Frans Michel Penning (1894-1953)
Frans Michel Penning, born December 12, 1894 at Gorcum, The Netherlands, studied Physics and Mathematics at the University of Leiden and graduated there on June 25, 1923 on a thesis entitled 'Metingen over isopyknen van gassen bij lage temperaturen' (Measurements on Isometric Density Lines of Gases at Low Temperatures). Promotor was Professor H. Kamerlingh Onnes. On March 15, 1924, Penning started to work at the Philips Natuurkundig Laboratorium at Eindhoven, where since 1914 Holst and Oosterhuis, and since 1920 also G. Hertz, had made investigations on electric discharges in rare gases. Penning and others were commissioned to carry on these investigations. Completely in line with his tutor Kamerlingh Onnes, Penning in the first place was an experimental physicist. But besides that he possessed the perspicacity to detect new phenomena, and the fantasy to coordinate the measured macroscopic discharge phenomena with the microscopic world of elementary processes, notably of the reactions between atoms, electrons and light quanta.
Penning's initial observations, among others, concerned high-frequency vibrations in gas discharge tubes and related abnormal electron velocities. These observations led to conclusions contradicting those of Irving Langmuir. During a personal discussion, Penning succeeded in convincing Langmuir of the correctness of his (Penning's) ideas. Relatively soon Penning started measurements on the liberation of electrons from metal surfaces by positive ions and metastable atoms, and especially on the effects related to ionisation by metastable atoms. Especially the latter effect, already conjectured from previous work by G. Hertz on energy levels of inert gases, was thoroughly investigated into all directions. Once the necessary data had been collected, Penning was also able to produce new effects, such as raising the discharge voltage of an inert gas containing an easily ionisable gas, the extinction of a discharge in an argon-neon mixture by radiation, etc. Also of interest are his measurements on discharge voltages in inert gases and inert gas mixtures, especially in helium, as function of gas density and electrode distance, and his corresponding reflections on the stability of discharges. Typical of Penning was his tenacity in pursuing the same subject, either because he was not completely satisfied with the published measurements, or because he saw possibilities for extending the subject into various new directions.
Perhaps his most important contribution was the study of the behaviour of discharges in a magnetic field. As a practical result a vacuum gauge emerged, in which the current through a discharge tube in a magnetic field is a measure of pressure. This gauge was marketed by several industrial firms because of its quick and direct indication. Also have to be mentioned the detailed measurements by Penning and his co-workers of Townsend's ionisation coefficient, leading to the insight that not only ions but occasionally also light quanta are able to liberate electrons from the cathode of a gas discharge tube.
In 1939 Penning, in co-operation with M.J. Druyvesteyn, wrote a review of his previous work and his new insights in the field of gas discharges in the (American) Review of modern Physics. The paper appeared in 1940 but then Holland had been occupied and Penning did not see the Journal until 1946. During the war Penning's assistance was asked in the development of new types of high frequency electron tubes in the Philips Tube Factory. After the war Penning returned to his original subject. His interest was drawn by the well-known fact that the data on normal cathode fall during a glow-discharge were so diverging that is was hardly possible to construct a table with generally acceptable values. Having learned how to master the purity of gases, Penning now devoted himself to the cathode material. After completely stripping it from its native oxide layer, he succeeded in producing consistent measurement of cathode fall. As a result a new tube of reliable voltage stability ensued.
In the meantime the above mentioned review paper of 1940 had made great impression in the United States and Penning was invited to come to that country in 1950 to see how his work was estimated, studied and pursued. An American paper humorously illustrated Penning's work and its often seemingly paradoxical results in an Alice-in-Wonderland presentation.
In the following years Penning's health gradually deteriorated and he died on December 6, 1953, at Utrecht, while recovering from an operation.
(Written by the late Dr. Wim de Groot, fellow-physicist of Frans Michel Penning at the Philips Laboratory at Eindhoven, The Netherlands. Appeared in Nederlands Tijdschrift voor Natuurkunde, vol. 20, January 1954. Translated and shortened by the late Prof. Dr. Lourens Penning, Neuroradiology at University of Groningen, The Netherlands)