3. Ernst Ruska: University Studies and His First Electron Microscope
After finishing high school in spring 1925, and after six months of practical training at the Brown-Bovery company in Mannheim, Ernst Ruska began his engineering studies at the Technische Hochschule in Munich. In 1928, he continued his studies at the Technological University in Berlin-Charlottenburg, where in the same year, Professor Adolf Matthias, head of High-Voltage Technology and Electrical Installations, had set up a working group to develop the old Braun tube (
Braun 1897) into a sensitive, fast-reacting cathode-ray oscillograph. Such an instrument was urgently needed by the electricity supply industry for the surveillance of high-voltage power lines. This working group, headed by Matthias’s assistant docent Dr. Max Knoll (1897–1969), included a number of dedicated Ph.D. students, diploma candidates, and advanced students, among them Ernst Ruska, Bodo von Borries, and Martin M. Freundlich (
Figure 3; for details, see
Freundlich 1963;
Ruska, E. 1979,
1980;
Lambert & Mulvey 1996). In December 1928, Ernst Ruska began with an undergraduate project: He was given the task of investigating experimentally the theoretical calculations made by Hans Busch on the electron-concentrating properties of electrostatic or magnetic fields (
Busch 1926,
1927).
Figure 3 Max Knolĺs group of doctoral candidates and undergraduates during a laboratory coffee break in the Technical University Berlin Charlottenburg in May 1931. From left to right, sitting: Carl Czemper, Ernst Ruska, Max Knoll, Robert Andrieux, Bodo von Borries, and Gustav-Adolf Blume. From left to right, standing: Kurt Schaudinn, Henning Knoblauch, Martin Freundlich (courtesy of Thomas Ruska, Ernst Ruska Archive).
It was already known that a bundle of electrons could be brought to a sharp focus using electrostatic or magnetic fields (
Busch 1926,
1927) comparable to visible light being focused by a magnifying glass lens. Supervised by Knoll, and later by the Ph.D. student Bodo von Borries as well, Ernst studied the properties of such focusing fields experimentally. He built and tested different designs and arrangements of electromagnetic lenses (
Ruska, E. 1929). Later, in his diploma work, begun July 18, 1930 and submitted on December 23, 1930, he also studied electrostatic fields (
Ruska 1930).
Figure 4 The first electron microscope, as published by Max Knoll and Ernst Ruska (1932). On April 7, 1931, this two-stage imaging instrument achieved sixteenfold enlarged images of metal aperture grids, thus proving the principle. A photograph of this instrument, as reconstructed in 1980, is included in the color insert (courtesy Thomas Ruska, Ernst Ruska Archive).
After obtaining his diploma in December 1930, Ernst Ruska continued working as an unpaid assistant. During these experiments—initially with Max Knoll and later with Bodo von Borries—he succeeded in constructing a hitherto-novel instrument. His apparatus consisted of a gas discharge tube with cold cathode emission, two magnetic lenses, and a “spherical condenser.” On April 7, 1931, using this two-stage magnification, Ruska recorded images of metal grids enlarged 16-fold (
Knoll & Ruska 1932a,
b; for reviews, see
Freundlich 1963;
Lambert & Mulvey 1996;
Ruska E. 1980;
von Borries 1991;
Wolpers 1991;
Figure 4). The term
electron microscope was born (
Knoll & Ruska, E. 1932a) and the instrument demonstrated that the principle worked, although it did not extend the resolving power of the existing light microscope. Because its performance was still poor, Ruska and Knoll hesitated to file a specific patent (
Ruska, E. 1980).
Figure 5 Bodo von Borries and Ernst Ruska during a cycling trip to the Baltic Sea in May 1931 (courtesy of Thomas Ruska, Ernst Ruska Archive).
It was clear that stronger collecting lenses with a shorter focal length and a more intense axial field were needed. In his Ph.D. dissertation, Ernst Ruska focused on the lens problem. By 1931, Max Knoll had became more and more involved in completing his habilitation treatise, and Ernst Ruska and Bodo von Borries began a close cooperation in designing and constructing electromagnetic lenses suitable for focusing a beam of electrons in a cathode ray oscillograph (
von Borries & Ruska, E. 1932).
Ernst Ruska developed a lens with an iron yoke round the coils and improved this design further in a collaboration with von Borries (
Figure 5) by adding iron pole pieces and making the inner bore cone-shaped. Now with a focal length of less than 3 mm and strong electron-collecting power, this advanced lens design became an essential component in the development of all further magnetic-type electron microscopes (
Ruska 1934b;
von Borries & Ruska, E. 1932). To protect their primacy, the engineers filed two patents on March 16, 1932, which were approved only in the middle of 1939, probably as a result of the existing disagreements. The titles were “Magnetische Sammellinse kurzer Feldlänge” (“Magnetic Converging Lens of Short Field Length”; DRP 680284); “Apparatur für Elektronenmikroskopie: Anordnung zur Beobachtung und Kontrolle der im Strahlengang eines Elektronenmikroskops mit zwei oder mehr elektronenoptischen Vergrösserungsstufen auftretenden elektronenoptischen Bilder (Arrangement for the Observation and Control of Electron-Optical Images Occurring in the Ray Path of an Electron Microscope with Two or More Electron-Optical Magnification Stages”; DRP 679 857), which were translated by Thomas Mulvey in
Ruska (1980).
Figure 6 Friedrich Krause (1937) used the instrument left behind by the engineers at Neubabelsberg to study biological samples. His work parallelled that of Ladislas Marton in Brussels (1934, 1937). (a) the magnetostatic electron microscope accompanied by two sketches on the technology of the new instrument and similarities with the light microscope. Below: Two images of bacteria fixed with formaldehyde and reproduced here at the original size. (b) Bacteria “embedded” in a stained support film appear in negative contrast. Magnification: x2.000-fold. Krause used various electron-dense stains to prepare these films and visualize bacteria in negative contrast. (c) Using transparent support films, the fixed bacteria appear with positive contrast due to their mass density. Magnification: x2.600-fold, original size (with the permission of Springer Science + Business Media, Berlin-Heidelberg). On April 18, 1932, the Institute for High-Tension Installations of the Technical University moved from Charlottenburg to Berlin-Neubabelsberg; and by December 1933, Ernst Ruska had built an advanced type of electron microscope there using these novel lenses (
Figure 6). Equipped with a condenser lens to conserve the electron current and two of the novel pole-piece lenses, it demonstrated up to 12,000-fold magnification on thin aluminium foils (
Ruska, E. 1934 a). Cotton wool fibers, however, used as test objects at only 8,000-fold magnification, were carbonized rapidly despite their small diameter of merely 1 micrometre in thickness (Ruska, E. 1934 a–c). As the resolution now clearly surpassed that of the light microscope, the instrument, though still far from being either perfect or versatile, was termed the “Übermikroskop” (hypermicroscope; abbreviated as ÜM). Its further development, however, was abandoned by the engineers because there were no prospects of industrial support or of commercial production.
By April 1, 1931, Max Knoll started work at the Telefunken factory; and in 1933, Bodo von Borries joined the RWE company in Essen and, later, from 1934 to 1937, the Siemens-Schuckertwerke in Berlin. After submitting his thesis, “Über ein magnetisches Objektiv für das Elektronenmikroskop,” on August 31, 1934 (
Ruska, E. 1934b), Ernst Ruska left academia as well, to join the Berliner Fernseh AG company and take over the development of television screens.
On December 12, 1933, Ernst Ruska submitted a paper called “Über Fortschritte im Bau und in der Leistung des magnetischen Elektronenmikroskops” (“On Progress in Construction and Performance of the Magnetic Electron Microscope”;
Ruska, E. 1934a). Soon after that, the close understanding between Ernst Ruska and Bodo von Borries fractured (
von Borries, H. 1991). These two gifted and strong-willed characters—despite having the same...
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