and VIIa. Another somewhat less complete one, was published with a brief apparently “authorized” biography by Lotte Bittner in her dissertation for the Philosophical Faculty of the University of Vienna, entitled “Geschichte des Studienfaches Physik an der Wiener Universität in den letzten hundert Jahren” (1949). Most of Ehrenhaft’s articles on the charge of electrons and subelectrons are listed in Holton (see below, 302–323).
The Center for the History of Physics of the American Institute of Physics in New York City has in its archives approximately three feet of original materials, mostly deposited by Ehrenhaft’s son, John L. Ehrenhaft (who still holds some of the more personal letters); these include handwritten autobiographical notes: the MS of an unpublished book, “Magnetismus und Licht” (ca, 1947): reprints of works by Ehrenhaft and his pupils; newspaper clippings; personal letters; lecture notes; and the typescriptǀ of ten unpublished lectures on his experiments (1946).
The Smithsonian Institution’s Dibner Library of the History of Science and Technology has correspondence between Ehrenhaft and other scientists, chiefly Einstein (1917–1941), and related papers. The AIP Center has four reels of microfilm containing most of the correspondence. An exchange of letters is in the R. A. Millikan archive at the California Institute of Technology. An unpublished typescript of ten lectures Ehrenhaft delivered in Vienna in 1947 was produced and distributed in 1967 by his student Paul Feyerabend (with J. Ferber) as “Single Magnetic Northpoles and Southpoles.”
II. Secondary Literature. See Paul A. M. Dirac, “Ehrenhaft, the Subelectron and the Quark,” in Charles Weiner, ed., History of Twentieth Century Physics (New York, 1977), 290–293; and Gerald Holton, The Scientific Imagination: Case Studies (New York, 1978), 25–83, 302–323. Evaluations of the dispute over the charge of the electron include R. Bär, “Der Streit um das Elektron,” in Die Naturwissenschaften, 10 (1922), 322–327, 340–350; J. Mattauch, “Zur Frage nach der Existenz von Subelektron,” in Zeitschrift für Physik, 37 (1926), 803–815, and “Antwort auf die Bemerkungen Herrn Ehrenhafts zu meiner Arbeit:’ Zur Frage nach der Existenz von Subelektron, ’” ibid., 40 (1926), 551–556; and E. Wasser, “Über Ladungmessungen an Selenteilchen bei hohen Gasdrucken,” ibid., 78 (1932), 492–509. For a summary of the physics involved, see Milton Kerker, “Movement of Small Particles by Light,” in American Scientist, 62 (1974), 92–98.