Wehnelt, Arthur Roudolph Berthold

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(b. Rio de Janeiro, Brazil, 4 April 1871; d. Berlin, Germany, 15 February 1944), Physics.

Wehnelt was the son of Berthold Wehnelt, an engineer and factory owner, who died at an early age, and Louise Muckelberg. He attended the Louisenstädter Gymnasium in Berlin and the Gymnasium in Landsberg an der Warthe (now Gorzów Wielkopolski, Poland), and graduated from secondary school in the spring of 1892. For a year he studied natural science at the Technische Hochschule in Berlin-Charlottenburg, and then from 1893 to 1897 at the University of Berlin. Next he went to the University of Erlangen, where he received the doctorate in the spring of 1898 under Eilhard G. H. Wiedemann. At Erlangen he became successively assistant at the Physics Institute (1900); Privatdozent (after obtaining the veniadocendi in 1901); and extra ordinary professor ofphysics (1904). In 1906 he was called to the University of Berlin as full professor, and in 1926 he was appointed director of its Physics Institute. He remained at the University of Berlin until 1939, becoming professor emeritus about 1938.

In his dissertation Wehnelt investigated the dark space near the cathode in gas-discharge tubes and established that the high resistance of the dark space of the cathode corresponds to that of a dielectric. Pursuing a remark made by Wiedemann and Ebert in 1891, Wehnelt demonstrated that the cross section of the cathode-ray bundle decreases with pressure, or with a decrease in the diameter of the tube placed around the bundle. These are essentially the characteristics of the epoch-making device (now known as the Wehnelt cylinder) that he later developed. In his Habilitationsschrift Wehnelt described the processes occurring in the discharge tube, taking into account the entire discharge from the cathode to the anode, and measured the current and voltage to analyze the discharge processes at various points of the tube. In the preface to his essay Wehnelt defended twelve theses, including the propositions that “electrical lighting systems of 220 volts should be avoided wherever possible,” and that “the refinement of modern electrical measuring methods has gone too far.”

Wehnelt became well known through his discoveries concerning discharge in rarefied gases. In the course of his research he studied cathode rays, canal rays, and Röntgen rays. He was involved in the technical development of the valve tube (the radio tube), Röntgen tubes, and the Braun tube (the oscilloscope); in this manner he made a fundamental contribution to modern electronics.

Three of Wehnelt’s discoveries deserve special mention. While investigating certain light and heat phenomena that had earlier been observed at very small electrodes by Davy, Wehnelt recognized in 1899 that current interruptions (up to 2200 sec-1) originate in rapid gas explosions at the electrode (for example, the platinum point). One such phenomenon, known as disruptive discharge, was demonstrated with the telephone by F. Richarz in 1892 and was applied to an electrical sweating process by Eugène Lagrange and P. Hoho in 1894. Wehnelt first applied an electrolytic interrupter–based on this phenomenon–to the induction coil, and then to short-exposure X-ray photographs.

In 1903 Wehnelt pointed out that a large potential drop exists between the cathode ray and the wall of the tube, and he used this knowledge to alter the hardness of the X rays, which he regulated with the help of an auxiliary tube placed over the cathode. To measure the hardness itself, he utilized the measurements published by Bénoist in 1902. concerning the comparative absorption of silver and aluminum. Wehnelt improved the measurements by replacing Bénoist’s aluminum disk with a continuous aluminum wedge (Wehnelt scale).

In 1903-1904 Wehnelt made his most important discovery, the “oxide cathode.” He observed a significant decrease in the cathode fall of glow discharge, occurring in the presence of platinum cathodes that had not been carefully enough cleaned; and then he noticed the same thing in the case of metal compounds, but especially in that of oxides. He found that the phenomenon was caused by an increase (which he estimated as 100 times) in the number of negative ions made available by the cathode metal when treated. A practical use for this insight was found in the production of very high current strengths (at 110 volts and 0.01 Hg pressure it is 3 amperes). More important, the exceptionally low exit potential at the cathode enabled Wehnelt to produce slow electrons, as well as soft canal rays. As a result, he could carry out measurements of velocity and of the ratio of charge to mass of very soft cathode rays. Since he knew how to obtain a good focus for the electron beam and how to make it visible at reduced pressure with the aid of the “Wehnelt cylinder,” he was especially successful in this undertaking. Wehnelt saw the theoretical interest of his discovery as possible confirmation of the hypothesis put forth by Kaufmann in 1902 and by Abraham in 1903, that is, that the mass of the electron is a purely electromagnetic quantity.

In subsequent research Wehnelt dealt with the photoelectric effect, secondary emission, mass spectra, thin metal layers, and the thermal conductivity of metals.


I. Original Works. A fairly complete bibliography of Wehnelt’s writings is in Poggendorff, IV, 1608-1609; V, 1345; VI, 2829; and VIIa, 887. See especially Strom-und Spannungsmessungen an Kathoden in Entladungsröhren. Habilitationsschrift zur Erlangung der Venia docendi der hohen philosophischen Fakultät der Friedrich-Alexander-Universität zu Erlangen, (Leipzig, 1901), which contains the 12 interesting theses that Wehnelt defended.

II. Secondary Literature. In addition to the titles cited by Poggendorff, the separately printed edition of Wehnelt’s Ph.D. (Leipzig, 1908) is reprinted (without the short biographical sketch originally included) in Annalen der Physik, 301 (1898), 511–542. Wehnelt’s MSS are held at the Staatsbibliothek Preussischer Kulturbesitz in Berlin; a few MSS also can be found at the Universitätsbibliothek in Erlangen.

Hans Kangro