The first electron microscope was built in the spring of 1931 in Berlin, when Ernst Ruska and his doctoral supervisor Max Knoll assembled a two-stage transmission instrument using magnetic lenses. That prototype could barely outperform the optical microscopes of its day, but it proved a radical idea: electrons, not light, could be harnessed to see the very small. What followed was not a clean, linear story of one inventor’s triumph but a tangled race involving rival engineers, competing patents, and parallel breakthroughs on three continents that collectively turned a physics demonstration into one of the most consequential instruments in modern science.
What Ruska and Knoll Actually Built
Ruska and Knoll’s 1931 instrument in Berlin was a transmission electron microscope, or TEM. It sent a beam of electrons through a thin specimen and focused the transmitted beam with magnetic coils acting as lenses. Their published report described the construction of this two-stage device and became the foundation for electron microscope designs worldwide.1Advances in Electronics and Electron Physics. Electron Microscopy in Berlin 1928–1945 The idea of using electron beams had been circulating among physicists since the late 1920s, after Louis de Broglie’s theoretical work showed that electrons have wavelengths far shorter than visible light. A shorter wavelength means the potential to resolve finer detail, and that potential is what drew Ruska, still in his twenties, into the project.
The 1931 prototype was crude. It could magnify objects, but its resolution was not yet better than what a good light microscope could achieve. Ruska continued improving the design, and by 1933 he had built a version that surpassed the resolution limit of optical microscopy for the first time. That crossing of the optical barrier is what made the electron microscope a genuinely new kind of instrument rather than an expensive novelty.
The Contested Invention
Crediting any single person with inventing the electron microscope oversimplifies the history, and the question of who deserved priority generated real disputes. Ernst Brüche, working at the German electronics firm AEG, was the first to construct an electron microscope by deliberately using electron-optical components in a cathode ray oscilloscope. Reinhold Rüdenberg, chief engineer at Siemens, filed a patent for an electron microscope concept. And Ruska designed and manufactured the first working instrument, later overseeing the development of the first commercially produced electron microscope.2Atlantis Press. A Historical Investigation of the Debates on the Invention and Invention Rights of Electron Microscope
The patent angle is especially contentious. Rüdenberg filed his patent in 1931, but critics argued he had not actually built a working instrument. Brüche’s approach used electrostatic lenses rather than Ruska’s magnetic ones, which represented a genuinely different technical path. Large corporations like Siemens and AEG competed fiercely for supremacy in the development and sale of this new technology, which gave the priority disputes a commercial edge that went well beyond academic credit.3Springer Link. Beyond Light: The Industrial Beginnings and Early Evolution of Electron Microscopy in Germany In practice, all three men contributed, and the instrument as it exists today descends from a braided lineage rather than a single inventor’s workshop.
Crossing the Atlantic
The German prototypes inspired work elsewhere almost immediately. By the late 1930s, researchers in Toronto had constructed their own high-resolution magnetic electron microscope, which they called a “supermicroscope.”4Canadian Journal of Research. THE CONSTRUCTION OF A MAGNETIC ELECTRON MICROSCOPE OF HIGH RESOLVING POWER The Toronto group, led by physicists Eli Franklin Burton and Cecil Hall along with graduate students James Hillier and Albert Prebus, pushed the practical resolution of the instrument further and helped establish North America as a center of electron microscopy development. Hillier went on to join RCA, where American commercial electron microscopes were produced through the 1940s.
This transatlantic spread mattered because it took the electron microscope out of physics labs and put it in the hands of biologists, materials scientists, and medical researchers. Once commercial models became available, the instrument stopped being a curiosity and started reshaping entire fields. Virologists could finally see the pathogens they had been studying indirectly for decades. Metallurgists could examine grain boundaries and crystal defects at scales that explained material failures. The 1940s and 1950s were a period of explosive adoption.
The Scanning Electron Microscope
The transmission electron microscope sends electrons through a sample, which means specimens must be sliced impossibly thin. A fundamentally different approach, the scanning electron microscope, works by sweeping a focused beam across the surface of a specimen and collecting the electrons that bounce back or are knocked loose. This produces detailed three-dimensional images of surfaces rather than flat cross-sections. The first draft of the SEM concept came from Manfred von Ardenne, another German physicist who was working in the same fertile period of the 1930s.5Advances in Imaging and Electron Physics. Reminiscences on the Origins of the Scanning Electron Microscope and the Electron Microprobe
Von Ardenne’s prototype demonstrated the principle, but the SEM did not become a practical laboratory tool until decades later. Shortly after World War II, Charles Oatley at the Cambridge University engineering laboratories began a long research program aimed at turning the scanning concept into a reliable, usable instrument.6CaltechAUTHORS. Persistence pays off: Sir Charles Oatley and the scanning electron microscope Oatley’s group, which included students who went on to become leaders in the field, worked through the 1950s and into the 1960s refining the design. The Cambridge Instrument Company eventually commercialized their work, and the modern SEM descends directly from those efforts. For anyone who has seen the dramatic, almost photographic images of insects, pollen grains, or fractured metal surfaces, those are SEM images, and the lineage traces back through Oatley to von Ardenne’s initial sketch.
Japan’s Rapid Rise in Electron Microscopy
While Europe and North America built the early prototypes, Japan became one of the world’s largest producers of electron microscopes with remarkable speed. In 1941, both Hitachi and Shimadzu began manufacturing commercial electron microscopes, drawing on the detailed construction information published in von Ardenne’s book and the theoretical framework in Zworykin’s work. After the war, the Japan Electron Optics Laboratory, now known globally as JEOL, started manufacturing magnetic electron microscopes in 1947. Akashi Seisakusho followed in 1950.7Advances in Imaging and Electron Physics. The Growth of Electron Microscopy in Japan
This was not simply copying Western designs. Japanese manufacturers quickly became innovators in their own right, and by the latter half of the twentieth century, JEOL and Hitachi were among the most important electron microscope makers in the world. Today, a significant share of the electron microscopes in research labs globally carry Japanese brand names. The rapid Japanese entry into the field also helped drive down costs and improve reliability, making the instruments accessible to a wider range of institutions than would have been possible if production had stayed concentrated in a few European and American firms.
Seeing Individual Atoms
For decades, electron microscopes could resolve structures far smaller than anything visible through light, but individual atoms remained out of reach. That changed with work by Albert Crewe at the University of Chicago in the late 1960s and 1970s. Crewe developed a new type of scanning transmission electron microscope, or STEM, that used a cold field emission source and specially designed low-aberration lenses. This combination revolutionized the field by making atomic-scale imaging and quantitative chemical analysis possible for the first time.8Ultramicroscopy. Albert Crewe–inventor of the modern STEM
Crewe’s instrument could produce images of individual heavy atoms sitting on a thin carbon film. The photographs were startling when they were first published. For the physics and chemistry communities, this was the equivalent of landing on the moon: atoms had been inferred, modeled, and theorized about for over a century, but now someone was looking at them. The STEM design Crewe pioneered remains the basis for most of the highest-resolution electron microscopy performed today, particularly in materials science and semiconductor research where knowing the position of individual atoms in a crystal lattice is essential.
Cryo-EM and the Problem of Water
Electron microscopes operate in a vacuum, which is fine for metal samples and dried biological preparations but catastrophic for anything that needs to stay wet. Living cells, proteins in solution, viruses in their native environment: all of these are destroyed by the vacuum and the electron beam. For decades this was a fundamental limitation. Biologists could look at dead, dried, metal-coated specimens but could not see the structures of life as they actually exist.
The breakthrough came in December 1981, when a short paper in the Journal of Microscopy described a method for vitrifying pure water, freezing it so rapidly that it forms a glass-like solid rather than ice crystals. That seemingly simple technique turned out to be profoundly important. It reversed water’s status in the electron microscopy community from enemy to ally and laid the groundwork for cryo-electron microscopy, or cryo-EM.9PubMed. Cryo-EM–the first thirty years By rapidly freezing biological samples in vitrified water, researchers could preserve the delicate three-dimensional structure of proteins and other biomolecules, then image them with the electron beam.
Cryo-EM did not become a dominant technique overnight. It took another two decades of improvements in detectors, image processing software, and computational power before the method reached near-atomic resolution. But when it did, the impact on structural biology was seismic. The technique earned its developers the Nobel Prize in Chemistry in 2017 and has since become the workhorse method for determining the shapes of proteins, including the spike protein of SARS-CoV-2 during the COVID-19 pandemic.
Breaking Through the Aberration Barrier
Even the best electron lenses suffer from a fundamental optical flaw called spherical aberration: electrons passing through the edges of a lens are bent differently than those passing through the center, blurring the final image. For most of the twentieth century, this was considered an unavoidable limitation, and electron microscopists designed their experiments around it. The theoretical possibility of correcting the aberration had been known since the 1940s, but building a practical corrector proved extraordinarily difficult.
That changed in the late 1990s, when a team implemented a hexapole corrector system on a commercial 200 kV transmission electron microscope equipped with a field emission gun. The corrector compensated for the spherical aberration of the objective lens, improving the instrument’s point resolution from 0.24 nanometers down to better than 0.13 nanometers.10Journal of Electron Microscopy. Towards 0.1 nm resolution with the first spherically corrected transmission electron microscope To put that in perspective, a hydrogen atom is about 0.1 nanometers across, so this brought the instrument tantalizingly close to resolving the smallest atom in the periodic table. The successful correction was also independently confirmed using the same general approach on another commercial instrument.11Ultramicroscopy. A spherical-aberration-corrected 200 kV transmission electron microscope
Aberration-corrected microscopes are now standard in top materials science and physics labs. They have enabled researchers to map the positions of atoms in complex alloys, study defects in semiconductors at the level of individual atomic columns, and image the interfaces between different materials with a clarity that was physically impossible before the correctors existed. The resolution race is not over: newer corrector designs and improved electron sources continue to push the limits.
The 1986 Nobel Prize and What It Recognized
Ernst Ruska waited over half a century for formal recognition. In 1986, the Nobel Prize in Physics was awarded partly to Ruska for his fundamental work in electron optics and the design of the first electron microscope. He shared the prize with Gerd Binnig and Heinrich Rohrer, who had developed the scanning tunneling microscope about fifty years after Ruska’s original work.12PubMed. Electron Microscope Inventors Share Nobel Physics Prize Ruska was 79 years old at the time, and he died just two years later.
The pairing of these two awards in the same year was telling. The scanning tunneling microscope, or STM, works on an entirely different principle from Ruska’s instrument: instead of sending electrons through a sample, it drags an atomically sharp needle across a surface and measures the quantum mechanical tunneling current between the needle and the surface atoms. The STM can image individual atoms on a surface and even manipulate them one by one. By honoring both instruments together, the Nobel committee drew a line from Ruska’s 1931 prototype through fifty years of development to the atomic-scale surface science of the 1980s.
Imaging Wet and Gaseous Samples Without Freezing
Cryo-EM solved the water problem by freezing samples, but another branch of development asked a different question: what if you simply let the sample stay wet and surrounded by gas? The environmental scanning electron microscope, or ESEM, does exactly that. Unlike conventional SEMs, which require a high vacuum around the specimen, the ESEM allows samples to sit in a gaseous environment at relatively low vacuum. This means wet biological specimens, oily industrial components, and even living insects can be imaged without the extensive preparation that traditional electron microscopy demands.13ScienceDirect (Elsevier). Foundations of Environmental Scanning Electron Microscopy
The ESEM trades some resolution for this versatility, but for many applications the tradeoff is worth it. Materials scientists use it to watch corrosion happen in real time. Geologists examine rock samples without drying out the clay minerals whose hydration state matters for understanding their behavior. Forensic analysts image fibers and biological evidence without the risk of altering them through dehydration or metal coating. The instrument expanded the range of questions electron microscopy could address, turning it from a tool that demands samples be dead and dry into one that can observe dynamic, real-world conditions.
Why 1931 Is the Answer, but Only Barely
If someone asks when the electron microscope was invented, 1931 is the correct short answer, and Ruska and Knoll are the names most commonly attached to it. But the fuller picture is that 1931 marks the construction of a prototype that barely worked, built in a period when several research groups were converging on the same idea from different directions. The instrument that researchers actually use today is the product of ninety years of refinement, from the aberration-plagued magnetic lenses of the 1930s to the aberration-corrected, cryo-cooled, computationally enhanced machines in labs right now. Each decade brought a genuinely new capability: higher resolution in the 1940s and 1950s, surface imaging with the SEM in the 1960s, single-atom visibility in the 1970s, cryo-preservation for biology in the 1980s, aberration correction in the 1990s, and the computational revolution in cryo-EM image reconstruction in the 2000s and 2010s. The electron microscope was invented once, in the sense that every technology has a first prototype, but it has been reinvented continuously ever since.