Replacing light rays and optical lenses, respectively, with electron beams and “electron lenses,” a group of German engineers pioneered electron microscopy.

A number of the most important inventions of the twentieth century had scattered origins, as did the first electron microscope, constructed by Ernst Ruska and Max Knoll in 1931. The electron microscope’s history can be traced along three lines: motivation, theory, and technology. Scientists who look into the microcosmic world always demand microscopes of higher and higher resolution, or resolving power—that is, the ability of an optical instrument to distinguish closely spaced objects. As early as 1834, George Airy, the eminent British astronomer, theorized that there should be a natural limit to the resolution of (optical) microscopes. In 1873, two Germans, Ernst Abbe, cofounder of the Karl Zeiss Optical Works at Jena, and Hermann von Helmholtz, the famous physicist and philosopher, independently published papers on this issue. Both arrived at the same conclusion as Airy. [kw]First Electron Microscope Is Constructed (Apr., 1931)
[kw]Electron Microscope Is Constructed, First (Apr., 1931)
[kw]Microscope Is Constructed, First Electron (Apr., 1931)
Microscopes;electron
Electron microscopes
Inventions;electron microscope
[g]Germany;Apr., 1931: First Electron Microscope Is Constructed[07820]
[c]Science and technology;Apr., 1931: First Electron Microscope Is Constructed[07820]
[c]Physics;Apr., 1931: First Electron Microscope Is Constructed[07820]
[c]Inventions;Apr., 1931: First Electron Microscope Is Constructed[07820]
Ruska, Ernst
Knoll, Max
Busch, Hans
Broglie, Louis de
Rüdenberg, Reinhold

Ernst Ruska.

(The Nobel Foundation)

When it was proved that the wavelength of the light is the ultimate obstacle in raising the resolving power of the microscope, scientists and engineers began to consider electromagnetic radiations of shorter and shorter wavelengths. At the beginning of the twentieth century, Joseph Edwin Barnard Barnard, Joseph Edwin experimented on microscopes using ultraviolet light. Such instruments, however, only modestly improved the resolution. In 1912, Max von Laue Laue, Max von considered trying X rays. At the time, however, it was hard to turn “X-ray microscopy” into a physical reality. The wavelengths of X rays were exceedingly short, but they mostly penetrated material objects. It was thus made clear in the early 1920’s that, in terms of resolving power, the optical microscope was approaching its limit. In a new microscopy, light—even electromagnetic radiation in general—as the traditional medium that carries image information, had to be replaced by a new medium. At the same time, progress in physics—theoretical as well as experimental—began to offer a prospective medium. In 1924, the French theoretical physicist Louis de Broglie advanced a startling hypothesis. His insight into the analogues between dynamic and optical phenomena and their mathematical formalities—a profound analogy first disclosed by Sir William Rowan Hamilton in the 1830’s—led him to state that there was something of a wave nature associated with material particles, particularly light microcosmic particles, such as electrons. His quantitative conclusions included a formula relating the particle’s motion—more exactly, momentum—to the wavelength of the particle’s associated wave. The faster the particle moves, the shorter the wavelength is.

Before the first electron microscope had been built, the technological possibility of electron microscopy occurred to some theoreticians along the line of the “matter wave.” According to Dennis Gabor, Gabor, Dennis in 1928 Leo Szilard suggested to him that an electron microscope should be made. Gabor, however, dismissed the idea with a forceful yet hypothetical statement: “Everything under the electron beam would burn to a cinder!” The electron microscope is another case of technological breakthrough that illustrates an interesting historical theme: Those who knew too much theory did not make the thing; those who made it were not aware of the latest theory.

When Knoll and Ruska built the first electron microscope in 1931, they had never heard about de Broglie’s “matter wave.” Ruska recollected that when, in 1932, he and Knoll first learned about de Broglie’s idea, he became “extremely disappointed” but then “was immediately heartened” because he realized that those matter waves “must be around five orders of magnitude shorter in wavelength than light waves.” It was based on two other lines of physical study—oscillography and electron optics—that Knoll and Ruska accomplished their invention. The core component of the two new subjects was the electron beam, or the cathode ray, as it was usually called then. Although for a long time the physical nature of the beam was not clear, some nineteenth century physicists succeeded in controlling and focusing it. As early as 1858, the mathematician and physicist Julius Plücker Plücker, Julius noticed that magnetic fields could deflect the “electric glow discharge.” Later, in 1869, Johann Wilhelm Hittorf Hittorf, Johann Wilhelm performed more and better experiments in controlling the cathode rays. In 1891, Eduard Riecke, Riecke, Eduard using Hittorf’s results and carrying out his own calculations, conjectured about the ultimate corpuscular nature of the phenomena involved. Five years later, in 1896, Olaf Kristian Birkeland, Birkeland, Olaf Kristian a Norwegian physicist, after experimenting with axially symmetric magnetic fields, arrived at a very encouraging conclusion: “Parallel light rays are not concentrated better to the focal point by a lens than are cathode rays by a magnet.”

From around 1910, the German physicist Hans Busch was the leading researcher in this field. In 1926, he published his theory on the trajectories of electrons in magnetic fields. His conclusion was that magnetic or electric fields possessing axial symmetry act as lenses for electrons or other charged particles. In 1927, he conducted experiments verifying his own theory with a magnetic lens. With these contributions, Busch has been recognized as the founder of a new field later known as electron optics. Electron optics His theoretical study showed, among other things, the exactness of the analogy between light rays and optical lenses on one side and electron beams and electromagnetic lenses on the other. One logical consequence thus should be the feasibility of electron microscopy. Busch’s experimental verification, however, was not a complete success. Ruska noticed that there existed an “order-of-magnitude discrepancy between the size of cathode image that he [Busch] found experimentally and that required by the imaging equation,” in short, between theory and measurement.

From 1928, Ruska, as a graduate student at the Berlin Institute of Technology, belonged to a group engaged in studying and building cathode-ray oscilloscopes, an instrument much in demand in the industry of electric power. Knoll and Ruska worked hard to find the physical laws of focusing electron beams by magnetic or electric fields. Ruska’s first project was the “bundling” of electron rays in the coaxial magnetic field of the short coil. On one hand, he had to find a method of calculation for the optimal design; on the other, he tried to build “powerful and compactly built oscillographs.” Because of this task, he was concerned with the discrepancies in Busch’s result. At this juncture, the difficulty of Busch became Ruska’s motivational force. Ruska carried out accurate measurements, especially with regard to the “lens theory of the short coil.” By doing so, he identified the major problem in Busch’s work, that is, the nonuniformity of the energy of the electrons in the beam. Beginning with certain nonuniformity, everything thereafter became increasingly diffused.

Knoll and Ruska’s effort ended in a series of successes: verification of Busch’s theory, design and materialization of a concentrated electron “writing spot,” and the actual construction of the electron microscope. By April, 1931, they established a technological landmark with the “first constructional realization of an electron microscope,” although when Knoll lectured about their work in June, he avoided the term “electron microscope” because he did not want to be “accused of showmanship.”

The world’s first electron microscope, which took its first photographic record on April 7, 1931, was rudimentary. Its two-stage total magnification was only sixteen times. Since that time, however, progress in electron microscopy has been spectacular. It is one of the prominent examples that illustrate the historically unprecedented pace of science and technology in the twentieth century. One comparison in the field of microscopy is that it took centuries for the simple magnifier, or the “burning glass,” to become the compound microscope, but the equivalent transition in the transmission electron microscope took only two years.

Ruska and Knoll’s achievement immediately motivated others to study further and experiment, although electron microscopes with better-than-light-microscope resolution seemed in 1932, according to many experts at the time, a pipe dream. Ruska had set his goal to create an electron microscope that would have a resolution better than that of the best optical microscope and that could observe every kind of specimen that had been observed previously in the light microscope. After Knoll left the team to work on the developing technology of television, Ruska found new coworkers and stimulated more. Reinhold Rüdenberg was then the chief of the scientific department of the Siemens-Schuckent-Werke. At the end of May, 1931, he applied for a patent in electron microscopy; later, after Knoll and Ruska’s first paper was published in 1932, he stated that similar work had been ongoing for some time at Siemens. According to Ruska, Rudenberg could not substantiate such claims with actual results.

In 1935, for the first time the electron microscope surpassed the optical microscope in resolution. The problem of damaging the specimen by the heating effects of the electron beam proved to be more difficult to resolve. In 1937, a team at the University of Toronto constructed the first generally usable electron microscope. In 1942, a group at the Radio Corporation of America (RCA) headed by James Hillier Hillier, James produced its commercial transmission electron microscopes. From 1939 and 1940, research papers on electron microscopes began to appear in Sweden, Canada, the United States, and Japan; from 1944 to 1947, papers appeared in Switzerland, France, the Soviet Union, the Netherlands, and England. Following research work in laboratories, commercial transmission electron microscopes using magnetic lenses with short focal lengths also appeared in these countries.

Despite some priority disputes, Ruska’s personal contribution has been generally recognized. In 1960, Ruska and Hillier were jointly presented the Albert Lasker Award in Medical Research for “their major contribution to the design, construction, development and perfection of the electron microscope, which led to the creation of an unique and much used research instrument.” In 1986, Ruska received the Nobel Prize in Physics Nobel Prize recipients;Ernst Ruska[Ruska] with Gerd Binnig and Heinrich Rohrer, two IBM physicist-engineers, for inventing the scanning tunneling microscope in the early 1980’s. Although the scanning tunneling microscope is a different microscope from Ruska’s transmission electron microscope, both types use the quantum mechanical characteristics of the electron beam.

The electron microscope has been described as one of the most important inventions of the twentieth century, and the long-range impacts of the instrument on science and engineering are self-evident. About Ruska’s work, John Reisner of RCA stated: “While electron-optics people knew of the idea after the work by Busch on electron trajectories, Ruska did it. It was tough technology, and his was the step that got everyone going.” Microscopes;electron
Electron microscopes
Inventions;electron microscope

• Bradbury, S. The Evolution of the Microscope. Elmsford, N.Y.: Pergamon Press, 1967. Detailed examination of the evolution of microscopy as a whole; ends with a history of the electron microscope.
• Burton, E. F., and W. H. Kohl. The Electron Microscope. New York: Reinhold, 1942. Although dated, explains the physics and technology of the electron microscope in a manner accessible to lay readers. Chapters titled “The Dual Theory of the Electron” and “The History of the Electron Microscope” are particularly informative.
• Hawkes, Peter W., ed. The Beginnings of Electron Microscopy. Orlando, Fla.: Academic Press, 1985. Substantial anthology covers the history of electron microscopy. Most of the contributions discuss its development in various nations, fields, and laboratories. Includes a preface by Ruska.
• Marton, Ladislas. Early History of the Electron Microscope. 2d ed. San Francisco: San Francisco Press, 1994. Brief historical introduction by an original contributor in the development of electron microscopy. Includes a preface by Dennis Gabor, the major constructor of the first electron lens and oscilloscope.
• Rasmussen, Nicolas. Picture Control: The Electron Microscope and the Transformation of Biology in America, 1940-1960. Stanford, Calif.: Stanford University Press, 1997. Discusses the rapid impact of the availability of the electron microscope on the direction of biological research in the United States. Includes index.
• Ruska, Ernst. The Early Development of Electron Lenses and Electron Microscopy. Translated by Thomas Mulvey. Stuttgart, Germany: S. Hirzel, 1980. Historical introduction by the acknowledged inventor himself begins with the ancestry of the optical microscope. Aware of his special position in the overall development, Ruska is careful to acknowledge other researchers’ contributions. This account extends to around 1940, when Ruska’s major contributions (magnetic lens with short focal lengths and the prototype transmission electron microscopes) still exerted significant influence.
• Wyckoff, Ralph W. G. Electron Microscopy. New York: Interscience, 1949. Although dated, provides an informative historical introduction to the technique and the applications of the electron microscope.

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